Colloids And The Ultramicroscope

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COLLOIDS

AND TOT

ULTRAMICROSGOPE

A MANUAL OF

Colloid Chemistry and Ultramicroscopy

BT

DR RICHARD ZSIGMONDY

Professor of Inorganic Chemistry in the university of Gdttingen

AUTHORIZED TRANSLATION

BY

JEROME ALEXANDER, M.Sc.

FIRST

SECOND THOUSAND

NEW YORK

JOHN WILEY & SONS,
LONDON: CHAPMAN & HALL, LIMITED
1914

Copyright, IP"

BT

JEROME ALEXANDER

BRAUNWORTM A CO.
BOOK MANUFACTURERS
IKL.VN, N. V.

PREFACE TO THE ENGLISH EDITION

THE expectation that this book would arouse the
interest of a wider circle has been agreeably fulfilled. I
am glad to be able to 'state that Mr. Jerome Alexander,
of New York, has undertaken to translate the book
because of interest in its contents, and has carried
out the work carefully and conscientiously. I have
myself read over the whole translation and found it in
excellent accord with the original text.

In so far as concerns the experimental basis for answer-
ing' the fundamental questions of Colloid Chemistry,
there is but little to be added to the German edition.
In but few places has it become necessary to amplify
the statements and extend the text because of later
publications. .

May the English edition receive the same friendly
reception, as the German edition.

R. ZSIGMONDY.

GOTTINGEN, November 14, 1908.

iii

TRANSLATOR'S PREFACE

AFTER discussing the basic principles governing the
colloidal condition, and the classification of colloids, and
reviewing the most important work already done in
this field, the author describes the development (upon
the principle originally conceived by himself) of the
ultramicroscope, which carries our range of vision well
towards molecular dimensions; and he furthermore
gives a detailed account of his own valuable pioneer
researches with the new instrument.

Giving as it does an actual insight into a sphere here-
tofore beyond the range of direct observation, the ultra-
microscope has proven to be a powerful weapon with
which to attack numerous problems confronting the
chemist, the physicist, and the biologist; and it will be
of special value in deciding many mooted questions in
theoretical and in technical colloid chemistry.

As the far-reaching ramifications of colloid chemistry
are better understood, its importance and the applicability
of its principles to a great variety of industrial problems,
become "more and more evident. There might, for ex-
ample, be mentioned agriculture, tanning, dyeing; rubber,
cement, ceramics; soaps, photography, sugar in fact,
almost every industry is directly or indirectly involved.
Professor Zsigmondy's work will, therefore, be of vital
interest not only to scientists concerned with theoretical

Vi TRANSLATOR'S PREFACE

questions, but also to chemists, engineers, and others
controlling technical processes. To physiological chemists
and physicians it is indispensible.

I must express my sincere thanks to Prof. Zsigmondy
and also to Alexander D. Ross, M.A., B.Se, (lecturer
on Natural Philosophy in the University of Glasgow),
both of whom read over the original manuscript of the
translation and made valuable suggestions. In the
English edition besides some additions to the text, there
are included two beautiful colored plates originally pub-
lished in Professor Zsigmondy 's paper on " Colloid
Chemistry/'

JEROME ALEXANDER.

NEW YORK, February 24, 1909.

PREFACE TO THE GERMAN EDITION

SOME of the conclusions and observations herein set
forth were originally intended for the Zeitschrift fur
physikalische Chemie; however, as the method of render-
ing visible ultramicroscopic particles which were developed
by Siedentopf and myself, has awakened the interest
of a larger circle, I have decided to m^ke readily accessible
to the representatives of other branches of science the
results of my ultramicroscopic investigation of fluids,
and the experiments associated therewith; all the more
so because they may be of some use to other workers in
the same field. Another reason that led me to take
this step was the fact that the problem I attacked is of
interest not only to the physicist and the chemist, but
to scientists in general.

It will be shown by a number of examples how the
properties of a solid metallic gold change with its pro-
gressive subdivision, especially if the subdivision be
carried as far as possible, to a degree approaching molec-
ular dimensions; and as near as may be the size; and
properties of the individual particles in each c&se are
accurately given.

As will be seen in the following chapters, the problem
is intimately connected with the question of the nature
of colloidal solutions, the latter in fact being the starting
point of this investigation.

Vlii PREFACE TO THE GERMAN EDITION

The paucity of our knowledge on this subject is shown
by the discussion of the question whether colloidal solu-
tions are homogeneous or heterogeneous, and whether
there is or is not progressive transformation from colloidal
to crystalloidal solution. As a preliminary discussion
shows that the words "solution " and " suspension "
are by no means synonymous, it is necessary, in order to
pave the way, to consider what these expressions leally
mean.

The title of this book, "Zur Erkenntnis der Kolloide,"
might lead to the belief that I intended to go into the
structure of hydrogels or jellies. Chapter XXI contains
some remarks on this subject. When working in a wide
field, however, it is best to proceed from the simple to
the complex. Solutions of colloids are of a simpler
nature than jellies; they contain to a certain extent the
structural units of the latter, and wherever before coagu-
lation begins, the appearance of ultramicroscopic parti-
cles is observed, it may be taken for granted that these
are constituents of the hydrogel. Finally, when we have
acquired more intimate knowledge of the actual constitu-
tion of such colloidal solutions, we can then successfully
theorize regarding the ultimate structure of jellies which
does not admit of direct observation.

But the constituent particles of hydrosols, which I
have just compared to structural units, show such varia-
tion in size, constitution, and properties, that it is neces-
sary at the outset to confine ourselves to one especially
simple case. As already intimated, the colloidal solutions
of metallic gold have been chosen. The minute particles
of gold composing them, their formation, their relation to
each other, and to the particles of other colloids, are
all closely considered. Reference to earlier work will
show that they are not indifferent to the action of other

PREFACE TO THE GERMAN EDITION ix

colloids, and that when they are mixed with other colloids,
products are often produced which have been in the
past frequently mistaken for chemical compounds.

In addition I have given, in more or less detail, an
account of my ultramicroscopic investigations on various
hydrosols, and also of the work already published by
other investigators.

On seeing the variety of these elementary relations, it
will at once be manifest that in the study of the colloids
we have outlined an enormous scientific structure, whose
foundation is as yet scarcely begun.

R. ZSIGMONDY.

JENA, May, 1905.

CONTENTS

INTRODUCTION

PAB

INTRODUCTION . 1

CHAPTER I
LIMITATION OP THE FIELD 11

CHAPTER II

CLASSIFICATION OP HYDROSOLS ACCORDING TO Two DIFFERENT

POINTS OF VIEW 19

CHAPTER III
HISTORY OF THE IRREVERSIBLE COLLOIDS 30

CHAPTER IV

FACTS POINTING TO THE HOMOGENEITY OF GOLD HYDROSOLS.

DEVELOPMENT OF ULTRAMICROSCOPY 95

CHAPTER V

DESCRIPTION OP THE APPARATUS FOR MAKING VISIBLE ULTRA-
MICROSCOPIC PARTICLES 103

CHAPTER VI
CERTAIN TERMS OFTEN USED HEREIN 109

Xii CONTENTS

CHAPTER VII

PAQB

PRINCIPLES OF THE ULTRAMICROSCOPIC INVESTIGATION OF
FLUIDS Ill

CHAPTER VIII
PREPARATION OP COLLOIDAL GOLD SOLUTIONS 124

CHAPTER IX
ULTRAMICROSCOPIC EXAMINATION OF THE SOLUTIONS OF GOLD. . 129

CHAPTER X
MOTION OF THE GOLD PARTICLES 134

CHAPTER XI
SIZE AND COLOR OF THE PARTICLES 141

CHAPTER XII
THE COLOR CHANGE OF COLLOIDAL GOLD 144

CHAPTER XIII
THE PRECIPITATION AND PROTECHOM OF COLLOIDAL GOLD 147

CHAPTER XIV
FILTRATION EXPERIMENTS 153

CHAPTER XV

THE SIZE OF THE GOLD PARTICLES COMPARED WITH THE SIZE
OF OTHER BODIES 157

CONTENTS xiii

CHAPTER XVI

PAB

SUPERIOR AND INFERIOR LIMITS OF THE SIZE OF THE PAR-
TICLES 180

CHAPTER XVII
AMICROSCOPIC NUCLEI IN COLORLESS RUBY GLASS 165

CHAPTER XVIII
GENERAL REMARKS CONCERNING METAL HYDROSOLS 175

CHAPTER XIX

ULTRAMICROSCOPIC EXAMINATION OF CERTAIN SOLUTIONS AND

SUSPENSIONS 188

CHAPTER XX

ULTRAMICROSCOPIC INVESTIGATIONS FROM THE PUBLICATIONS OF

OTHER SCIENTISTS 198

CHAPTER XXI

ON THE FORMATION OF HYDROSOL AND HYDROGEL 212

RECAPITULATION 236

COLLOIDS AND THE ULTRAMICROSCOPE

INTRODUCTION

Regarding Solution and Homogeneity. The original
conception of the word "solution," and the one still extant,
does riot completely coincide with that developed from
the generalization of the laws of gases.

As considerable misunderstanding may arise by using
the term "solution" to express different ideas, a few
words on this subject may not be amiss.

Solutions are more easily described by limitation than
by definition. Originally they included solutions not only
of crystalloids, but also many of the earlier known colloids.
Graham included for instance solutions of albumen, glue,
rubber, ferric oxid, Prussian blue, etc., and because of its
uniform appearance Berzelius considered the purple of
Cassius to be a solution. Ordinarily we speak of dilute
aqueous solutions of zinc chlorid and ferric chlorid.
although these solutions are partially hydrolyzed and lack
homogeneity.

An example of an old definition of " solution" is found
in Fehling's " Handworterbuch der Chemie":

"Solution is the equal distribution of a body in a liquid,
the resulting mass being in 'all parts homogeneous and
fluid enough to form drops. According as the dissolved
body is solid, fluid, or gaseous, the product is termed a

2 INTRODUCTION

solution (in the restricted sense of the word), a mixture,
or an absorption."

Ostwald and Nernst also define solution as a homo-
geneous mingling, 1 a physical mixture, 2 a homogeneous
phase. 3 Consequently homogeneity is even now regarded
as the distinguishing characteristic of solutions.

Because of the brilliant results following the general
application by van't Hoff of the laws of gases, physical
chemists came to regard these laws as typical attributes
of solutions, although as enunciated they apply only to
solutions in the more limited sense, viz., those of a crys-
talloidal nature. As characteristic properties of crystal-
loid solutions, may be mentioned: diffusibility, osmotic
pressure, the capability of the solute (in the case of solids)
to crystallize from supersaturated solutions.

Regarding the above as the most important character-
istics of solutions, 4 these scientists must have naturally
considered the question whether those mixtures which
completely or partially lack these characteristics (colloidal
solutions or hydrosols) could be classed as solutions at all. 5

1 Ostwald, Grundriss der allg. Chem., 2d ed., p. 119.

2 Nernst considers solutions to be physical mixtures, and defines
them as complexes of different substances, in every part physically
and chemically homogeneous. Theoret. Chem., 3d ed., p. 99.

3 Ostwald, Grundriss, 3d ed., p. 313.

4 Some scientists apparently failed to observe the fact that in addi-
tion to the conception of solution as used by them, there exists another
with a much wider meaning which is hi general use. I do not there-
fore consider it necessary to put the word " solution'' in quotation
marks when speaking of colloidal solutions.

5 It might here be mentioned that research on this point has lead
to the conclusion: typical colloidal solutions are not solutions. It
will, of course, be seen that this only means that typical colloidal
solutions are not crystalloidal solutions, which statement hardly
expresses any new idea of importance. The physico-chemical inves-
tigation of colloids however, has lead to most important results, and
very largely extended our knowledge of them.

INTRODUCTION 3

The definitions, however, are not based upon the proper-
ties above referred to, but give homogeneity as the most
important characteristic. Because of this state of affairs
the moot question as to the nature of colloidal solutions
hinges on the question of their homogeneity or heterogeneity.
This very circumstance has lead to an important exten-
sion of our knowledge. We must therefore next consider
the concept "homogeneity"

In his book: " Heterogeneous Equilibrium," Bakhuis
Roozeboom l states :

"We call a system homogeneous if all its mechanically
separable parts show the same chemical constitution and
the same chemical and physical properties.

"Gases or liquids which have been well mixed possess
this homogeneity of constitution because of the smallness
of molecules and the grossness of our means of observa-
tion."

We still are unable to observe directly particles the size
of a crystalloid molecule. However, by means of the
Faraday-Tyndall convergent beam of light, we are able
in an indirect manner, with the aid of the Nichol prism,
to demonstrate the existence of very small boundary
planes, and therefore of particles of unusually small size.
As Bredig 2 has already pointed out, TyndalPs method
is the most sensitive for the recognition of the lack of
optical homogeneity. By this method, as we shall see,
we can detect smaller quantities of substances than by
spectrum analysis; but just because of its enormous sensi-
tiveness, the most extreme care must be taken in forming
conclusions as to the nature of a solution, upon the basis
of the results of an examination by it.

1 B. Roozeboom, Die heterogenen Gleichgewichte, Part I, p. 9,
Braunschweig, 1901.
2 Bredig, Anorganische Fermente, Leipzig, 1901, p. 21.

4 INTRODUCTION

This I pointed out several years ago. 1 Recently a similar
view was advanced by Konowalow, 2 who, because of its
enormous sensitiveness, denies the value of the Tyndall
method, and regards solutions in the critical state, and
colloidal solutions as well, to be homogeneous solutions,
which owe their inhomogeneity to a partial separation of
the substance otherwise homogeneously dissolved.

Consequently Konowalow took the stand that there
are homogeneous solutions in which, as compared with
crystalloid solutions, the work necessary to separate sol-
vent and solute is excessively small. 3

This standpoint coincides to some extent, with the one
I have formerly taken. 4 Because of observations, to which
we will later on return, I regarded optical inhomogeneity
as an incidental characteristic of colloidal solutions,
attributing their opalescence (Lichtzerstreuung) to the
presence of a smaller number of larger particles, while I
regarded the bulk of the colloid as being in homogeneous
solution.

From the following remarks it will be evident that the
importance ascribed to the optical ifthomogeneity of
colloidal solutions by Faraday, Picton and Linder, Spring,
Bredig, and others, is well founded. Furthermore, we
will find confirmation for the compromise view of Lobry
de Bruyn, that a particle the size of a starch molecule

1 Zeitschr. f. physik. Chem., Vol. XXXIII, p. 64.

'Konowalow, Drude's Ann., Vol. X, p. 360-392, 1903, and Vol.
XII, p. 1160-34, 1903.

8 The cause of opalescence in liquids, according to Konowalow, is
the "very fine dust," which is found everywhere. The reason it is so
strongly marked 'in the critical state and in colloidal solutions, is due
to the fact that in- these cases the vapor pressure, as well as the osmotic
pressure, is practically independent of the constitution.

4 Zsigmondy, Liebigs Ann., Vol. CCCI, p. 53; Zeitschr, f. physik
Che*m., Vol. XXXIII, p. 64.

INTRODUCTION 5

can within its own mass diffract light of sufficient intensity
to be observed.

The method of Tyndall was further extended by means
of a method developed by II. Siedentopf and myself, of
rendering visible ultramicroseopic particles. 1 With its
aid we can readily determine whether the inhomogeneity
recognized by the Tyndall method is due to largor or to
smaller particles. It also gives us within certain limits
an idea of the size and color of the particles, and infor-
mation furthermore if they are at rest or in motion. By
this method we can even determine the major size limit
of individual particles, which are beyond the bounds of
visibility, providing we first fix the limit of ultramicro-
seopic visibility of the particles of the body in ques-
tion.

The research described herein, made according to this
method, has shown, in conformity with prior publica-
tions, 2 that the optical inhomogeneity which appears as a
distinct -haze when the particles are about 100 /*/* in size
becomes less and less as the particles become smaller;
that fluids containing particles measuring 20 w or less
appear clear by ordinary daylight, 3 and that the optical
inhomogeneity recognizable by the method of Tyndall
is still to be seen with particles approaching the size
which the kinetic theory of gases ascribes to the molecules
of crystalloids, but vanishes upon further subdivision.

Hence solutions of crystalloids, unless they contain very

1 Siedentopf and Zsigmondy, Drudc's Ann., Vol. X, pp. 1-39, 1903.
See also Chapter VI.

'Zsigmondy, Zeitschr. f. Elektrochemie, Vol. VIII, pp. 684-687,
1902; Siedentopf and Zsigmondy, Drude's Annal., Vol. X, pp. 30-39,
1903. H. Siedentopf, Journ. Roy. Microscop. Soc., pp. 573-578, 1903.

3 These facts apply to gold; with other substances noticeable opal-
escence vanishes much sooner-

6 INTRODUCTION

large molecules 1 or are reduced to hydrosols by colloid
formation, 2 should appear optically homogeneous when
examined with the convergent beam of light (Lichtkegel.)

Since, therefore, the hypothetical molecules of crystal-
loids of low molecular weight escape direct observation
because of their small size, and the best optical instru-
ments that we have at present are powerless against them;
and since some crystalloid solutions appear perfectly
homogeneous optically, the question naturally arises if we
cannot by other means observe the lack of homogeneity
which the molecular theory presumes to exist in every
solution.

A means to this end might be the use of light waves of
the greatest specific intensity, or else of ultraviolet light
for illumination, in conjunction with the photographic
plate to observe the diffracted light.

It may be that we can also make use of differences of
density which escape direct observation, by the applica-
tion of mechanical force to a separation of the constituents.

Some recently published work along this line by van
Calcar and Lobry de Bruyn should be of great interest.
These two Dutch investigators, by the action of centri-
fugal force on salt solutions (KI, KCyS, etc.), have been
able to produce a considerable change of concentration,
and in the case of a saturated solution of Glaubers salt,
were even able to force three eighths of the dissolved salt
to crystallize at the periphery of the rotating vessel. 3
Van Calcar and Lobry de Bruyn regard this fact as new

1 Lobry de Bruyn and Wolff, Rec. Trav. chim. Pays-Bas., Vol.
XXIII, p. 155, 1904.

8 Spring, Bull, d 1'Acad. roy. de Belgique, No. 4, pp. 300-315, 1899.

8 v. Calcar and Lobry de Bruyn, Rec. Trav chim. Pays-Bas., Vol.
XXIII, pp. 218-223, 1904. Earlier work in this field was done by
Th. des Coudres, Bredig, Colley, and Quincke.

INTRODUCTION 7

evidence of the continuity of the transition from colloid
to crystalloid solution, which idea had been repeatedly
advanced by the latter as well as by Picton and Linder
To me these experiments seem to support the view that
even the homogeneous solutions of the crystalloids are
not in a strict sense homogeneous, for in no fluid which is
physically and chemically uniform in all its parts, and
which therefore has the same specific gravity in every
conceivable little section, could such a difference of con-
centration be produced, no matter how great the speed of
rotation of the centrifuge.

Finally, we must mention that, according to Lord Ray-
leigh, part of the polarized blue light of the sky may be
attributed to the diffraction of .sunlight by the molecules
of the atmosphere. 1 There are, therefore, media which,
although in small volume they may appear entirely
homogeneous and optically clear, may nevertheless possess
an inhomogeneity so small that it can be observed only
by looking through a depth comparable to the height of
the atmosphere.

After these remarks, on returning to the definition of
solution, we see that, just as other investigators have said,
when solutions are spoken of as homogeneous distributions,
mixtures, etc., it cannot be meant that they are absolutely
homogeneous mixtures. If such great homogeneity is
demanded of solutions that we can detect no inhomo-
geneity in them by our most sensitive methods, we would
thereby exclude altogether from this classification solutions 2
not only of many colloids, but also of numerous crystal-
loids, for example, fuchsin, ferric chloric!, chromic chloric!, 3

1 Philos. Mag., Vol. XLVII, pp. 375-884, 1899.

2 It need hardly be said that polarized diffraction is referred to, not
fluorescence.

8 Spring, loc. cit., Picton and Linder, Journ. Chern. Soo., Vol. LXI,
pp. 114-136.

8 INTRODUCTION

saccharose, raffinose, 1 and solutions in the critical state. 2
We would thus run a risk of reducing the sphere of solu-
tions every time we increase the sensitiveness of our
methods of investigation. This danger can be easily
avoided if we use the word " solution 7 ' in its usual chemical
acceptation, meaning thereby subdivisions which appear
clear in ordinary daylight, and which cannot be separated
into their constituents by the ordinary mechanical means
of separation (filtration and decantation). The word solu-
tion will be used here in this sense. The question of the
heterogeneity of colloidal solutions seems rather to resolve
itself into the question of the heterogeneity of solutions in
general; and this standpoint may lead to many investi-
gations of the highest importance.

Regarding Suspension and Colloidal Solution. While
most chemists regard suspension as a coarse me-
chanical mixture, the solid or semi-solid parts of which
can be separated from the fluid by decantation and
filtration, 3 some physicists Faraday for example went
much further, and considered as suspensions the smallest
particles that can by any manner of means be recognized
optically, even if the individual particles cannot be seen.
They knew not how far this brought them into the domain
of the colloidal solutions of the chemist, the physiologist,
and the physician.

While several investigators differentiate between sus-
pensions and emulsions, according as the subdivided
substance is solid or fluid, others have made no such dis-

1 Lobry de Bruyn and Wolff, Rec. Trav. chim. Pays-Bas, Vol. XXIII,
p. 155, 1904.

2 Konowalow, loc. cit.

8 Examples: Suspended stannic acid, alumina, etc.; suspended
ferric oxid in contrast to: colloidally dissolved stannic acid, alumina;
solutions of colloidal ferric oxid.

INTRODUCTION 9

tinction. According to Quincke, 1 suspensions contain solid
or fluid particles, while emulsions contain only fluid
particles, or else solid floating particles that are coated
with a thin oily skin.

From what precedes, 2 it is evident that there is no uni-
form conception of " suspension/ 7 and that therefore,
because the multifarious meaning of this word can easily
lead to misunderstanding, even to the extent of consider-
ing as suspensions only the coarse mechanical ones, we must
hold as erroneous the definition in most general use, that
colloidal solutions and ^Jso colloidal metals are suspen-
sions. 3

I believe that all misunderstanding could be avoided
if in this definition we use the word subdivision 4 instead
of- suspension. No one could object to the conception of
colloidal solutions or hydrosols as very fine subdivisions,
especially as this involves no assumption regarding the
nature of the aggregation of the material subdivided, or
the forces affecting the individual particles, or their stereo-
metric nature, or their size and limits. These are ques-
tions that cannot be answered by general definition or by
hypotheses, but each case must be differentiated by
experimentation, which will make the investigation of

1 Drude's Ann., Vol. IX, pp. 1009 and 1010, 1902.

2 Here might be mentioned the hypothesis of Lemery (Cours de
chimie, Leyden, 1716) which at present seems to us extremely odd,
that gold chlorid solution is a suspension of gold particles carried by a
network of aqua regia. (See Ostwald, Lehrb. der allgem. Chem.,
2d cd., Vol. II, 2, p. 5.)

8 Cf. B. A. MOller, Zqitschr. f. anorg. Chem., Vol. XXXVI, p. 340,
1903.

4 The term " subdivision" is frequently misunderstood. It is a
generic term, "suspension" arid "solution" being specific terms
included within it, and it does not express any definite degree of fine-
ness, as is commonly supposed.

10 INTRODUCTION

colloids the theme of a new, far-reaching science, built
upon the present promising foundation.

We may further characterize colloidal solutions and
distinguish them in an unobjectionable manner from
crystalloiclal solutions, by incorporating into the definition
of the former the characteristic noted by Graham, who
showed that colloidal solutions lack the property (or else
possess it very slightly) of diffusing through parchment
membranes. l

A safe distinction between coarse mechanical suspensions
and colloidal solutions is that* the latter have a more
complete and thorough mixture of the constituents, and
possess greater homogeneity, as compared with the
former. The claim that colloidal solutions must show
osmotic pressure, small though it may be, in order to be
called solutions at all, does not appear to me to be essen-
tial, for it adds but little to the characteristics of the
hydrosols, whether their solutions possess measurable
osmotic pressure or not.

In this book we will designate as suspensions only the
coarser subdivisions of solids, such as fine powders which
undergo sedimentation. Finer subdivisions which no
longer settle, will on the other hand be called colloidal
solutions if they exhibit the other essential characteristics
of such solutions.

1 There might also be incorporated into the definition, the criterion
noted by Bredig (Anorganische Fermente) "that the work necessary
to separate solvent and solute is extremely small in comparison with
crystalloid solutions."

CHAPTER I
LIMITATION OF THE FIELD

i. Limitation as to Crystalloidal Solutions

THE chief characteristics distinguishing colloidal from
crystalloidal solutions, have already been given by Graham.
According to him colloidal solutions are distinguished
from crystalloidal solutions by their small diffusibility,
and the concomitant inability of the majority of them
to pass through parchment membranes or jellies. (See
Chapter lit)

Numerous other distinguishing characteristics were
noted by Graham as well as by other investigators, and
Bredig 1 has done us the service of compiling them and
adding some new ones.

According to Bredig solutions of crystalloids and col-
loids may be distinguished by means of: (a) diffusibility;
(b) the work necessary to remove the solvent; (c) elec-
trical migration; (d) coagulation; (e) absorption; (/)
irreversible changes of constitution and hysteresis; (g) im-
permeability to other colloids; (h) optical inhomogeneity;
(i) electrical formation of sols.

It is evident from this brief resum6 that there are many
ways of distinguishing colloids from crystalloids. Not-
withstanding this, no sharp line of demarcation can be
established, for there are numerous intermediaries between
both kinds of solutions.

1 Anorganische Fermente, loc. cit., pp. 10-12.

11

12 LIMITATION OF THE FIELD

2. Limitation as to Suspensions

But little has been said of the characteristics which
distinguish colloidal solutions from undoubted suspensions;
a few words on this topic are therefore added.

In opposition it might be said that there is such a mani-
fest difference between real suspensions, for instance, sedi
ments of powdered substances like quartz, starch, metal
dust, suspensions of bacteria, etc., and clear, colloidal
solutions, that no words need be wasted on this subject.
Nevertheless I do not regard it unnecessary to consider
the matter, because here too there are intermediate forms
which lead so gradually from one class to the other, that
an observer, having in mind only the intermediate forms,
is apt to miss the remarkable difference between the mem-
bers of both groups.

(1) In the first place there is an enormous difference in
size between the particles in hydrosols and those found in
real suspensions. A glance at Plate III, Chapter .XV,
will make this more evident than words.

The following properties of colloids will serve in part
to show the difference :

(2) Irreversible change of condition.

(3) Change of the total energy of the system by coagu-
lation.

(4) Absorption of the subdivided substance by porous
bodies, such as charcoal.

(5) The property of colloids to enter into reactions among
themselves, which bear a deceptive resemblance to chemical
reactions, etc.

To avoid possible misunderstanding it might right here
be stated that most of these differences between hydrosols
and suspensions are dependent upon the degree of sub-

LIMITATION AS TO SUSPENSIONS 13

division of th$ material, and they are therefore not very
great if closely adjoining members of the series be com-
pared; but they become enormous when we compare
typical representatives of true suspensions with those of
colloidal solutions. These differences justify the classifi-
cation into colloidal solutions and suspensions long ago
established by chemists, physiologists, and physicians.

Let us now consider the differences (2) and (3) above
noted. ,

But little expenditure of mechanical energy is necessary
to stir up the fine powder of a practically insoluble sub-
stance into a suspension. If this is allowed to stand,
gravity separates the powder from the bulk of the fluid.
The sediment can again be stirred up and again allowed to
settle. This process can be repeated as often as desired,
the powder remaining unchanged, except that the dis-
tances between the particles will be larger after stirring
them up, and smaller when they have settled.

In the case of coarse suspensions, the addition ot elec-
trolytes does not appreciably change the above described
process; this can readily be shown by allowing a suspen-
sion of potato starch or quartz whose particles measure
0.06 mm. and over to settle first in pure water and then
in a dilute solution of table salt. Finer suspensions, such
as the turbidity due to clay, are precipitated by the addi-
tion of electrolytes, just as are colloidal solutions, but even
in this case, as we shall see, characteristic differences can
be determined.

If the suspended powder is completely separated by
desiccation from the water in which it was distributed,
the original subdivision can be reproduced by stirring,
provided that certain colloids which tend to cement
together the individual particles are absent.

But if an attempt is made to separate a colloidal metal,

14 LIMITATION OF THE FIELD

gold for instance, from the medium in which it is dis-
tributed (which can easily be done by evaporating the
fluid or adding electrolytes), it undergoes an irreversible
change of condition, or coagulates as we say. The metal
thereby completely loses its capability of resuming its
original condition of subdivision by shaking or stirring,
and the coagulation involves profound changes in its
relations to light waves, as evidenced by change of color,
polarization, etc. 1

Regarding these changes produced by coagulation, we
understand with certainty only one, that is that numerous
small submicroscopic 2 particles, which I will call a-par-
ticles, are combined into larger, but still submicroscopic
particles (/3-particles). Comparison with the flocculation
of fine suspensions, carefully studied by many investi-
gators, 3 has led to the conclusion that the submicroscopic /?-
gold particles are grouped together like the flocks in
kaolin, clay, etc. ; but in order to avoid unwarranted con-
clusions based upon analogy, see the relative sizes given
in Plates III and IV, Chapter XV. To see the vast
difference just imagine flocks made up of the clay par-
ticles or anthrax bacilli shown in Plate III, and compare
them with imaginary flocks made up of gold particles
which, though magnified 10,000 times, appear as tiny
points. It will at once be seen that a-gold particles in
such a "gold flock/' must be much closer together than
the kaolin particles in a clay flock, and that the attractive
forces which we suppose to exist between the smallest

1 The difference between suspensions and irreversible colloidal
systems was known to Hardy, although not sharply characterized.

2 For definition, see Chapter VI.

8 See Bams and Schneider, Zeitschr. physik. Chem., Vol. VIII,
p. 278, 1891; Quincke, Drude's Ann., Vol. VII, p. 59, 1902; Bod-
lander, G6tt. Nachr, 1893.

LIMITATION AS TO SUSPENSIONS 15

particles of a substance (cohesion forces), will effect a
much more powerful union of the a-particles than in
ordinary flocks of coarser material.

But even without this exposition, just by the aid of
rough experiments, we can easily establish the difference
between the flocculation of fine clay particles, and the
coagulation of colloidal metals.

As far back as 1870, Ch. Schlosing showed l that the
turbidity due to clay could, after its precipitation by salts
of calcium or magnesium, be once more suspended in
water and again flocculated by the addition of salts. 2 On
the contrary, coagulated colloidal gold cannot be brought
into colloidal solution by stirring it up in pure water,
even if all salt solution has been previously washed out.
There results only a suspension of /?-gold particles, which
soon deposits its gold.

In order to change /?-gold into a-gold we must make
use of chemical or electrical energy, for instance solution
in aqua regia and reduction of the gold chloric! to colloidal
gold, or electric atomization by Bredig's method.

Finally coagulation involves change, considerable at
times, in the total energy of the system, which can be
directly determined by the calorimeter. Thus Prange
(Chapter III) found that one gram of colloidal silver on
precipitation by ammonium citrate, developed 250.98 to
126.73 calories, depending on whether a dilute or a con-
centrated solution was precipitated. Every gram-atom
of silver accordingly liberates 27,100 to 13,700 calories.
Even this does not yield the ordinary metallic condition.
The conversion of the hydrogel into the metal (by means
of H 2 S0 4 ) sets free more heat, 60 calories per gram of
silver, according to Prange.

1 Ch. Schlosing, Compt. rend., Vol. LXX, p. 1345, 1870.

2 Sometimes this flocculation is erroneously called coagulation.

16 LIMITATION OF THE FIELD

Barus and Schneider, in commenting on Prange's obser-
vations, say:

"In his experiments on colloidal silver, Prange found
that the transformation of the almost solid colloid into
normal silver liberated about 60 calories. Prange attri-
butes this to the change of colloidal into normal silver.
But the liberation of heat can be explained in the following
manner: Work is necessary to transform a quantity of
silver, already finely subdivided (normal silver) into a
considerably finer state of subdivision (colloid) that is,
heat is necessary. Reversing the process, the transforma-
tion of colloidal silver into the precipitate of normal silver
(consisting of larger particles of the substance) liberates
the heat again. "

On this point I agree entirely with Barus and Schnei-
der. The size of the coagulated silver particles, as
figured by these authors, approximates the true value,
and is rather too small than too large.

On the other hand the difference between the colloidal
solution and suspension containing the same percentage
of the same substance is evidence to me of a remarkable
difference between the two kinds of subdivision. It is
to be noted that the energy difference between suspended
and colloidal silver is greater per gram-atom than that
between the allotropic modifications of phosphorous,
carbon, sulphur, etc.

Intermediate Forms between Suspensions and Colloidal
Solutions. Between the coarse mechanical suspensions
and colloidal solutions there are many intermediate
forms. In general the finer a body is subdivided the
more it assumes the properties of a colloid.

Even with potato and wheat starch we can readily see
the influence of subdivision. While the settlement of the
former is scarcely influenced at all by the addition of an

LIMITATION AS TO SUSPENSIONS 17

electrolyte, the finer particles of the latter exhibit a dis-
tinct flocculation and accelerated settlement upon the
addition of table salt.

Turbidity due to clay, ultramarine, the much investi-
gated mastic-turbidity, and others, form good examples of
the intermediate forms between colloidal solutions and
suspensions.

By pointing out the relations between fine suspensions
and colloidal solutions, and thus establishing a new point
of view, Ebell, Qstwald, Barus and Schneider, Spring,
Lottermoser and von Meyer, and particularly Bredig,
have rendered a great service which has already proved
useful in leading the supporters as well as the opponents
of this view to a series of experiments and the establish-
ment of important facts.

Summary. Colloidal metal solutions are differentiated
from true suspensions of pulverized substances, not only
by the dissimilar subdivision of the solid body but also
by the irreversible change of constitution invariably
shown by colloidal solutions^ and further by change of
the total energy liberated upon the separation of the
atomized substance from the medium. There are many
intermediate forms between the two classes of subdivision,
so that there is a continuous series, one leading to the
other.

The colloidal metal solutions have been purposely con-
sidered, to establish the distinguishing characteristics
between colloidal solutions or hydrosols and true suspen-
sions, because they show a striking similarity to certain
fine suspensions in their behavior with reagents. This
similarity has led to the attempt on the part of some
investigators to remove them from the group of colloidal
solutions and classify them as suspensions. We must
oppose such a conclusion, based as it is on one-sided

18 LIMITATION OF THE FIELD

observation, because of the distinguishing characteristics
before given. Real metal hydrosols rather tend to form
a class of colloidal solutions, which is distinguished from
the majority of the others, not by a lesser degree of sub-
division but by the nature of the original substance.
We will take this up more fully later.

CHAPTER II

CLASSIFICATION OF HYDROSOLS ACCORDING TO TWO
DIFFERENT POINTS OF VIEW l

ONE of the most important tasks of a nascent science
appears to me to be the arrangement of the available
facts upon one single basis.

If only an imperfect attempt in this direction can be
made, it is because the point of view is so new, and the
facts are so incompletely known. I consider it of import-
ance, however, to direct the attention of investigators to
this subject.

Just as geologists distinguish, according to the size of
their' constituent particles, between boulders, rubble,
pebbles, gravel, sand, and dust, so too in dealing with
more finely subdivided bodies, a classification based on
the size of the particles will be of corresponding value. 2

1 The best classification of colloids yet suggested is that of Hardy
(Zeitschr. f. physik. Chem., Vol. XXXIII, p. 326). Recently Muiler
(Zeitschr. f. anorg. Chem., 1903, Vol. XXXVI, p. 340) suggested a
classification with which I cannot agree, for reasons that may in part
be gathered from the following chapters. Nevertheless I fully agree
with Mailer that colloidal chemistry must establish a classification
of hydrosols. Independently of Hardy, the changes of condition of
colloids have been divided by Wolfgang Pauli (Arch. f. d. ges. Phy-
siologic, 1899, Vol. LXXVIII, p. 315), into those easily and those
with difficulty reversible.

* A classification of the various kinds of sand, pebbles, and dust,
according to the size of their gramiles, has been attempted by Dr.
Albert Atterberg, of Kolmar (Schwed. landw. Akad., 1903, and Chem.
Ztg., 1905, Vol. XXIX, p. 195). The capillarity (the height in milli-

19

20 CLASSIFICATION OP HYDROSOLS ACCORDING TO

Pieces of like size have certain properties in common; for
instance, sand and dust are carried by the wind, while
cobble-stones and pebbles are unmoved; it makes no
difference of what material the sand, dust, or pebbles
consist. Sand will pass through a 10 mm. mesh sieve,
but cobblestones and pebbles will not.

Weight has a noticeable effect on suspensions, pro-
ducing the deposition of the suspended particles, and its
influence can be traced down to particles as small as
one tenth of a wave length of light. With particles smaller
than 60 /t/*, 1 and even often with larger ones, its influence
is masked by that of the electrical charge of the particles
and by the influence of other kinds of energy. Further-
more in the case of much finer subdivisions, which we will
take up later, the size of the particles immediately deter-
mines a whole series of properties. We would be anticipat-
ing if we discussed here the change of properties dependent
upon the size of the particles. Part of the experimental

meters to which water will ascend in sand) as well as the perme-
ability wjth respect to water, are intimately dependent upon the size
of the granules, both of which facts have great significance in agricul-
ture. Atterberg (a Swede) makes the following classification of the
fragmental pieces:

Diameter

Block (boulders) 2 m. to 20 cm.

Klapper (pebbles) 20 cm. to 2 cm.

Grus (gravel) 2 cm. to 2 mm.

Sand (sand) 2 mm. to 2 mm.

Mo (earth) 0.2 mm. to 02 mm.

Lattler (loam) .02 mm. to . 002 mm.

Ler (clay) smaller than . 002 mm.

With each of these various sizes are associated definite physical
properties. Loam shows great capillarity, but little permeability;
sand 2 mm. in diameter has a capillarity of only 25 mm. ; gravel and
pebbles have no perceptible capillarity; earth (mo) allows water to
soak through quickly. (Capillarity, 428-2000 mm.)
1/1,000,000 mm.

TWO DIFFERENT POINTS OF VIEW 21

section of this book deals with this subject. Polarization,
the color of the particles in many cases, their behavior
under the action of gravity and during filtration, the
energy-content of the system, and the movement of the
individual particles all these vary with their increasing
or diminishing size.

One change must be mentioned here. While suspensions
of larger particles, for example, 1-10 /*, always possess a
particularly inhomogeneous appearance, 1 fluids containing
particles smaller, than 20 /*/< appear clear by ordinary day-
light, even if the indices of refraction of the medium and
subdivided substance are as different as those of water
and metallic gold. While particles of the former kind
can be easily and completely separated from the fluid in
which they float, by filtration through filter paper, the very
actively moving particles of the latter cannot by this means
be separated from the fluid. It has been customary for
chemists to designate as solutions, 2 subdivisions of such
perfect homogeneity, that, like the latter class above

1 It is here assumed that the particles and medium have different
indices of refraction, as is generally the case.

2 These solutions with particles of about 20 /</ and less are not without
influence on light, although by ordinary daylight they appear clear
and transparent. With the intense transverse illumination of direct
sunlight their characteristic inhomogeneity is easily recognized by
their characteristic scattering and polarization of light. Although
this method has long been known to physicists, it was neglected by
chemists; for the slight turbidity almost always present, even if visible
without direct sunlight, was ascribed to contamination of the solution
by larger suspended particles. Finally more recent investigations (Pic-
ton and Linder, Spring, Lobry de Bruyn, and others) have made
evident the universality of this phenomenon, and shown that a large
majority of colloidal and crystalloidal solutions are optically inho-
mogeneous But even this inhomogeneity, most conspicuous at
20 w, becomes less and less, and almost or entirely vanishes with all
substances in the neighborhood of the molecular dimensions. (See
Introduction and Chapter IX.)

22 CLASSIFICATION OF HYDROSOLS ACCORDING TO

described, they are clear, transparent, and do not settle.
Subdivisions of the former class are, on the other hand,
considered as sediments or suspensions. Accordingly, ' if
we arbitrarily omit the intermediate members (between
1000 and 20 /*//) we can differentiate according to the
size of the particles, two sharply defined groups of sub-
divisionscoarse mechanical suspensions with particles
from about 1 p. and over, and apparently homogeneous
solutions with particles of 20 /</< and less. Between these
two classes of subdivisions come the transition forms
which, according to their properties, are classed sometimes
as colloidal solutions, sometimes as fine suspensions.

These remarks suffice to show the importance of a
classification based on the size of the particles.

Before taking this up, we must first consider the classifi-
cation established by Graham, and also that of Hardy.
Graham divides homogeneous looking subdivisions, that is
solutions, according to their greater or lesser diffusibility,
into solutions of crystalloids and colloids. This classifica-
tion of Graham is an exceptionally happy one, for although
it establishes no sharp boundary between the two groups, 1
it brings into prominence a series of contrasts between the
members of both classes of solutions.

In general, crystalloidal solutions are regarded as the
complete subdivision of the substance, and colloidal solu-
tions as the less complete. But it cannot reasonably be
held that all crystalloidal solutions without exception
possess a higher degree of homogeneity than colloidal
solutions. In the following chapters we will learn of one
case of colloidal solution where the subdivision is evidently
carried as far as in the case of a crystalloid solution, and
is probably even more complete.

1 Graham himself knew of the transition forms between crystalloid
aod colloid solutions.

TWO DIFFERENT POINTS OF VIEW 23

(Compare Chapters IX, XVII, and XVIII (6).

On the other hand, coarse subdivisions, such for ex-
ample as are formed by washing out sulphids, and which
pass through filter paper, would be classed as colloidal
solutions, and it was just in the case of such subdivisions
that the heterogeneity of hydrosols was first discovered.
Regarding these relative proportions, reference should be
made to the diagram, Plate I, which is based upon the
size of the particles. If the subdivisions arc arranged in
successive order according to the size of their particles,
beginning above, having the largest at the top and the
smallest at the bottom, the relative sizes may then be
seen at a glance.

Several investigators have expressed the opinion that
those colloidal solutions (such as the metal hydrosols)
which, upon the addition of electrolytes, act as do sus-
pended clay particles, should be considered as suspensions,
and not as colloidal solutions at all. From this it might
be assumed that, in the diagram, the metal hydrosols
would lie next to or be included in with the suspensions;
while those hydrosols which are not precipitated by traces
of electrolytes would be nearer the solutions that is,
they would be in a finer state of subdivision. Such
is not the case, however. Another classification than
the preceding, one which, up to a certain pointy is
independent of the size of the particles, would be
made by studying the effect of the addition of
electrolytes, and of changes of temperature on the
subdivisions. .

Such a classification was first successfully introduced by
Hardy. He divided the colloids into those reversible and
those irreversible, according as the change from sol to gel
could or could not be made retrogressive by reversing the
conditions of its production. The conditions studied by

24 CLASSIFICATION OF HYDROSOLS ACCORDING TO

Hardy l as causing hydrogel formation were chiefly
changes of temperature and the addition of electrolytes.
But difficulties arise on attempting a classification of
colloids in general, according to Hardy's method. Only
in individual cases do ordinary changes of temperature lead
to the formation of gels, as in the case of gelatin solutions
which were closely studied by Hardy. Extreme changes
of temperature (complete freezing, for instance), almost
always lead to gel formation, but they sometimes cause
irreversible change of constitution in the case of colloids
classed by Hardy as reversible (gelatin, for example).

If the changes produced by the addition of electrolytes
are studied, it will be found that a certain hydrosol under-
goes sometimes a reversible and sometimes an irreversible
change of condition, according to the nature of the elec-
trolyte added, so that it may appear doubtful to which
class the hydrosol belongs. 2 Thus the hydrosol of stannic
acid is converted into a hydrogel by the addition of the
slightest trace of most electrolytes. However, while acids
and many salts give a hydrogel which remains insoluble
after the elimination of the precipitant, 3 other precipitants
for example table salt or caustic alkalis, yield precipitates
which redissolve in water upon the elimination of the
reagent.

In order therefore to make practical a classification
along the lines laid down by Hardy, it is necessary to con-
sider only such influences on the hydrosol as always lead
to formation of gels or of solid colloids, and which, as far

1 Hardy, Zeitschr. f . physik. Chem., 1900, Vol. XXXIlI, pp. 326 and
385.

8 Pauli first referred to this ; loc. cit.

8 Lottermoser calls water-soluble colloids solid hydrosols. According
to Hardy, however, we must regard jellies and solid colloids as hydro-
gels, even if they are soluble in water. The word hydrogel is used
here in this sense.

TWO DIFFERENT POINTS N OF VIEW 25

as possible, are independent of chance or outside influences,
as well as of the personal equation.

Evaporation is a method of treatment which in every
case leads to the formation of a gel or of a solid residue.
It is sufficient for our purpose to class as reversible those
colloids whose solutions leave a residue soluble in water,
when desiccated at ordinary temperatures; 1 and to consider
as irreversible those colloids which, under the same cir-
cumstances, yield a residue insoluble in water.

According to this classification, dextrin, gum arabic,
most albumens, molybdic oxid, molybdic acid, etc., are
reversible colloids, while, on the other hand, stannic acid
and many other colloidal oxids and sulphids, as well as
the pure colloidal metals, etc., are irreversible colloids.

While the former are frequently insensitive to the addi-
tion of electrolytes, the latter for the most part are ex-
tremely sensitive, and are easily coagulated by the slightest
addition of electrolytes. If solutions of reversible are
added to those of the irreversible colloids, the latter may
become reversible, in which case they are often protected
against precipitation by salts (Schutzwirkung). Fre-
quently a very small addition of a reversible colloid is
sufficient to produce these effects in a striking manner.
Carey Lea's colloidal silver, which can never be prepared
free from foreign colloids, commercial argentum Cred6,
collargol, PaaPs colloidal metals, Mohlau's colloidal indigo,

1 As will be seen, Hardy's basis of classification is adhered to ; but
instead of numerous conditions which lead to gel formation, here only
one is taken into consideration. Evaporating the hydrosol is a very
efficient means of causing gel formation ; on increasing concentration
many colloids are transformed into insoluble hydrogels, even though
wet, while others lose their solubility on further evaporation. The
reversible hydrosols, on the other hand, even after evaporation to
dryness, retain the property of distributing themselves in water
in their original condition.

26 CLASSIFICATION OF HYDROSOLS

all owe their solubility in water to the presence of pro-
tective- colloids. 1

The accompanying diagram, Plate I, shows how colloidal
solutions and suspensions may be grouped according to
the size of their particles. Above, in the microscopic field
and below, even extending into the ultramicroscopic field,
are to be found true suspensions, whose sphere extends
much further up than is shown in the diagram.

Colloidal solutions or hydrosols all contain ultramicro-
scopic particles. No sharp line of demarcation can be
drawn between suspensions and colloidal solutions; their
spheres mutually invade each other; that of the hydrosols
being here extended as far upwards as possible, to include
everything which has formerly been classed as a colloidal
solution. Several typical hydrosols are mentioned in the
table. A perpendicular through the cross-hatching lines
divides the whole area into two sections; on the right
are the irreversible, on the left the reversible colloids.
This division simplifies the diagram. It will be seen that
on the right are those hydrosols which are distinguished
by especial sensitiveness to electrolytes. This does not
mean, however, that colloidal metals (and sulphids, etc.),
more nearly approach suspensions than do reversible

1 By the classification here followed two groups will be distinguished,
Whose typical representatives differ from each other in several import-
ant particulars. As everywhere in nature there^ are also transitions
here; nevertheless -a sharp separation could be made if the conditions
upon desiccation, such as temperature, moisture-content of the gas-
phase, etc., are predetermined. An advantage of this classification
is that outside influences which might act chemically upon the colloid,
such as increase of temperature, chemical action of foreign electrolytes,
etc., are as far as possible avoided. On the other hand, possible
chemical changes upon desiccation of the colloid, are left out of con-
sideration, for it is almost impossible to determine such changes
satisfactorily and to separate them clearly from the physical changes
that always occur.

PLATE I

O.l/A

50 MM

SO MM

5 MM

0.1MM

-S-u-s p -e-n-&-i-o-n-s-

idal Solutions-

e HytlTOHol

o

PL,

Classification of Colloidal Solutions

according to the size of the particles contained in them and according
to their behavior upon dessication .

IToface page 20.1

TWO DIFFERENT POINTS OF VIEW 27

hydrosols (glycogen, albumen, etc.), as has often been
maintained, but that they tend of themselves to assume
the stable form that is, the solid compact form of the
substance (metal, sulphid, or oxid). This tendency is
realized upon evaporation, as well as upon the addition
of electrolytes. Such addition hastens the process, which
in most cases takes place of itself. 1 The very finest sub-
divisions of metals are sometimes the most sensitive and
coagulate very readily.

It must, however, be noted that the behavior of hydro-
sols upon evaporation cannot always be foretold from their
reaction with electrolytes. Most reversible colloids for
instance, molybdic acid, tungstic acid, gum, albumen,
gelatin are not sensitive to small additions of electrolytes.
On such addition either they are not precipitated 01 else
they undergo a reversible precipitation, which, upon
elimination of the precipitant, can be made retrogressive.
(On this subject sec W. Pauli. 2 ) But there are also col-
loids, reversible according to the above classification, which
are extremely sensitive to the addition of electrolytes of
every kind, Graham's caramel, for example. (Chapter
III). In this case the specific influence of the subdivided
substance or of its constituents, is distinctly changed.

In the table sulphids, oxids, etc., are placed with the
colloidal metals, while glycogen, albumen, etc., are grouped
with the reversible colloids. The names written vertically
indicate that these fluids may contain particles of the
most diverse sizes, but not, however, that any one fluid
must necessarily contain all sizes.

Linder and Picton have already differentiated various

1 Electrolytes are not, however, to be here considered as catalysers.
(See Chapter XVIII, also W. Pauli, Ergebnisse der Physiologic, 1904,
p. 157).

W. Pauli, Pflugers Archiv., 1899, Vol. LXXVIII, p. 315.

28 CLASSIFICATION OF HYDROSOLS ACCORDING TO

kinds of arsenic sulphid according to their method of
preparation, which they called arsenic sulphid a, /?, 7-,
and d, and they have shown that the fluid As 2 S 3 a: contains
the largest, while As 2 S 3 <5 contains the smallest particles. 1

Ultramicroscopy enables us to fix the approximate sizes
when the particles can be rendered visible, and when these
are known, the diagram, which is only outlined here, can
be then partially filled in. As an illustration several
colloidal gold solutions described in detail in the experi-
mental section of this book (see Chapter IX), are arranged
according to the sizes of their particles.

At the bottom following the fluids Au 73a , Au 92 , etc., are
gold solutions with amicroscopjc particles, which con-
clude the series beginning with Bredig's colloidal gold,
having particles from 20 to 80 /*// in size. Above is the
gold suspension c which deposits its gold; above this
should be placed the well-known suspensions of gold,
obtained by precipitating a solution of gold chlorid with
FeS0 4 or oxalic acid.

By way of further explanation of the diagram, it might
be said that the individual particles of most colloidal inor-
ganic oxids cannot be seen, for they, as well as most organic
colloids, are not so easily rendered visible as are colloidal
metals, and therefore I have formed no idea as to the
size of their particles. Colloidal stannic acid contains,
according to its method of preparation, submicrons 2 Or
amicrons. 2 The same is the case with the purple of
Cassius and colloidal silicic acid.

Among the reversible colloids are included: Mohlau's

1 Picton and Linder (loc. cit. in Chapter III) based their conclusions
regarding the difference of the sizes of the particles, upon the behavior
of the fluids when filtering through porcelain and upon diffusion
experiments.

2 Definition, see Chapter VI.

TWO DIFFERENT POINTS OF VIEW 29

colloidal indigo and PaaPs blue colloidal gold, which
contain quite large copper-red particles 1 ; Lea's colloidal
silver with particles of various size; and further, on the
basis ot the work of Raehlmann, Much, Romer and Sie-
bert, Biltz and Gatin-Gruzewska, as well as Michaelis
(Chapter XX), glycogen, several kinds of albumen, dyes
with particles which are ultramicroscopically visible, but
whose size is not yet determined.

Lower down are soluble starch and crystallized albumen
with question marks attached, because, although they show
a light-cone (lichtkegel), their individual particles cannot
be distinguished, but probably are about 5 pp. Finally,
at the side are dextrin, many dyes such as Congo red, etc.,
and molybdic acid, etc., which from their general proper-
ties are all to be regarded as intermediate between crystal-
loidal and colloidal solutions, or else, like soluble starch,
as crystalloid solutions with high molecular weight. If a
3 .per cent solution of soluble starch is allowed to stand,
in course of time particles are formed in it, which may
grow up to 100 fjL/j. (see Chapter XXI).

At the very bottom of the reversible colloids I have
placed the crystalloidal solutions and gas mixtures, not
because the sizes of their particles can as yet be demon-
strated experimentally by any method independent of
theory, but just far the sake of completeness. In the
present state of our knowledge, the hypothetical molecules
of the crystalloids will serve quite well and be unobjection-
able for the purpose of this table.

1 The respective sizes of these particles are not yet determined,
their appropriate place in the table is therefore still uncertain.

CHAPTER III
HISTORY OF THE IRREVERSIBLE COLLOIDS

i. Graham's Investigations

SINCE the time when chemists commenced the examina-
tion and description of animal and vegetable materials,
colloidal substances have been investigated and described,
and that in much greater detail in the older than in the
modern text books of organic chemistry. In addition,
many isolated observations on inorganic colloids have been
made and recorded in recent years.

To Thomas Graham is due the credit of having recog-
nized and described in detail the similarity in the various
phenomena, and of having first proposed a classification
that gave an insight into the essential differences between
the two separate classes of solutions. Here, as in many
other instances, progress followed the discovery of a new
method. While working on diffusion experiments, Graham
found that those substances which easily crystallize from
their solutions, would readily diffuse through jelly-like
membranes, while on the contrary, amorphous substances
either lacked this property or possessed it to only a very
limited degree. Membranes of parchment paper are par-
ticularly suitable for this work. By their aid Graham
could not only easily separate crystalloids from colloids,
but could also prepare a large number of the latter in a
sufficient state of purity. He also introduced the expres-
sions now in general use; colloid, crystalloid, hydrogel,

80

LIQUID DIFFUSION APPLIED TO ANALYSIS 31

hydrosol, etc. * Graham's work must therefore be dis-
cussed in detail, especially as it is not well enough known. 1
A considerable part will be given in Graham's own words: 2

Liquid Diffusion Applied to Analysis

By THOMAS GRAHAM, F.R.S., Master of the Mint

(Received May 8, Read June 13, 1861)

"The property of volatility, possessed in various degrees
by so many substances, affords invaluable means of separa-
tion, as is seen in the ever-recurring processes of evapora-
tion and distillation. So similar in character to volatility
is the diffusive power possessed by all liquid substances,
that we may fairly reckon upon a class of analogous
analytical resources to arise from it. The range also in the
degree of diffusive mobility exhibited by different sub-
stances appears to be as wide as the scale of vapor ten-
sions. Thus hydrate of potash may be said to possess
double the velocity of diffusion of sulphate of potash, arid
sulphate of potash again double the velocity of sugar,
alcohol, and sulphate of magnesia. But the substances
named, belong all, as regards diffusion, to the more "vola-

1 In contradistinction to this fundamental work, other publications
regarding irreversible hydrosols are rather briefly treated here. In
the quotations there is no claim to completeness; much important
work is passed over, the main object being to direct attention to
certain points. The complete literature of the subject, in so far as
not here given, can be found in A. Lottermoser, "Uber anoganische
Kolloide," Stuttgart, 1901, and A. Miiller, "Die Bibliographic der
Kolloide" (Zeitschr. f. anorg. Chemie, 1904, Vol. XXXIX, p. 121).
Work concerning the theory of colloids is referred to only in so far
as germane. See also Bredig, " Anorganische Fermente"; A. Muller,
"Die Theorie der Kolloide," Leipzig and Vienna, 1903; Billitzer
(Zeitschr. f. physikal. Chemie, 1905, Vol. LI, pp. 129-166).

8 Thomas Graham, Philosophical Transactions of the Royal Society
of London, 1861, Vol. 151, pp. 183-224. (Also Liebig's Annalen, 1862,
Vol. CXXI, pp. 1-77).

32 HISTORY OF THE IRREVERSIBLE COLLOIDS

tile" class. The comparatively "fixed" class, as regards
diffusion, is represented by a different order of chemical
substances, marked out by the absence of the power to
crystallize, which are slow in the extreme. Among the
latter are hydrated silicic acid, hydrated alumina, and
other metallic peroxids of the aluminous class, when they
exist in the soluble form; with starch, dextrin, and the
gums, caramel, tannin, albumen, gelatin, vegetable, and
animal extractive matters. Low diffusibility is not the
only property which the bodies last enumerated possess in
common. They are distinguished by the gelatinous
character of their hydrates. Although often largely
soluble in water, they are held in solution by a most feeble
force. They appear singularly inert in the capacity of
acids and bases, and in all the ordinary chemical relations.
But, on the other hand, their peculiar physical aggregation
with the chemical indifference referred to, appears to be
required in substances that can intervene in the organic
processes of life. The plastic elements of the animal body
are found in this class. As gelatin appears to be its
type, it is proposed to designate substances of the class
as colloids, and to speak of their peculiar form of aggrega-
tion as the colloidal condition of matter. Opposed to the
colloidal is the crystalline condition. Substances affecting
the latter form will be classed as crystalloids. The dis-
tinction is no doubt one of intimate molecular constitution.
"Although chemically inert in the ordinary sense,
colloids possess a compensating activity of their own,
arising out of their physical properties. While the rigidity
of the crystalline structure shuts out external impressions,
the softness of the gelatinous colloid partakes of fluidity,
and enables the colloid to become a medium for liquid
diffusion, like water itself. The same penetrability appears
to take the form of cementation in such colloids as can

LIQUID DIFFUSION APPLIED TO ANALYSIS 33

exist at a high temperature. Hence a wide sensibility on
the part of colloids to external agents. Another and emi-
nently characteristic quality of colloids, is their mutability.
Their existence is a continued metastasis. A colloid may
be compared in this respect to water, while existing liquid
at a temperature under its usual freezing-point, or to a
supersaturated saline solution. Fluid colloids appear to
have always a pectous modification; and they often pass
under the slightest influences from the first into the second
condition. The solution of hydrated silicic acid, for
instance, is easily obtained in a state of purity, but it
cannot be preserved. It may remain fluid for days or
weeks in a scaled tube, but is sure to gelatinize and become
insoluble at last. Nor does the change of this colloid
appear to stop at that point. For the mineral forms of
silicic acid, deposited from water, such as flint, are often
found to have passed, during the geological ages of their
existence, from the vitreous or colloidal into the crystalline
condition. (II. Rose). The colloidal is/ in fact, a dynam-
ical state of matter; the crystalloidal being the statical
condition. The colloid possesses Energia. It may be
looked upon as the probable primary source of the force
appearing in the phenomena of vitality. To the gradual
manner in which colloidal changes take place (for they
always demand time as an element), may the character-
istic protraction of chemico-organic changes also be
referred." * (Pp. 183-184).

1 Some of Graham's general statements are at the present day open
to criticism. Thus it is not quite right to state that colloids are
incapable of passing over into the crystalline condition. Not all colloids
give jellies; van Bernmelen has also shown that in the case of the
hydrogels there is hardly a real chemical hydrate. Chemical inactivity
and the slowness of reaction often referred to by Graham are not
always characteristic of colloids. Objection might also be made to
many other statements, but it should, however, be remembered that

34 HISTORY OF THE IRREVERSIBLE COLLOIDS

Graham then describes the simple jar diffusion experi-
ment and remarks that separation by diffusion is due to
the property possessed by crystalloids of easily passing
through jelly-like masses of mucus, starch, gelose, etc.,
which offer great resistance to or completely prevent the
passage of colloids. For instance, with a mixture of sugar
and gum arabic, thin French letter paper will practically
allow only the former to pass through. This separation is
analogous to that of carbonic acid and hydrogen by a thin
film of water. He then continues :

"It may perhaps be allowed to me to apply the con-
venient term dialysis to the method of separation by
diffusion through a septum of gelatinous matter. The
most suitable of all substances for the dialytic septum
appears to be the commercial material known as vegetable
parchment, or parchment-paper, which was first pro-
duced by M. Gaine, and is now successfully manufactured
by Messrs. De la Rue." (P. ISO.)

The experiments made by the jar-diffusion method,
the importance of which in physical chemistry has long
been known, are described in detail on pages 186-199.
Graham then takes up dialysis proper, describes the
dialyzer and discusses the precautionary measures to
be taken in carrying out the process. (Pp. 199-204.)

PREPARATION OF COLLOID SUBSTANCES BY DIALYSIS

"The purification of many colloid substances may be
effected with great advantage by placing them on the
dialyzer. Accompanying crystalloids are eliminated, and
the colloid is left behind in a state of purity. 1 The purifi-

Graham was the first to recognize the similarity underlying the great
variety of colloids, and also that his papers were written more than
forty years ago.
1 This should not be regarded as meaning that by dialysis a com-

LIQUID DIFFUSION APPLIED TO ANALYSIS 35

cation of soluble colloids can rarely bo effected by any
other known means, $nd dialysis is evidently the appro-
priate mode of preparing such substances free from
crystalloids.

"Soluble Silicic Acid. A solution of silica is obtained
by pouring silicate of soda into diluted hydrochloric
acid, the acid being maintained in large excess. But in
addition to hydrochloric acid, such a solution contains
chlorid of sodium, a salt which causes the silica to gela-
tinize when the solution is heated, and otherwise modifies
its properties. Now such soluble silica, placed for twenty-
four hours in a dialyzer of parchment-paper, to the usual
depth of 10 millimetres, was found to lose in that time
5 per cent of its silicic acid and 86 per cent of its hydro-
chloric acid. After four days on the dialyser, the liquid
ceased to be disturbed by nitrate of silver. All the
chlorids were gone, with no further loss of silica. In
another experiment 112 grams of silicate of soda, 67.2
grams of dry hydrochloric acid, and 1000 cc. of water were
brought together, and the solution placed upon a hoop
dialyzer, 10 inches in diameter. After four days the solu-
tion had increased to 1235 cc, by the action of osmose,
colloid bodies being generally highly osmotic. The solution
now gave no precipitate with nitrate of silver, and con-
tained 60.5 grams of silica, 6.7 grams of that substance
having been lost. The solution contained 4.9 per cent
of silicic acid. 1

plete elimination of crystalloids is always secured, although the puri-
fication is so perfect that the impurities often fall below the limit of
analytical determination (Zs).

1 Jordis and Kanter (Zeitschr. f. anorg. Chemie., 1903, Vol. XXXV,
p. 18), have found, contrary to Graham, that in the dialysis of colloidal
silicic acid, the chlorin reaction of the contents of the dialyser taken
from one to three weeks to disappear. Variation!-} in the quality of the
parchment membranes and in the conditions of the dialysis influence

36 HISTORY OF THE IRREVERSIBLE COLLOIDS

"The pure solution of silicic acid so obtained may be
boiled in a flask and considerably concentrated, without
change; but when heated in an open vessel a ring of
insoluble silica is apt to form round the margin of the
liquid, and soon causes the whole to gelatinize. The pure
solution of hydrated silicic acid is limpid and colorless, and
not in the least degree viscous, even with 14 per cent of
silicic acid. The solution is the more durable the longer
it has been dialyzed and the purer it is. But this solution
is not easily preserved beyond a few days, unless consider-
ably diluted. It soon appears slightly opalescent, and
after a time the whole becomes pectous somewhat rapidly,
forming a solid jelly, transparent and colorless, or slightly
opalescent and no longer soluble in water. This jelly
undergoes a contraction after a few days, even in a close
vessel, and pure water separates from it. The coagulation
of the silicic acid is effected in a few minutes by a solu-
tion containing I/ 10,000th part of any alkaline or earthy
carbonate, but not by caustic ammonia, nor by neutral or
acid salts. Sulphuric, nitric, and acetic acids do not
coagulate silicic acid, but a few bubbles of carbonic acid
passed through the solution produce that effect after the
lapse of a certain time. Alcohol and sugar, in large quan-
tity even, do not act as precipitants; but neither do they
protect silicic acid from the action of alkaline carbonates,
nor from the effect of time in pectizing the fluid colloid.
Hydrochloric acid gives stability to the solution; so does
a small addition of caustic potash or soda.

"This pure water-glass is precipitated on the surface

its course in a high degree, and perhaps this is the cause of their lack
of agreement with Graham. Neither in the English nor German text
of Graham's work does it appear that he tested for chlorin only the
exterior water and not the dialyser contents, as Jordis and Kanter
seem to assume.

LIQUID DIFFUSION APPLIED TO ANALYSIS 3?

of a calcareous stono without penetrating, apparently
from the coagulating action of soluble lime-salts. The
hydrated silicic acid then forms a varnish, which is apt
to scale off on drying. The solution of hydrated silicic
acid has an acid reaction somewhat greater than that of
carbonic acid. It appears to be really tasteless (like
most colloids), although it occasions a disagreeable per-
sistent sensation in the mouth after a time, probably from
precipitation.

"Soluble hydrated silicic acid, when dried in the air-
pump receiver, at 15, formed a transparent glassy mass
of great luster, which was no longer soluble in water. It
retained 21.99 per cent of water after being kept two
days over sulphuric acid.

"The colloidal solution of silicic acid is precipitated by
certain other soluble colloids, such as gelatin, alumina,
and peroxid of iron, but not by gum nor caramel. As
hydrated silicic acid, after once gelatinizing, cannot be
made soluble again by either water or acids, it appears
necessary to admit the existence of two allotropic modifi-
cations of that substance, namely, soluble hydrated silicic
acid, and insoluble hydrated silicic acid, the fluid and
pectous forms of this colloid." (Pp. 204-205).

Ordinary sodium silicate is not a colloid; it diffuses
through membranes. Colloidal silicic acid has an acid
reaction which can be neutralized by alkalis. 1

"The acid reaction of 100 parts of soluble silicic acid
is neutralized by 1.85 part of oxid of potassium, and by
corresponding proportions of soda and ammonia. The
coZK-silicates or co-silicates thus formed are soluble and
more durable than fluid silicic acid, but they are pectized
by carbonic acid or by an alkaline carbonate, after standing
for a few minutes. The co-silicate of potash forms a
1 See Jordis (Zeitschr. f. Electroehemie, 1902, No! 36.)

38 HISTORY OF THE IRREVERSIBLE COLLOIDS

transparent hydrated film on drying in vacuo, which is not
decomposed by water, and appears to require about ten
thousand parts of water to dissolve it. The silicate of
soda which Forchhammer obtained by boiling freshly
precipitated silicic acid with carbonate of soda, and
collecting the precipitate which falls on cooling, contains
2.74 per cent of soda, and is represented by NaO + 36Si02
(Gmelin). This silicate is probably a co-silicate of soda in
the pectous condition. Soluble silicic acid produces a
gelatinous precipitate in lime-water, containing six per cent
and upwards of the basic earth. This and the other
insoluble earthy co-silicates appear not to be easily obtained
in a definite state. They gave out a more basic silicate to
water on washing. The composition of these salts and that
also of the co-silicate of gelatin, were found to vary
according as the mode of preparation was modified. When
a solution of gelatin was poured into silicic acid in excess,
the co-silicate o gelatin formed gave, upon analysis,
100 silicic acid with 56 gelatin, or a little more than half
the gelatin stated above as found in that compound pre-
pared with the mode of mixing the solutions reversed.
The gallo-tannate of gelatin is known to offer the samfe
variability in composition." (1?. 206).

"Soluble Alumina. We are indebted to Mr. Walter
Crum for the interesting discovery that alumina may be
held in solution by water alone in the absence of any acid.
But two soluble modifications of alumina appear to exist,
alumina and metalumina. The latter is Mr. Crum's
substance.

" A solution of the neutral chlorid of aluminium (A^Cla),
placed on the dialyzer, appears to diffuse away without
decomposition. But when an excess of hydrated alumina
is previously dissolved in the chlorid, the latter salt is
found to escape by diffusion in a gradual manner, and the

LIQUID DIFFUSION APPLIED TO ANALYSIS 39

hydrated alumina, retaining little or no acid, to remain
behind in a soluble state. A solution of alumina in chlorid
of aluminium, consisting at first of 52 parts of alumina
to 48 of hydrochloric acid after a dialysis of six days, con-
tained 66.5 per cent of alumina; after eleven days 76.5 per
cent; after seventeen days 92.4 per cent; and after twenty-
five days the alumina appeared to be as nearly as possible
free from acid, as traces only of hydrochloric acid were
indicated by an acid solution of nitrate of silver. But in
such experiments^the alumina often pectizes in the dialyzer
before the hydrochloric acid has entirely escaped.

"Acetate of alumina with an excess of alumina gave
similar results. The alumina remained fluid in the dialyzer
for twenty-one days, and when it pectized was found to
retain 3.4 per cent of acetic acid, which is in the propor-
tion of one equivalent of acid to 28.2 equivalents of alumina.

"Soluble alumina is one of the most unstable of sub-
stances, a circumstance which fully accounts for the
difficulty of preparing it in a state of purity. It is coagu-
lated or pectized by portions, so minute as to be scarcely
appreciable, of sulphate of potash and, I believe, by all
other salts; and also by ammonia. A solution containing
two or three per cent of alumina was coagulated by a few
drops of well-water, and could not be transferred from one
glass to another, unless the glass was repeatedly washed
out by distilled water, without gelatinizing. Acids in
small quantity also cause coagulation; but the precipi-
tated alumina readily dissolves in an excess of the acid.
The colloids gum and caramel also act as precipitants.

"This alumina is a mordant, and possesses indeed all
the properties of the base of alum and the ordinary alu-
minous salts. A solution containing one half per cent of
alumina may be boiled without gelatinizing, but when
concentrated to half its bulk it suddenly coagulated.

40 HISTORY OF THE IRREVERSIBLE COLLOIDS

Soluble alumina gelatinizes when placed upon red litmus
paper and forms a faint blue ring about the drop, showing
a feeble alkaline reaction. Soluble alumina is not pre-
cipitated by alcohol nor by sugar. No pure solution of
alumina, although dilute, remained fluid for more than a
few days.

"Like hydrated silicic acid, then, the colloid alumina
may exist either fluid or pectous, or it has a soluble and
insoluble form, the latter being the gelatinous alumina as
precipitated by bases. It is evident that the extraordinary
coagulating action of salts upon hydrated alumina must
prevent the latter substance from ever appearing in a
soluble state when liberated from combination by means
of a base/' (P. 207.)

" Soluble metalumina" can be produced, according to
Crum's method, by boiling off the acetic acid from a solu-
tion of aluminium acetate, or according to Graham by
dialyzing a solution of aluminium acetate altered by pro-
longed boiling. After six days the contents of the dialyzer
still contains some acetic acid.

"The alumina exists in an allotropic condition, being no
longer a mordant; and forming, when precipitated, a jelly
that is not dissolved by an excess of acid. Metalumina
resembles alumina in being coagulated by minute propor-
tions of acids, bases, and of most salts. Mr. drum found
the solution of metalumina to require larger quantities of
acetates, nitrates, and chlorids to produce coagulation
than of the former substances. The solution of metalu-
mina is tasteless, and entirely neutral to test-paper,
according to my own observation.

"Like alumina, the present colloid has therefore a fluid
and a pectous form, the liquid soluble metalumina, and
the gelatinous insoluble metalumina/' (P. 208.)

Graham then describes soluble ferric oxid and sesqui-

LIQUID DIFFUSION APPLIED TO ANALYSIS 41

oxid, which are analogous to the corresponding aluminium
colloids. The hydrosol of copper ferrocyanid is formed by
mixing very dilute solutions of potassium ferrocyanid with
copper sulphate (1:3000) and then dialyzing. Precipi-
tated copper ferrocyanid is insoluble in potassium oxalate
and oxalic acid, but is soluble in a quarter of its weight of
neutral ammonium oxalate. A solution containing 3-4
per cent of copper ferrocyanid is dark red-brown in color,
transparent to transmitted light, but cloudy to reflected
light. Iron sesquioxid has the same appearance. On
dialysis a little ammonium salt is strongly retained.

The solutions of copper ferrocyanid can be heated, but
arc easily made pectous by foreign substances, such as
nitric, hydrochloric, or sulphuric acid (and also after
gentle heating with oxalic and tartaric acid). Acetic
acid does not have this effect. Metallic salts coagulate
the solution. The colloidal solution of Prussian blue is
made pectous by zinc sulphate, etc., but requires a large
quantity of alkali salts for precipitation. Graham further
describes soluble compounds of sugar with copper oxid,
iron oxid, uranium oxid, and of sugar with lime. The
latter upon heating forms a solid coagulum. The solution
of copper tartarate in alkali is also colloidal. (Pp. 208-
212.)

Soluble chromium oxid (p. 212) is produced similarly to
soluble alumina; after thirty-eight days the solution in
the dialyzer was partly gelatinous and contained 1.5 parts
acid to 98.5 oxid or one equivalent of acid to 31.2 equiva-
lents of chromium oxid. Traces of salts make it gela-
tinous, and then it is insoluble even in hot water.

"It appears, then, that the hydrated peroxids of the
aluminous type, when free, are colloid bodies; that two
species of each of these hydrated oxids exist, of which
alumina and metalumina are the types; one derived from

42 HISTORY OF THE IRREVERSIBLE COLLOIDS

an unchanged salt, and the other from the heated acetate
of the base; further, that each of these species has two
forms, one soluble and the other insoluble or coagulated.
This last species of duality should be well distinguished
from the, preceding allotropic variability of the same
peroxid. The possession of a soluble and an insoluble
(fluid and pectous) modification is not confined to hydrated
silicic acid and the aluminous oxids, but appears to be
very general, if not universal, among colloid substances.
The double form is typified in the fibrin of blood/ 1
(Pp. 212, 213.)

The ammoniacal solutions of copper and zinc oxids
diffuse through a colloidal septum and are therefore not
to be considered colloids.

Dialysis of organic colloid substances (pp. 213-217).
Tannic acid passes through parchment paper about 200
times slower than sodium chlorid; gum arable 400 times
slower.

"The separation of colloids from crystalloids by dialysis
is, in consequence, generally more complete than might
be expected from the relative diffusibility of 'the two
classes of substances." (P. 214.)

Vegetable gum, according to Fremy, is a gummate of
lime, and after the addition of HC1, can be freed from ftme
by dialysis. One hundred parts of the remaining gummic
acid was neutralized by 2.85 parts of potash. This quan-
tity of potash is about equivalent to the lime originally
present in the gum. The gummic acid, if dried at 100,
becomes insoluble in water. .Gummic acid and glue give
oily drops, which upon standing yield a colorless
jelly, which becomes fluid at as low a temperature as
25 C.

Dextrin is colloidal, but slowly diffuses through animal
mucus.

LIQUID DIFFUSION APPLIED TO ANALYSIS 43

Caramel. Crude caramel, produced by heating raw
sugar to 210-220, when dialyzed allows a colored substance
to pass through, while the substance richest in carbon
remains behind. A ten per cent solution of this substance
is gum-like and forms a weak jelly completely soluble in
water. On evaporation in a vacuum, it dries to a black,
shining mass, which still contains water and is tough and
elastic. When thoroughly dry it can be heated to 120
and still remains completely soluble. If, however, the
first solution is evaporated to dryness on the water-bath,
it becomes insoluble. Soluble and insoluble caramel have
the same constitution, represented by the empirical
formula, C 24 Hi 5 Oi5. Liquid caramel is quite tasteless,
neutral in reaction, and is extremely sensitive to crystal-
loid reagents. Traces of mineral acids, alkali salts, and
alcohol make it pectous ; the precipitated caramel yields a
brownish-black powdery substance insoluble in hot or
cold water. "The presence of sugar and of the inter-
mediate brown substances protects the liquid caramel in
a remarkable degree from the action of crystalloids and
accounts for the preceding properties not being observed
in crude caramel." 1 Pectous caramel is easily rendered
soluble again by dilute potash, in which it becomes gela-
tinous and then dissolves on heating. By acetic acid
and dialysis, the potash can be removed once more, and
pure, soluble caramel remains in the dialyzer. Caramel is
600 times less dialyzable than sodium chlorid and 200
times less than sugar. Graham refers to the analogy
between caramel and anthracite coal: "Caramelization

1 This is the first example of a protective action, recognized as such,
and it is worthy of remark that substances capable of diffusion pro-
duce the same effect. I should here state that caramel acts similarly
to colloidal gold, from which it differs, among other points, in the
water-solubility of its dry residue (Zs).

44. HISTORY OF THE IRREVERSIBLE COLLOIDS

appears the first step in that direction the beginning
of a colloidal transformation to be consummated in the
slow lapse of geological ages." (P. 216.)

Albumen. Acetic acid was added to a solution of
egg albumen, and the solution dialyzed; entirely ash-
free albumen remained behind, which had a weak acid
reaction. Pure albumen prepared according to Wurtz's
method is 1000 times less dialyzable than sodium chlorid;
if to it is added ^ its weight of sodium hydrate, the lat-
ter alone will dialyze from the solution; and its separa-
tion from the albumen is complete. A film of albumen
coagulated by heat is totally impermeable to albumen
solution.

Neither gelatinized starch, nor an aqueous solution of
animal gelatin, nor meat extract, diffuses through colloid
septa.

Graham thereafter describes the well-known separa-
tion of arsenious acid from colloidal fluids by dialysis.
(Pp. 217-219.)

"Colloidal Condition of Matter. I may be allowed to
advert again to the radical distinction assumed in this
paper to exist between colloids and crystalloids in their
intimate molecular constitution. Every physical and
chemical property is characteristically modified in each
class. They appear like different worlds of matter, and
give occasion to a corresponding division of chemical
science. The distinction between these kinds of matter
is that subsisting between the material of a mineral
and the material of an organized mass.

"The colloidal character is not obliterated by lique-
faction, and is therefore more than a modification of the
physical condition of solids. Some colloids are soluble in
water, as gelatin and gum-arabic; and some are insoluble,
like gum-tragacanth. Some colloids, again, form solid

LIQUID DIFFUSION APPLIED TO ANALYSIS 45

compounds with water, as gelatin and gum-tragacanth,
while others, like tannin, do not. In such points the
colloids exhibit as great a diversity of property as the
crystalloids. A certain parallelism is maintained between
the two classes , notwithstanding their differences.

"The phenomena of the solution of a salt or crystalloid
probably all appear in the solution of a colloid, but
greatly reduced in degree. The process becomes slow;
time, indeed, appearing essential to all colloidal changes.
The change of temperature, usually occuring in the act
of solution, becomes barely perceptible. The liquid is
always sensibly gummy or viscous when concentrated.
The colloid, although often dissolved in a large propor-
tion by its solvent, is held in solution by a singularly
feeble force. Hence colloids are generally displaced and
precipitated by the addition to their solution of any sub-
stance from the other class. Of all the properties of
liquid colloids, their slow diffusion in water, and their
arrest by colloidal septa, are the most serviceable in dis-
tinguishing them from crystalloids. Colloids have feeble
chemical reactions, but they exhibit at the same time a
very general sensibility to liquid reagents, as has already
been explained.

"While soluble crystalloids are always highly sapid,
soluble colloids are singularly insipid. It may be ques-
tioned whether a colloid, when tasted, ever reaches the
sentient extremities of the nerves of the palate, as the
latter are probably protected by a colloidal membrane,
impermeable to soluble substances of the same physical
constitution. (P. 220.)

"A tendency to spontaneous change, which is observed
occasionally in crystalloids, appears 'to be general in the
other class. The fluid colloid becomes pectous and
insoluble by contact with certain other substances, with-

46 HISTORY OF THE IRREVERSIBLE COLLOIDS

out combining with these substances, and often under the
influence of time alone. The pectizing substance appears
to hasten merely an impending change. Even while
fluid a colloid may alter sensibly, from colorless becoming
opalescent; and while pectous the degree of hydration
may become reduced from internal change. The gradual
progress of alteration in the colloid effected by the
agency of time, is an investigation yet to be entered
upon.

"The equivalent of a colloid appears to be always high,
Although the ratio between the elements of the substance
piay be simple. Gummic acid, for instance, may be
represented by C^HnOn, but judging from the small
proportions of lime and potash which suffice to neutralize
this acid, the true numbers of its formula must be sev-
eral times greater. It is difficult to avoid associating
the inertness of colloids with their high equivalents, par-
ticularly where the high number appears to be attained"
by the repetition of a smaller number. The inquiry
suggests itself whether the colloid molecule may not be
constituted by the grouping together of a number of
smaller crystalloid molecules, and whether the basis of
colloidality may not really be this composite character
of the molecule.

"With silicic acid, which can exist in combination
both as a crystalloid and colloid, we have two series of
compounds, silicates and cosilicates, the acid of the
latter appearing to have an equivalent much greater
(thirty-six .times greater in one salt) than the acid of the
former. The apparently small proportion of acid in a
variety of metallic salts, such as certain red salts of iron,
is accounted for by the high colloidal equivalent of their
bases. The effect of such an insoluble colloid as Prus-
sian blue in carrying down small proportions of the pre-

ON THE PROPERTIES OF SILICIC ACID 47

cipitating salts, may admit of a similar explanation. (P.
221).

"The hardness of the crystalloid, with its crystalline
planes and angles, is replaced in the colloid by a degree
of softness, with a more or less rounded outline. The
water of crystallization is represented by the water of
gelatination. The water in gelatinous hydrates is aptly
described by M. Chevreul as retained by 'capillary affin-
ity/ that is, by an attraction partaking both of the phy-
sical and chemical character." (P. 222).

Graham, at the conclusion of his important work,
gives an explanation of osmosis, which in the main agrees
with the views of Liebig, published some time previously.

The contents of another valuble work of Graham will
for the most part be given in the author's own words.

" On the Properties of Silicic Acid and other Analogous
Colloidal Substances "

By THOMAS GRAHAM, F.R.S., Master of the Mint
(Received June 16, 1864) l

PRELIMINARY NOTICE

"The prevalent notions respecting solubility have been
derived chiefly from observations on crystalline salts, and
are very imperfectly applicable to the class of colloidal
substances. Hydrated silicic acid, for instance, when in
the soluble condition, is, properly speaking, a liquid body,
like alcohol, miscible with water in all proportions. We
have no degrees of solubility to speak of with respect to
silicic acid, like the degrees of solubility of a salt, unless
it be with reference to silicic acid in the gelatinous con-

1 Thomas Graham, Proceedings of the Royal Society, June 10,
1864. (Also Pogg, Ann., 1864, Vol. CXXIII, pp. 529-641.)

48 HISTORY OF THE IRREVERSIBLE COLLOIDS

dition, which is usually looked upon as destitute of solu-
bility. The jelly of silicic acid may be more or less rich
in combined water, as it is first prepared, and it appears
to be soluble in proportion to the extent of its hydration.
A jelly containing 1 per cent of silicic acid, gives with
cold water a solution containing about 1 of silicic acid
in 5000 water; a jelly containing 5 per cent of silicic
acid gives a solution containing about 1 part of acid in
10,000 water. A less hydrated jelly than the last men-
tioned is still less soluble; and finally, when the jelly is
rendered anhydrous, it gives gummy-looking, white
masses, which appear to be -absolutely insoluble, like
the light dusty silicic acid obtained by drying a jelly
charged with salts, in the ordinary analysis of a silicate.

"The liquidity of silicic acid is only affected by a
change, which is permanent (namely, coagulation or pec-
tization), by which the acid is converted into the gela-
tinous or pectous form, and loses its miscibility with
water. TShe liquidity is permanent in proportion to the
degree of dilution of silicic acid, and appears to be favored
by a low temperature. It is opposed, on the contrary,
by concentration, and by elevation of temperat^e. A
liquid silicic acid of 10 or 12 per cent pectizes spontane-
ously in a few hours at the ordinary temperature, and
immediately when heated. A liquid of 5 per cent may
be preserved for five or six days; a liquid of 2 per cent
for two or three months; and a liquid of 1 per cent has
not pectized after two years. Dilute solutions of 0.1 per
cent or less are no doubt practically unalterable by time,
and hence the possibility of soluble silicic acid existing
in nature. I may add, however, that no solution, weak
or strong, of silicic acid in water has shown any disposi-
tion to deposit crystals, but always appears on drying as
a colloidal glassy hyalite. The formation of quartz cry-

ON THE PROPERTIES OF SILICIC ACID 49

stals at a low temperature, of so frequent occurrence in
nature, remains still a mystery. I can only imagine that
such crystals are formed at an inconceivably slow rate,
and from solutions of silicic acid which are extremely
dilute. Dilution no doubt weakens the colloidal char-
acter of substances, and may therefore allow their crys-
tallizing tendency to gain ground and develop itself,
particularly where the crystal once formed is completely
insoluble, as with quartz.

"The pectization of liquid silicic acid is expedited by
contact with solid matter in the form of powder. By
contact with pounded graphite, which is chemically
inactive, the pectization of a 5 per cent silicic acid is
brought about in an hour or two, and that of a 2 per
cent silicic acid in two days. A rise of temperature of
l.l C. was observed during the formation of the 5 per
cent jelly.

"The ultimate pectization of silicic acid is preceded
by a gradual thickening in the liquid itself. The flow
of liquid colloids through a capillary tube is always slow
compared with the flow of crystalloid solutions, so that a
liquid-transpiration-tube may be employed as a colloido-
scope. With a colloidal liquid alterable in viscosity,
such as silicic acid, the increased resistance to passage
through the colloidoscope is obvious from day to day.
Just before gelatinizing, silicic acid flows like an oil.

"A dominating quality of colloids is the tendency of
their particles to adhere, aggregate, and contract. This
idio-attraction is obvious in the gradual thickening of
the liquid, and when it advances leads to pectization. In
the jelly itself, the specific contraction in question, or
synaeresis, still proceeds, causing separation of water,
with the division into a clot and serum; and ending in
the production of a hard stony mass, of vitreous struc-

50 HISTORY OF THE IRREVERSIBLE COLLOIDS

ture, which may be anhydrous, or nearly so, when the
water is allowed to escape by evaporation. The intense
synaeresis of isinglass dried in a glass dish over sulphuric
acid in vacuo enables the contracting gelatin to tear up
the surface of the glass. Glass itself is a colloid, and
the adhesion of colloid to colloid appears to be more
powerful than that of colloid to crystalloid. The gelatin,
when dried in the manner described upon plates of calc-
spar and mica, did not adhere to the crystalline surface,
but detached itself on drying. Polished plates of glass
must not be left in contact, as is well known, owing to
the risk of permanent adhesion between their surfaces.
The adhesion of broken masses of glacial phosphoric acid
to each other is an old illustration of colloidal synaeresis.

"Bearing in mind that the colloidal phasis of matter
is the result of a peculiar attraction and aggregation of
molecules, properties never entirely absent from matter,
but more greatly developed in some substances than in
others, it is not surprising that colloidal characters
spread on both sides into the liquid abd solid conditions."
(Pp. 335, 336.)

Graham goes on to say that certain fluid substances
which exercise no pectizing influence on colloids,*- can
totally or partially displace the combined water of col-
loidal silicic acid, without causing the coagulation of the
colloid. Hydrochloric, nitric, acetic, and tartaric acids,
sugar syrup, glycerin, and alcohol are in this class. He
then describes the production of such a solution of silicic
acid in alcohol and the alcogel.

"The.cdcogel, or solid compound, is readily prepared
by placing masses of gelatinous silicic acid, containing
8 or 10 per cent of the dry acid, in absolute alcohol, and
changing the latter repeatedly till the water of the hydro-
gel is fully replaced by alcohol. The alcogel is generally

ON THE PROPERTIES OF SILICIC ACID 51

slightly opalescent, and is similar in aspect to the hydro-
gel, preserving very nearly its original bulk. The follow-
ing is the composition of an alcogel carefully prepared
from a hydrogel which contained 9.33 per cent of silicic
acid :

Alcohol 88.13

Water 0.23

Silicic acid 11 .64

100.00

"Placed in water, the alcogel is gradually decomposed
alcohol diffusing out and water entering instead, so that
a hydrogel is reproduced. (Pp. 337, 338.)

"The compound of sulphuric acid, sulphagel, is also
interesting from the facility of its formation, and the
complete manner in which the water of the original hydro-
gel is removed. A mass of hydrated silicic acid may be
preserved unbroken if it is first placed in sulphuric acid
diluted with two or three volumes of water, and then
transferred gradually to stronger acids, till at last it is
placed in concentrated oil of vitriol. The sulphagel
sinks in the latter fluid, and may be distilled with an
excess of it for hours without losing its transparency or
gelatinous character. It is always somewhat less in
bulk than the primary hydrogel, but not more, to the
eye, than one fifth or one sixth part of the original vol-
ume. This sulphagel is transparent and colorless. When
a sulphagel is heated strongly in an open vessel, the last
portions of the monohydrated sulphuric acid in com-
bination are found to require a higher temperature for
their expulsion than the boiling-point of the acid. The
whole silicic acid remains behind, forming a white, opaque,
porous mass, like pumice. A sulphagel placed in water

52 HISTORY OF THE IRREVERSIBLE COLLOIDS

is soon decomposed, and the original hydrogel repro-
duced. No permanent compound of sulphuric and
silicic acids, of the nature of a salt, appears to be formed
in any circumstances. A sulphagel placed in alcohol
gives ultimately a pure alcogel. Similar jellies of silicic
acid may readily be formed with the monohydrates of
nitric, acetic, and formic acids, and are all perfectly
transparent.

"The production of the compounds of silicic acid now
described indicates the possession of a wider range of
affinity 1 by a colloid than could well be anticipated.
The organic colloids are no doubt invested with similar
wide powers of combination, which may become of
interest to the physiologist. The capacity of a mass of
gelatinous silicic acid to assume alcohol, or even olein,
in the place of water of combination, without disinte-
gration or alteration of form, may perhaps afford a clue
to the penetration of the albuminous matter of mem-'
brane by fatty and other insoluble bodies, which seems
to occur in the digestion of food. Still more remarkable
and suggestive are the fluid compounds of silicic acid.
The fluid alcohol or olein compound favors the possi-
bility of the existence of a compound of the colloidKalbu-
men with olein, soluble also and capable of circulating
with the blood.

"The feebleness of the force which holds together two
substances belonging to different physical classes, one
being a colloid and the other a crystalloid, is a subject
deserving notice. When such a compound is placed in
a fluid the superior diffusive energy of the crystalloid may
cause its separation from the colloid. Thus, of hydrated

1 This is not an instance of chemical affinity and chemical combina-
tion in the now adopted meaning of the word. See further on in this
Chapter (Zs).

ON THE PROPERTIES OF SILICIC ACID 53

silicic acid, the combined water (a crystalloid) leaves
the acid (a colloid) to diffuse into alcohol; and if the
alcohol be repeatedly changed, the entire water is thus
removed, alcohol (another crystalloid) at the same time
taking the place of water in combination 1 with the silicic
acid. The liquid in excess (here the alcohol) gains
entire possession of the silicic acid. The process is
reversed if an alcogel be placed in a considerable volume
of water. Then alcohol separates from combination, in
consequence of the opportunity it possesses to diffuse
into water; and water, which is now the liquid present
in excess, recovers possession of the silicic acid. Such
changes illustrate the predominating influence of mass.

"Even the compounds of silicic acid with alkalies yield
to the decomposing force of diffusion. The compound
of silicic acid with one or two per cent of soda is a col-
loidal solution, and, when placed in a dialyzer over water
in vacuo to exclude carbonic acid, suffers gradual decom-
position. The soda diffuses off slowly in the caustic
state, and gives the usual brown oxid of silver when
tested with the nitrate of that base.

"The pectization of liquid silicic acid and many other
liquid colloids is effected by contact with minute quan-
tities of salts in a way which is not understood. On the
other hand, the gelatinous acid may again be liquefied,
and have its energy restored by contact with a very
moderate amount of alkali. The latter change is grad-
ual, 1 part of caustic soda, dissolved in 10,000 water,
liquefying 200 parts of silicic acid (estimated dry), in
60 minutes at 100 C. Gelatinous stannic acid also is
easily liquefied by a small proportion of alkali, even at
the ordinary temperature. The alkali, too, after lique-
fying the gelatinous colloid, may be separated again

1 See note on p. 52.

54 HISTORY OF THE IRREVERSIBLE COLLOIDS

from it by diffusion into water upon a dialyzer. The
solution of these colloids, in such circumstances, may be
looked upon as analogous to the solution of insoluble
organic colloids witnessed in animal digestion, with the
difference that the solvent fluid here is not acid, but
alkaline. Liquid silicic acid may be represented as the
'peptone' of gelatinous silicic acid; and the liquefac-
tion of the latter by a trace of alkali may be spoken of as
the peptization of the jelly. The pure jellies of alumina,
peroxid of iron, and titanic acid, prepared by dialysis,
are assimilated more closely to albumen, being peptized
by minute quantities of hydrochloric acid.

11 Liquid Stannic and Metastannic Acids. Liquid stan-
nic acid is prepared by dialyzing the bichlorid of tin
with an addition of alkali, or by dialyzing the stannate
of soda with an addition of hydrochloric acid. In both
cases a jelly is first formed on the dialyzer; but, as thg
salts diffuse away, the jelly is again peptized by the
small proportion of free alkali remaining; the alkali
itself may be removed by continued diffusion, a drop
or two of the tincture of iodin facilitating the separation.
The liquid stannic acid is converted on heating^it into
liquid metastannic acid. Both liquid acids are remark-
able for the facility with which they are pectized by a
minute addition of hydrochloric acid, as well as by salts.

"lAqwd Titanic Add is prepared by dissolving gela-
tinous titanic acid in a small quantity of hydrochloric
acid, without heat, and placing the liquid upon a dialyzer
for several days. -The liquid must not contain more
than one per cent of titanic acid, otherwise it spontane-
ously gelatinizes, but it appears more stable when dilute.
Both titanic and the two stannic acids afford the same
classes of compounds with alcohol, etc., as are obtained
with silicic acid.

COLLOIDAL SULPHIDS AND METALS 55

"Liquid Tungstic Acid. The obscurity which has so
long hung over tungstic acid is removed by a dialytic
examination. It is, in fact, a remarkable colloid, of
which the pectous form alone has hitherto been known.
Liquid tungstic acid is prepared by adding dilute hydro-
chloric acid carefully to a five per cent solution of tungstate
of soda, in sufficient proportion to neutralize the alkali,
and then placing the resulting liquid on a dialyzer. In
about three days the acid is found pure, with the loss of
about twenty per cent, the salts having diffused entirely
away. It is remarkable that the purified acid is not
pectized by acids or salts even at the boiling temperature.
Evaporated .to dryness, it forms vitreous scales, like
gum or gelatin, which sometimes adhere so strongly to
the surface of the evaporating dish as to detach portions
of it. It may be heated to 200 C. without losing its
solubility or passing into the pectous state, but at a
temperature near redness it undergoes a molecular
change, losing at the same time 2.42 per cent of water.
When water is added to unchanged tungstic acid it
becomes pasty and adhesive like gum; and it forms a
liquid with about one fourth its weight of water, which
is so dense as to float glass. The solution effervesces
with carbonate of soda, and tungstic acid is evidently
associated with silicic and molybdic acids. The taste
of tungstic acid dissolved in water is not metallic or
acid, but rather bitter and astringent. Solutions of
tungstic acid containing 5, 20, 50, 66.5, and 79.8 per cent
of dry acid possess the following densities at 19: 1.0475,
1.2168, 1.8001, 2.396, and 3.243. Evaporated in vacuo
liquid tungstic acid is colorless, but becomes green ir
air from the deoxidating action of organic matter
Liquid silicic acid is protected from pectizing
mixed with tungstic acid, a circumstance

56 HISTORY OF THE IRREVERSIBLE COLLOIDS

connected with the formation of 1>he double com-
pounds.

"Molybdic Acid has hitherto been known (like tungstic
acid) only in the insoluble form. Crystallized molybdate
of soda dissolved in water is decomposed by the gradual
addition* of hydrochloric acid in excess without any
immediate precipitation. The acid liquid thrown upon
a dialyzer may gelatinize after a few hours, but again
liquefies spontaneously, when the salts diffuse away.
After a diffusion of three days, about 60 per cent of the
molybdic acid remains behind in a pure condition. The
solution of pure molybdic acid is yellow, astringent to
the taste, acid to test-paper, and possesses much sta-
bility. The acid may be dried at 100, and then heated
to 200 without losing its solubility. Soluble molybdic
acid has the same gummy aspect as soluble tungstic
acid, and deliquesces slightly when exposed to damp
air. Both acids lose their colloidality when digested
with soda, for a short time, and give a variety of crystal-
Uzable salts." (P. 338-341.)

2. Colloidal Sulphids and Metals: Order of Their Discovery

If, in our present state of knowledge we consider the
3hief moot question as to the nature of colloidal solu-
tions, it will at once be seen that the conflict of opinions
had its origin in the irreversible colloids, and is in part
due to a mutual misunderstanding. As stated in Chapter
I, chemists designated as .suspensions, only coarse, het-
erogeneous subdivisions, which settle of their own
accord, but considered clear non-settling subdivisions as
solutions; wEile some physicists called subdivisions of
this latter class suspensions. Soon, however, fluids were
found which were intermediate between the suspensions

COLLOIDAL SULPHIDS AND METALS 57

and the solutions of the chemists. These formed the
subject of differences of opinion, and at the same time
were the starting-point of investigations which afforded
a partial, but still very incomplete, insight into the
nature of hydrosols.

Thus Berzelius regarded the hydrosol of arsenic sul-
phid as a suspension, because of its property of settling
after standing a while.

. Colloidal Sulphids. H, Schulze, whom we must
thank for the first thorough investigation on colloidal
sulphids (1882), concluded from his observations on
colloidal arsenic sulphid that this fluid is by no means
identical with ordinary levigated arsenic sulphid; but
rather that its action and properties coincide in many
important points with the ,hydrosols of ferric oxid,
aluminium, etc., described by Graham, and therefore had
no hesitation in calling them solutions. The marked
cloudiness of his fluid, Schulze considered fluorescence;
for he should have been able to see suspended particles
of arsenic sulphid under the microscope, while his fluid
by microscopic examination appeared perfectly homo-
geneous. 1 In order to explain the existence of solutions
of the insoluble As 2 S3, Schulze (just as had Graham),
assumed the existence of allotropic modifications.

Schulzc's communication on the colloidal sulphids of
arsenic and antimony were soon followed by further
work on colloidal sulphids, especially that of Spring
(1883), and later by that of Winssinger. Spring's article
on colloidal copper sulphid led Ebell (1883) 2 to point

1 Schulze cannot be blamed for not differentiating between djffuse
dispersion and fluorescence, for the method of differentiating these
two was practically unknown to the chemical fraternity.

'Berichte d. Deutschen chem. Gesell., 1883, Vol. XVI, pp. 1142-43;
also Spring and de Boeck, Bull, de la Soc. chim., 1887 [2], Vol. XLVIII,
p. 165.

58 HISTORY OF THE IRREVERSIBLE COLLOIDS

out the analogy between colloidal sulphids arid the finest
levigated ultramarine, which no longer settles. 1 Ebell
fortunately had at his disposal a fluid which may be
considered as an intermediary between hydrosols and
suspensions. This fluid contained particles which were
still visible under the microscope, but which upon long
standing settled only partially, or not at all. In layers
about 2 cm. thick the fluid was transparent, and appeared
clear to transmitted light. As is the case with many
irreversible hydrosols, it left, when evaporated on the
water bath, a shining varnish which could jiot be Jeyi-
gated in water to the original state of fineness; having
in mind that its particles are microscopically visible, its
relation to suspensions is indicated by the fact that pre-
cipitates produced by salts assume their original state
of subdivision in pure water. (This fluid should be further
examined ultramicroscopically for the presence of finer
particles.) Remembering that this fluid is obtained by
the levigation of the crude product in the second step in
ultramarine manufacture, and that in the first step clay
is for a long time roasted with alkali sulphate and car-
bon, it will not appear improbable that we have here a
partially coagulated hydrosol, and that the particles
visible under the microscope are already flocks, formed
by the clotting together of incomparably much smaller
particles under the influence of salts, which arc at once
brought into solution upon leaching the melt. Further
evidence of this is its slow settling and the transparency
of the fluid, a property common to levigated hydrogels.

Spring, who originally held with Schulze that colloidal
sulphids were especial water soluble modifications, later
agreed with the opinion of Ebell and other investigators,
that in this case there was an extreme subdivision of the

1 Berichte d. Deutschen chem. Gesell., 1883, Vol. XVI, pp. 2429-2432.

COLLOIDAL SULPHIDS AND MENTALS 56

material; but in addition ho made the most important
observation, 1 based upon the work of Picton and Linder,
and his own work on optically clear water, that many
other colloidal solutions and also solutions of many crys-
talloids show the same lack of optical homogeneity as
do the irreversible hydrosols, which were chiefly sus-
pected of being heterogeneous.

Colloidal Silver. An excellent experimental investiga-
tion of the water-soluble modifications of silver was worked
out by Carey Lea, and at that time it attracted general
attention. In 1889 Lea succeeded in producing a water-
soluble substance containing over 95 per cent of silver.
It was natural to regard this water-soluble form as an
allotropic modification of the metal. The name chosen
by Lea assumed as much.

Prior to Lea, in 1887, O. Loew 2 and Muthmann 3 had
prepared colloidal silver, and Muthmann had observed
that the substances assumed by Wohler and others to
contain silver suboxid, were mixtures of silver oxids with
metallic silver. He had furthermore dissolved out with
ammonia the colloidal silver contained in such reduc-
tion products, established its constitution, and described
its properties.

Muthmann also observed that certain other amorphous
bodies, such as ferric hydroxid and molybdic dioxid
hydrate had similar properties to the finely divided sil-
ver described by him, and he considered it not improb-
able that the various modifications of silver which he
noticed betokened variations in molecular condition.
The connection of this work with Graham's appears to

1 W. Spring, Bull, de TAcad. roy. de Belgique, 1899 (Cl. de. sc.),
No. 4, pp. 300-315.

2 Loew, Berichte, 1883, Vol. XVI, p. 2707.

8 Muthmann, Berichte, 1887, Vol. XX, p. 983.

60 HISTORY OF THE IRREVERSIBLE COLLOIDS

have escaped Muthmann, for he makes no reference to it
in his paper, and besides, the only correct terminology
for his fluids " colloidal solutions or hydrosols " is not
used by Muthmann.

In the year 1890 Prarige observed that the allotropic
silver of Carey Lea could be regarded as a water-soluble
colloidal form of this metal, and gave a series of proofs
as to the colloidal nature of its solution. Since Prange's
publication Lea's silver has been generally and properly
called colloidal silver.

^Conflicting Views Regarding the Nature of Irreversible
Hydrosols. Barus and Schneider 1 in a most thorough
paper, oppose the idea that colloidal silver is an allotropic
modification of the metal. Without being quite able to
disprove the presence of an allotropic modification, both
investigators pointed out the analogy between the
properties of the colloidal silver and those of suspended
clay particles, and adduced much evidence to show
that the silver is present in a fine state of subdivision.
Barus and Schneider regard colloidal solutions as true
suspensions, in which the sedimentation can be calcu-
lated from the mechanical laws of falling bodies based
upon the radius of the particles, viscosity of the medium,
difference in density between both substances, and the
acceleration due to gravity.

They came to the following conclusion:

"Taking all facts into consideration, we think we may
conclude that the view that colloidal silver consists of
extremely finely divided little particles of normal silver
which may be considered as continuing to float, because
of the viscosity of the solvent, is in no way contrary to
the observed properties of colloidal solutions. Inas-
much as there are hardly any good reasons for the assump-

1 Zeitschr. f. physikal. Chemie, 1891, Vol. VIII, p. 297.

COLLOIDAL SULPHIDS AND METALS 61

tion of an allotropic molecule, it is simpler to adhere to
the normal molecule. The same applies to colloidal
solutions in general." l

From a little known criticism of this work by two
English scientists, Picton and Linder (1892), who,
among other things, deserved credit for having shown
that by continued subdivision of sulphids, colloidal
solutions exhibiting osmotic pressure and diffusibility
nearly approaching those of the lower colloids can be
obtained from #iechanical suspensions, we quote only
one part 2 having reference to the work of Barus and
Schneider :

"The evidence upon which most stress is laid in the
above quoted paper (of Barus and Schneider) seems to
us to ,be decidedly inconclusive. It is also noteworthy
that throughout this paper there is an assumption that
if the colloidal solutions contain very finely divided
particles, the solution is a fact of mere mechanical sus-
pension and nothing more. This is evident, too, from
their method of calculating the size of the suspended
particles. An assumption of this kind seems to us quite
irreconcilable with fact."

Although Picton and Linder completely agree with
Barus and Schneider that in colloidal solutions there is
the most extreme subdivision of the substance, and in
their beautiful experimental work give much evidence
therefor, they by no means favor the explanation given
by the latter. They oppose the conception that col-
loidal solutions are mere mechanical suspensions, and
conceive the particles contained in hydrosols to be

1 Wernicke has' also expressed the same idea. Wied. Ann., 1894,
N. F., Vol. LII, p. 515.

2 Picton and Linder, Solutions and Pseudo-Solutions, J. of Chem.
Society, 1892, p. 148.

62 \ HISTORY OF THE IRREVERSIBLE COLLOIDS

large molecular aggregations. (Picton and Linder, p.
169.)

Picton and Linder produced four kinds of arsenic
sulphid: As2Ss a, /?, 7-, d, of which a is the largest parti-
cle (still visible microscopically) and 8 is the smallest.
As2Sa or, /? and r are retained upon filtration through
a porcelain cell, while d is not. In this case no adsorp-
tion takes place, for if As2Sa were adsorbed by porce-
lain, d would also be retained by it; the particles of
the first three hydrosols are much too large to pass
through the filter.

While hydrosols a and /? do not diffuse, hydrosols
Y and d exhibit distinct diffusibility. A four per cent
solution of arsenic sulphid had an osmotic pressure
equivalent to 17 mm. of water.

Picton and Linder are of the opinion that the parti-
cles which diffuse and produce osmotic pressure are
identical with those which, upon Tyndall's test, betray
their existence by polarization. This has an important
bearing on the objections which can be urged (sec this
chapter further below) against the applicability of the
osmotic method for the determination of molecular
weight. '**

If diffusibility were due to the presence of crystalloids
or else to a partial crystalloid solubility of the arsenic
sulphid, then the same cause would also operate in the
case of the coarser hydrosols, which, however, show no
trace of diffusion and no osmotic pressure.

That in As 2 S 3 <? the whole mass of the arsenic sulpid
consists of smaller particles than the others, is self-
evident from the fact that it passes through a clay cell,
while the other three hydrosols arc held back.

It should be observed that Linder and Picton, even
in their first communication, pointed out that all col-

COLLOIDAL SULPHIDS AND METALS 83

loidal sulphids contain a certain quantity of sulphur-
rctod hydrogen which can be partially driven off by
the addition of acid. Linder and Picton regard this
sulphureted hydrogen as chemically combined, and
consider the colloidal sulphids to be complex chemical
compounds of sulphid with H 2 S. Thus on the basis of
their analyses they assign to cupric sulphid the formula
9CuS,HS or 22CuS,H 2 S; to zinc sulphid the formula
7ZnS,H 2 or 12ZnS,H 2 S; to mercuric sulphid 31HgS,H 2 S
and 62HgS,H 2 S. , Further, colloidal arsenic sulphid, for
which above the * abbreviated formula As 2 S 3 has been
used, contains combined sulphureted hydrogen in the
proportion of 8 (or 16) As 2 S 3 :lH 2 S. The solubility of
some sulphids in sulphureted hydrogen is attributed to
the formation of complex hydrosulphids.

Jordis, as is well known, recently adopted a quite
similar view concerning colloidal silicic acid (and col-
loids in general), in that he regards, for instance, the
silicic acid hydrosol to be not a colloidal solution of
silicic acid, but a solution of a chemical compound of
the same. 1 Besides, even Graham regarded solutions of
colloidal silicic acid in a little alkali as silicates, and
called them colli-silicates or co-silicates.

New Hydrosols of the sulphid group have been made in
large numbers by Winssinger (almost all of them colloidal
sulphid solutions), by E. A. Schneider (colloidal gold
sulphid and tin sulphid among others), colloidal oxids
by Grimaux, Spring, Biltz, and others.

Hydrosols of Metals have been produced by Faraday,
Lea, 0. Loew, Muthmann, Lobry de Bruyn, Zsigmondy,
Lottermoser (Hg, Bi, Cu, Pt, Pd, Rh), and by Bredig, by
the very original method of atomizing metal by means

1 Jordis, loc. cit., Chapter III above and Chapter XXI.

64 HISTORY OF THE IRREVERSIBLE COLLOIDS

of an electric arc under water (Pt, Au, Pt, Ag, Cd, etc.).
The numerous metallic hydrosols which Brcdig has pre-
pared in this manner were used by him in a very valuable
chemico-contact investigation, the object of which was
to throw new light on a series of hitherto unexplained
phenomena which had been observed with organic fer-
ments and enzymes. The analogy is so close that many
of the same substances which are known as powerful
blood poisons, and inhibit the catalytic action of the
blood, also act as powerful " poisons" in hindering the
catalytic action of colloidal platinum or gold.

An important method for the manufacture of high
percentage and reversible hydrosols is that of Paal, 1
who succeeded in producing among other things a col-
loidal gold soluble in alkali and containing more than
93 per cent Au.

Recent investigators, Gutbier, Kiispert, Henrich, Bil-
litzer, Blake, Donau, and others, by extending known
methods, have largely increased the number of methods
for the production of metallic hydrosols, and among
other things have shown that numerous reducing agents
are suitable for this purpose, for example, any energetic
reducing agent which does not of itself coagulate the
hydrosol, or which exercises a protective action (Schutz-
wirkung), is adaptable for the production of hydrosols
of the noble metals or analogues of the purple of Cassius,
providing the essential well-known conditions are observed :
sufficient dilution and extreme purity of the reagents,
or the use of protective colloids which prevent the indi-
vidual particles from coalescing.

Worthy of note is the method discovered by Donau
for the production of colloidal* gold by bubbling carbon

1 Paal, Berichte, 1902, Vol. XXXV, p. 2236.

COLLOIDAL SULPHIDS AND METALS 65

monoxid through a dilute solution of chlorid of gold, in
which case no protective colloids can be formed. 1

Purple of Cassius. A whole literature exists regarding
the ammonia-soluble deep red precipitate known as the
Purple of Cassius, which, is obtained by mixing dilute
solutions of chlorid of gold and stannous chlorid. As
this substance has acquired significance because it gives
an insight into many colloids, I must devote some space
to it, even if I can mention only the most important
points. 2

Even in the time of Berzelius there existed two differ-
ent ideas regarding the nature of this substance. Berze-
lius himself writes regarding it in his "Text Book of
Chemistry " : 3

"It is not yet established what is the condition of
the gold in the purple of Cassius. Some chemists con-
sider it to be metallic and only mixed with the tin oxid.
When, however, a gold salt which is mixed with any
powder is reduced, the mixture gives a brick red powder
which does not appear metallic and does not, further-
more, have the clear, transparent color of the purple;
and if the purple is roasted in a small retort, it acquires
the same brick-red color, indicating that the gold, upon
roasting, has again assumed the metallic form."

And further (page 245):

"That the purple contains neither metallic gold nor

1 The resemblance of this gold solution to those here described
(Chapter VIII) is very great. The somewhat greater sensitiveness of
Donau's gold soluton might be ascribed to its larger content of hydrogen
ions. Donau, Wien. Akad. Ber., Vol. CXIV. Abt. Ha, 1905.

3 A more detailed discussion will be found in my article : The Chemi-
cal Nature of the Purple of Cassius. Liebig's Ann., 1898, Vol. CCCI,
pp. 362-387.

8 Second Edition, translated by Bldde and Palmstedt, 1823, Vol. II,
p. 244.

66 HISTORY OF THE IRREVERSIBLE COLLOIDS

oxid is evident from the fact that it dissolves in ammo-
nium hydrate, which would leave undissolved the admixed
metal, or would form fulminate of gold with the oxid.
The ammonia solution is a dark red fluid, from which
the purple precipitates as a. jelly upon spontaneous
evaporation of the ammonia.''

On the basis of his investigations, Berzelius regarded
the purple as a chemical combination of tin sesquioxid
(zinnoxyduloxid) with purplish oxid of gold. As may
be seen, Berzelius laid great stress upon the homogeneity
of the purple, and just because of the clear, transparent
color of the precipitate and its solubility in ammonia,
he was led to hold that the purple is not a mixture of
finely divided gold and stannic acid, the color of which
was known to him.

It is of the utmost importance to bear in mind that
a greater part of inorganic chemistry was developed at
a time when such ideas held sway, and a considerable
number of substances which were called chemical com-
pounds, would to-day be considered as colloidal mix-
tures or compounds, or as colloidal solutions, or as
absorption compounds.

Later, many workers took up the moot (Question as
to the nature of the purple of Cassius I mention only
Gay-Lussac, Debray, Golfier Besseyre, Miiller without
reaching any final conclusion, for the chief point of Ber-
zelius, the solubility of the purple in ammonia, was not
cleared up. E. A. Schneider was the first correctly to
explain the purple of Cassius as a mixture of the hydro-
sols of gold and stannic acid, and quite independently I
was able to prove that the purple is not a chemical com-
bination, by synthesizing it from its constituents.

By simple mixture of the hydrosols of gold and stan-
nic acid I produced the purple of Cassius with all of its

COLLOIDAL SULPHIDS AND METALS 67

usual properties. That the stannic acid in this case
does not enter into a chemical reaction with the gold,
is evident, firstly, from the indifference of metallic gold
to colloidal stannic acid and similar bodies, and secondly,
from the fact that by the presence of the stannic acid
neither the absorption spectrum nor the intensity of
the color of the colloidal gold changed in the least.
These changes would have appeared if the gold had
entered into chemical combination (as by treatment
with chlorin, potassium cyanid, etc., whereby the red
color vanishes in a short time). Nevertheless the pres-
ence of stannic acid has effected a profound change in
the behavior of the gold toward reagents; it suffers no
color change l upon the addition of electrolytes, but
precipitates together with the stannic acid as a purple-
red ammonia-soluble gelatinous precipitate, and in this
respect acts as if it were chemically combined with the
stannic acid.

Colloidal Mixtures which have Simulated Chemical
Compounds. The extensive experiments which led to
the result above set forth, were begun with the belief
that they were to settle a principle of general applica-
tion. When, therefore, it was possible to prove beyond
doubt that the purple of Cassius is not to be considered
a chemical compound, as the most brilliant chemist of
his time had assumed, it at once became questionable
whether a very large number of substances, which had
been called chemical compounds really were such. At
the beginning of my investigations, I myself considered
it probable that this was a case of a chemical compound

1 As is well known, pure red gold hydrosols are immediately coagu-
lated by the addition of electrolytes with the production of a blue
color, and subsequently deposit metallic gold; the purple of Cassius
acts quite differently (see pp. 13, 14, and 78).

68 HISTORY OF THE IRREVERSIBLE COLLOIDS

of acid character, which owed its solubility in ammonia
to the formation of a soluble salt; but I was led to the
conclusion that it was an intimate mixture of colloidal
gold and colloidal stannic acid, or a " colloid-compound "
of gold and stannic acid, owing its solubility to the pep-
tisizing action of the alkali. Making use of the language
of the old chemists, I expressed the generalization of
these facts in the following sentence: A mixture of col-
loidal substances can, under certain conditions, act like
a chemical compound, and the properties l of the one
substance in such a mixture can be masked by the other. 2
I chose the expression "colloidal mixture " in order
to lay particular stress on the fact that this is not a case
of combination in a chemical sense. Later, in conjunc-
tion with van Bemmelen's expression "absorption com-
pound/' I used the expression colloid-compound 3 for

1 By "properties" is here meant chiefly behavior with reagents.

2 Bredig (Anorganische Fermente, 1st ed., p. 20) has taxed me
with basing my definition upon a loose conception of chemical com-
bination. In reply I might simply remark, that a long-established
idea cannot be overturned by the mere dictum of a new definition.
A chemist whom it was desired to convince, upon the basis of a defi-
nition, that a substance described as a chemical compound by the
highest authorities, really was not one, would rather d^ubt the accu-
racy of the definition in question than assume that the authorities
were in error, especially when the proof was not based upon experi-
mental grounds. An old-established means of discrimination between
a mixture and a chemical compound, and one used by Berzelius 3n
this case, is that the constituents of the former can be easily separated
by means of solvents, while those of the latter cannot. I recall only
the well-known example of iron and sulphur: from the mixture of
both substances the sulphur (or the iron) may be removed by a variety
of solvents, which is not the case with the combination FeS. That
in the case of colloids every characteristic cannot be applied to dis-
tinguish between mixtures and chemical compounds will be shown by
the substance of my work, and is indirectly expressed by the above
definition.

3 Zsigmondy, Verh. d. Gesell. Deutscher Naturforcher u. Aerate
(Hamburg), 1902, pp. 168-172.

COLLOIDAL SULPHIDS AND METALS 69

colloidal mixtures in which both constituents are col-
loids, in order to express the intimate union of both
colloidal substances.

It must here be noted, however, that not only have
colloid compounds or colloidal mixtures, in which two
colloids are united, been erroneously described as chemi-
cal compounds, but so also have mixtures or absorption
compounds of crystalloids with colloids, as well as col-
loidal solutions of various substances, especially of
metals in fused salts.

Thus Kirchhoff and Bunsen, 1 and later H.Rose, 2 describe
a blue potassium subchlorid K 2 C1, which, upon stirring
in water, decomposes into KC1 and KOH with the evo-
lution of hydrogen. It is very probable that this is a
case of a colloidal solution of potassium in potassium
chlorid, all the more so as Bronn 3 has shown that in all
probability the red and blue solutions of potassium,
sodium, etc., in ammonia are most probably colloidal.
The same applies to sodium subchlorid Na 2 Cl, described
by H. Rose, and to rubidium and caesium subchlorids
(Kirchhoff and Bunsen). These subchlorids interfere
with the electrolysis of fused salts, and the metals dis-
solve in the molten mass producing the color. It is
worthy of notice that Bunsen and Kirchhoff (as well as
H. Rose) were led by the homogeneous appearance of
their molten salts, "which neither to the naked eye nor
under the microscope show the slightest trace of metal-
lic substance/' to assume that they were chemical com-
pounds.

Recently R. Lorenz 4 has pointed out the analogy

1 Kirchoff and Bunsen, Pogg. Ann., 1861, Vol. CXIII, p. 345.

2 H. Rose, Pogg. Ann., 1863, Vol. CXX, p. 1.

8 Bronn, Drude's Ann., 1905, Vol. XVI, p. 166.

1 R. Lorenz. A complete collection of the observations of R. Lorenz

70 HISTORY OF THE IRREVERSIBLE COLLOIDS

between these " compounds " and the " metal fog"
(which also interrupts the electrolysis of fused salts),
and it can be considered as almost proved that in this
case too there is a colloidal solution of the metal in the
fused salt.

From the above it seems probable that with a number
of substances which have been described as chemical
individuals, the chemical compounds in question do not
exist. In part, the proof of this has already been fur-
nished by means of individual substances, by various
investigators each independent- of the other.

As examples may be mentioned : l

(1) All substances in which one constituent is a metal,
and the other a colloidal oxid, a colloidal salt or a pro-
tective colloid:

Purple of Cassius as a colloidal mixture of colloidal
gold and colloidal stannic acid.

The purplish gold oxid and related compounds (Berze-
lius, Buchner), as colloidal gold with impurities.

The solution of aurous sulphite (observed by Krtiss and
Clemens Winkler), as colloidal gold reduced by SO2
(Zsigmondy).

Silver-purple, as a mixture of colloidal silveftand col-
loidal stannic acid, produced synthetically by Lotter-
moser.

Argentous oxid and its salts, described by Wohler,
v. d. Pfordten and others; explained by Muthmann as
mixtures which owe their color to colloidal silver.

To this group of colloid compounds belong the many
analogues of the purple of Cassius, which on account of

upon metal fogs, is found in his monograph, " Electrolysis of Molten
Salts," Part II, published by W. Knapp, Halle a. S. (See p. 57 of his
book.)

1 The references to the literature are given partly in my article
above cited, partly in Lottermoser "On Inorganic Colloids," Stuttgart.

COLLOIDAL SULPHIDS AND METALS 71

their high metal content may be called colloidal metals.
For example:

The colloidal silver of Carey Lea, the colloidal gold,
silver, platinum, palladium, etc., of Paal; the colloidal
mercury of Lotterrnoser and others.

(2) Many substances formed by the union of two col-
loidal oxids, and sometimes regarded as sesquioxids or
as chemical compounds of an acid oxid with a basic one.

Thus Berzelius describes tin sesquioxid, and considers
its solubility in ajnmonia as evidence of the existence of
a chemical compound. 1 This substance can, however, be
regarded as purple of Cassius in which the gold is replaced
by stannic oxid. 2

There are, besides, a whole series of stannic acids
intermediaries between ortho- and metastannic acids,
each with its peculiar reaction, which may be regarded
as colloidal mixtures or colloid compounds of the two
extreme members of this group. 3 The number of ap-
parent chemical compounds would be indefinitely in-
creased if every such body were described as a special
hydrate or as a special allotropic modification of stannic
acid, and even to-day this is often done with other col-
loidal precipitates.

The characteristic behavior of tin towards nitric acid
containing ferric nitrate, described by Lep6z and Storch,
can be explained on the assumption of the presence of a
colloidal mixture of ferric oxid and stannic acid in solu-
tion.

By mixing solutions of positive and negative hydro-
sols, as Picton and Linder, and Lottermoser, have shown,
precipitates can be obtained which, when one of the

1 Stannous oxid does not dissolve in ammonia.

2 Zsigmondy, Liebig's Ann., 1898, Vol. CCCI, p. 386
8 Zsigmondy, Ibid, p. 372. .

72 HISTORY OF THE IRREVERSIBLE COLLOIDS

constituents contains an acid, and the other a basic oxid,
may easily be mistaken for a salt-like chemical combina-
tion of the two which, indeed, has often been the case.

This point in particular has been elucidated by the
research of W. Biltz, 1 who demonstrated the generality
of these reactions. As Biltz very justly observes, the
assumption of a salt-like compound is excluded if the
negative constituent is metallic gold, which gives pre-
cipitates when mixed with positive colloids just the
same as do other negatively conducted colloids.

The most important results of Biltz are the following:
oppositely conducted hydrosols mutually precipitate each
other without the addition of electrolytes; similarly
conducted ones do not (with some exceptions in the
case of negatively conducted colloidal solutions, of selen-
ium and gold, for example, in which case Biltz assumes
the existence of chemical reactions).

Biltz further found that in the mutual precipitation
of oppositely conducted colloids, an optimum of the
precipitative action is to be observed. If the favorable
conditions of precipitation are exceeded on either side,
no precipitation at all takes place. (Protective action.)

(3) Here belong the organic compounds of*oxids with
albuminoid substances and the like. Thus Paal 2 has
shown that the alkaline solutions of the salts of heavy
metals in protein substances contain the heavy metal,
not, as has been assumed, in so-called organic combina-
tion, but as colloidal oxids, which are held in solution by
the protective action of the albuminoid or its decom-
position products.

All these determinations and facts serve to round out
our knowledge of the absorption compounds, which is

1 W. Biltz, Berichte, 1904, Vol. XXXVII, p. 1111.

2 Paal, Berichte, 1902, Vol. XXXV, p. 2205.

COLLOIDAL SULPHIDS AND METALS 73

based upon the extended and basic research of van
Bemmelen, 1 the pertinent results of which we will try
to express in two sentences:

Most of the hydrogels contain their water, not chemi-
cally combined, but absorbed (or " adsorbed"); hydro-
gels of ferric oxid. alumina, silicic acid, stannic acid, etc.,
are not hydroxids of these elements, but "absorption
compounds " of oxid and water in varying proportions.

By absorption, hydrogels may take up and hold acids,
alkalis, salts, etc v dissolved in water, in such quantity
sometimes that the resulting products may be mistaken
for chemical compounds.

Following the investigations of van Bemrtielen, Biltz
and Behre have recently shown 2 that the compound of
arsenic acid with ferric oxid, which Bunsen considered
basic ferric arsenite, 4Fe 2 3 , As 2 O 3 , 5H 2 0, is as a matter
of fact an absorption 3 compound in van Bemmelen's
sense.

W. Biltz has added to this result some interesting
observations on the action of toxins and antitoxins, and
has shown that these reactions may be considered as
absorption phenomena.

Further W. Pauli, developing the basic work of Ehr-
lich, makes evident the fact 4 that reaction between

1 van Bemmelen, Landw. Vers. Stat., 1888, Vol. XXXV, pp. 69-136;
Rec. Trav. Chim. Pays-Bas, 1888, Vol. VII, pp. 37-118; Zeit. f.
anorgan. Chemie, 1894, Vol. V, p. 466; Ibid, 1896, Vol. XIII, p. 283;
1898, Vol. XVIII, pp. 14 and 98; 1899, Vol. XX, p. 185; 1900, Vol.
XXIII, pp. Ill and 321; 1903, Vol. XXXVI, p. 380; 1904, Vol.
XLII, p. 265.

3 Berichte, 1904, Vol. XXXVII, p. 3138, communication from W.
Biltz.

3 Biltz and other investigators use for the most part the word
" adsorption "; in order to honor van Bemmelen who has attained
special prominence in this field, I use the customary term "absorp-
tion" chosen by him.

4 Wolfgang Pauli, Wandlungen in der Pathologic durch die Fort-

74 HISTORY OF THE IRREVERSIBLE COLLOIDS

toxin and antitoxin may be traced to the mutual influence
of different kinds of colloids, and that this opens up a
wide field for colloid chemistry.

According to the above research the number of apparent
chemical compounds, in which colloids play a role, is
very large. We may here distinguish between:

(1) Colloidal solutions of metals in crystalloids (for
example, "sodium subchlorid," salts of "silver suboxid,"
"aurous sulphite/' etc.);

(2) Absorption compounds, like the hydrogcls them-
selves, or those in which a crystalloid is taken up by a
colloid, which in particular have been carefully studied
by van Bemmelen;

(3) Colloid compounds, in which two colloids combine
to form a new mixture which cannot be separated by
solvents without destroying the colloidal character.

Incidentally we might here remark that many, though
by no means all of the phenomena of dyeing may be
attributed to just such reactions of colloids with each
other, or between colloids and crystalloids. 1 Color lakes
are to be considered, some as colloid compounds, some
as absorption compounds; only rarely is it the case
that real chemical compounds or even soliS solutions
are formed.

Colloidal Gold. In the year 1857 Michael Faraday 2
described various colored liquids which he had pro-
'duced by reducing gold chlorid solutions with phos-
phorus, liquids which to-day would be called colloidal so-
lutions or hydrosols of gold, terms that Faraday could

schritte der allgemeinen Chemie. Jubilee Address. Vienna, 1905, M
Perles.

." Compare for example Krafft, Berichte, 1899, Vol. XXXII, p. 1008;
Zacharias, Zeitsch. f. phys. Chem., 1902, Vol. XXXIX, p. 468;
Biltz, G6tt. Nachr., 1904, No. 1.

* Faraday, Phil. Trans., 1857, p. 154.

COLLOIDAL sULPfiibs AND METALS 75

not use since the idea "colloidal solution" did not yet
exist.

His method was very crude, and yielded liquids appearing
sometimes red, sometimes blue or violet, liquids which
mostly deposited their gold content; but sometimes very
stable hydrosols. Upon boiling, all these liquids under-
went a marked change; they became darker, the red
gave place to violet, the violet to blue ; their cloudiness
increased, and their permanence was correspondingly
lessened; and a Jittle while after boiling they all gave
a deposit. To this deposit Faraday directed his par-
ticular attention; he proved in many cases that whether
it was obtained from a boiled liquid or not, whether it
showed a blue, violet, or red color, it was metallic gold,
notwithstanding its lack of resemblance to the noble
metal. These investigations led Faraday to the con-
clusion that the liquids owed their color to the extremely
finely subdivided gold contained in them. He further
showed by direct experiment (concentrating the sun's
rays with a burning glass) the existence of a diffuse
dispersion, which as often as not showed a gold
color.

Faraday found excellent confirmation for the correct-
ness of his conclusion in the fact, brought out indepen-
dently of the above research, that pure metallic gold
which he produced as a thin coating on quartz and
glass by the "electrical evaporation" of a gold wire
in hydrogen, etc. showed to transmitted light the
same colors as the above described gold-containing
liquids.

To that group of chemists for whom it would have
had particular interest, Faraday's pregnant work has
apparently remained entirely unknown until 1898. I
have read no paper on gold ruby glass or purple of Gas-

76 HISTORY OF THE IRREVERSIBLE COLLOIDS

sius, in which Faraday was quoted. 1 For instance, a
quotation may be given from the paper of E. A. Schnei-
der 2 on the purple of Cassius:

"Of itself the hydrosol of gold is probably not perma-
nent for more than a few minutes. Up to the present
time all attempts to produce it direct, or from the purple
solution, have been fruitless. Thus it was thought that
by treating the purple solution with concentrated hydro-
chloric acid or concentrated sulphuric acid, and then
dialyzing, that the gtannic acid would be separated from
the gold. As a matter of fact, this did take place, but
the gold in the dialyzer was found to be in a coagulated
condition, after the acid and resulting tin compound had
diffused out. The presence of the tin hydrosol is there-
fore absolutely necessary for the existence of the gold
hydrosol."

This was written by E. A. Schneider, who not only
showed how to prepare colloidal silver in a particularly
pure form, but also was very ingenious in producing
other colloidal solutions. He discovered the hydrosols of
several sulphids of the noble metals, organosols of silver,
of gold sulphid, colloidal iron phosphate, etc.

From this it will be seen that it was by no^neans easy,
as long as the essential conditions were unknown, to
prepare the hydrosol of gold free from stannic acid, and
as I did not know of Faraday's work, and was working
with less energetic reducing agents than the phosphorus
used by him, I was obliged to surmount all the difficul-
ties which stood in the way of the production of a stable,
sufficiently pure and homogeneous red colloidal gold
solution.

1 Just as little was he quoted by those investigators who investi-
gated the nature of "silver suboxid" and similar substances,

2 Zeitschr. f. anorg. Chem.. 1894, Vol. V, p. 82.

COLLOIDAL SULPHIDS AND METALS 77

A gold solution of such properties was absolutely
necessary in order to finally settle the question of the
chemical nature of the purple of Cassius.

It was only by a thorough search of the literature,
which I undertook after the conclusion of my experi-
mental work on this subject, but before its publication, 1
that I found Faraday's work, which I rescued from the
oblivion into which it had fallen.

My colloidal gold solutions exhibited the necessary
properties in the- way of homogeneity and permanence,
which chemists are wont to attribute to a solution in
the usual acceptation of the term (see Introduction).
The experiments which led to their production also
taught me to know the influence exerted by certain
impurities in the water on the properties of the gold
hydrosol.

While certain colloids favor the formation of the red
gold hydrosol remarkably, others, even if present only
in traces, make it almost impossible to obtain a usable
colloidal gold solution. To the former belong colloidal
stannic acid, for example; because of its presence, the
gold formed by reducing gold chlorid solution with stan-
nous chlorid, yields, not a brown or black precipitate,
but a red precipitate, the purple of Cassius. Another
colloid which protects the nascent colloidal gold was dis-
covered by Faraday, and called by him "jelly." Lobry
de Bruyn 2 (1898), characterized gelatin jelly as a pro-
tective colloid (Schutzkolloid). Paal (1900) showed that
lysalbinic acid and protalbinic acid possess the same
protective properties.

For my purpose the absence of foreign colloids was
necessary; it was only when I had secured a gold hydro-

1 Liebig's Annalen, 1898, Vol. CCCI, p. 29.

2 Rec, des Trav. Chim. d. Pays-Bas, 1900, Vol. XIX, p. 236,

78 HISTORY OF THE IRREVERSIBLE COLLOIDS

sol free from detectable quantities of foreign colloids,
that I could synthesize the purple of Cassius, and show
how the reactions of colloidal gold solutions could be
influenced by the presence of other colloids.

By the exclusion of protective colloids, however, the
sensitiveness of the solution was increased to such an
extent that I was obliged carefully to keep out the col-
loids present in most kinds of distilled water, which
would destroy the hydrosol as soon as it was formed.
(See also Chapter VIII.) This I did by repeated redis-
tillation, using a silver condenser, or by freezing the
distilled water several times.

The colloidal gold solution obtained by carefully fol-
lowing my directions, is bright red and very stable; in
contradistinction to Faraday's fluids, it stands heating
to the boiling-point without being thereby altered, and
it remains practically unchanged for months, even years.
Upon the addition of most electrolytes l it quickly
changes to a blue color, and then deposits gold as an
extremely fine powder. The presence of foreign colloids
of certain kinds prevents the color change as well as the
precipitation of gold. Very minute quantities some-
times answer, 0.0001 per cent of gelatin? for instance,
which can by no means alter the viscosity of the fluid
in any appreciable manner. Thus was disproved a
correlated assumption which was thought to follow from
certain observations of Faraday 2 or Lobry de Bruyn,

1 Exceptions to this rule are ammonia, and potassium cyanid and.
ferrocyanid.

2 Faraday had allowed gold chlorid to dry out with " jelly," and
found that the jelly colored red by metallic gold showed no color
change with salt in contradistincton to the "ruby fluids" made without
jelly. Faraday, however, has not stated from what colloid the jelly
was made. The fact also escaped him that the same action exerted
by the stiff jelly can also be exerted by minute traces of a glue-like
substance.

COLLOIDAL SULPHIDS AND METALS 79

that the added colloid works by increasing the viscosity
of the medium, and preventing the sedimentation of the
gold; for it is not the sedimentation but the coagula-
tion of the metallic gold that is prevented. In this case
the gold and the protective colloid exercise on each other
a reciprocal influence, to which I shall later recur. 1

Protective Action of Many Colloids and the Gold Figure.
By looking further into these reactions I was able to
bring to light an almost unknown general property of
many colloids, of preventing the coagulation of typical
irreversible hydrosols; and further to establish a rela-
tive measure of this protective action on bright red col-
loidal gold solutions, by fixing the gold figure. 2

It must here be mentioned, however, that A. Lotter-
moser and E. von Meyer had previously found that egg
albumen and blood serum could completely or partially pre-
vent the precipitation of colloidal silver by sodium chlorid; 3

1 Facts in this field of work have long been known (for example, the
solubility in alkali of albuminates of the heavy metals; the property
possessed by gelatin, of preventing the precipitation by hydrogen
eulphid of sulphids of the heavy metals), but such protective action
had for the most part either been designated as chemical reaction,
or else it had been assumed that the viscosity of the colloid in ques-
tion held in suspension the precipitate produced. That we are here
dealing with a general property of colloids, for which a quantitative
expression can be found, was, as far as my knowledge goes, first brought
to light in the work referred to.

2 Zeitsch. f. analyt. Chem., 1901, Vol. XL, p. 697. Not all col-
loidal gold solutions are suitable for this purpose. That obtained by
reducing gold chlorid with phosphorus is not, for reasons to which I
will later refer. It gives very different gold figures.

1 Journal f. prak. Chem, 1897, N. F., Vol. LVI, p. 242. On this
subject the authors say: "The cause of this fact we cannot yet state;
the formation of silver albuminate seems to be excluded." In a
reference to this paper in the Zeitsch. f. physik. Chem., 1898, Vol.
XXVI, p. 368, signed with the initials W. O., it is stated as follows:
"It is a medical fact that colloidal silver in salve form, etc., is taken
up by the organism, although the salts present in the body-fluids

80 HISTORY OF THE IRREVERSIBLE COLLOIDS

and that Lottermoser has followed up this reaction
further. 1

By the "gold figure" is meant the number of milli-
grams of colloid which are just insufficient to prevent
the change to violet of 10 ccm. of bright red colloidal
gold solution, by the addition of 1 cm. of a ten per cent
solution of sodium chlorid. 2

There is thus made evident an enormous difference
between the individual colloids, as may be seen from the
table on the next page.

In the paper referred to are given the conditions which
must be followed in order to secure for any particular
protective colloid, by the method mentioned, concordant
gold figures. The influence exercised upon the gold
figure by dilution, temperature, and length of the experi-
ment, is stated. It is also shown that all solutions of
protective colloids undergo change with the lapse of
time, so that their effectiveness diminishes, in some cases
so much that the protective effect vanishes.

Fr. N. Schulz and myself established the gold figure*
for several constituents of egg albumen obtained bjf
fractional precipitation according to the practice ^f
physiological chemists. 3 It appeared thai; the gold figure
well serves the purpose of characterizing more exactly

precipitate the solutions of colloidal silver. The authors first state,"
etc., and W. O. in his remarks on this point says: "In this case there
is probably a formation of complex silver compounds similar to the
silver compounds with the imids; to such substances silver does not
act like a noble metal, finally dissolving with ease under the influence
of free oxygen."

1 Journal f. prak. Chem., 1905, N. F., Vol. LXXI, p. 296.

8 The protective action may be better expressed by the reciprocal
of the gold figures. Thus carrageen would have a protective action
of 1-2, gelatin 100-200, raw sugar 0.

3 Hofmeister's Beitrage zur chem. Physiol. u. Pathol., Vol. Ill, pp
138-160 (1902).

COLLOIDAL SULPHIDS AND METALS 81

TABLE I. GOLD FIGURES OF SOME COMMERCIAL
COLLOIDS

(According to ZSIGMONDY)

Colloid.

Gold Figures.

Remarks.

Gelatin

0.005-0.01

Russian glue

0.005-0.01

Cologne glue

0.005-0.01

Bone glue

0.005-0.01

Isinglass

01 -0 02

Casein

Ol 1

Aqueous solution made

Egg albumen 1 . . .

JO. 15-0. 25)

with a few drops of NH S .
Two different commercial

Gum arabic, la

\0.1 -0.2 J
0.15-0.25

varieties.

" " f Ha

0.1

" " , Ilia ....

5-4

Irish mo KS

5-1

Tragacaiith

about 2

Dextrin

1 0-121

Two commercial varieties.

Wheat starch

I 10-20 J
about 4-G 2

Potato starch

about 25 2

Sodium stearate

10

At about 60 C.

it (i

01

At boiling point

Sodium oleate .

4-1

Old stannic acid solution . .
Urea . {

00
00

At ordinary temperature.

Raw susar

less than 500

on

At boiling point.

Gum arabic, selected in
pieces

0.4-0.6

the globulin, albumen, etc., and of testing for the pres-
ence of certain impurities. The great difference in the
value of the gold figure of crystallized and amorphous
albumen, will be seen from the table on the following page. 2
Very marked was the action of a substance usually
obtained in large quantity in the second albumin frac-

1 Only once determined, without control.

2 The addition of NaOH to albumins having low protective action
increases it very considerably.

82

HISTORY OF THE IRREVKRSIBLE COLLOIDS

tion and called by us " impurity 7 ' This substance alone
(without the presence of salts) colored the colloidal gold
solution blue, and made it impossible to determine the
gold figure of crystallized albumen, so long as the impurity
was contained in it.

TABLE IL GOLD FIGURES OF SEVERAL ALBUMIN
FRACTIONS AND ALBUMOSES

Colloid.

Gold Figure.

Author.

Remarks.

Globulin

0.02-0.05

Schulz & Zsigrnondy

Obtained from

Ovomucoid

0.04-0.08

1 1

white of egg

Crystallized albumen

2-8

tt

by fractional

Amorphous albumen

0.03-0.06

tt

precipitation

Fresh white of egg .

0.08-0.15

Schulz & Zsigmondy

Albumin from Merck

0.1-0.3

1 1

Heteralbumose ....

0.01-0.075

E. Zunz

Primary albu-

!Protalbumose

1 .60-3.36

moses from

Synalbumose *

u

Witters pep-

tone, isolated

by P i c k ' s

method.

Witte's peptone 2 . . .

By the excellent work of Zunz 3 have been determined
the gold figures of the primary albumoses (product of
the peptic digestion of albumen, which, in addition to
secondary albumoses and peptones, occur in Witte's
peptone for example). Zunz determined with great

1 Synalbumose does not protect colloidal gold but colors it blue.
According to Zunz, 0.64-2.24 mg. of synalbumose is enough to turn
10 ccm. of gold solution violet in the absence of salts. This reaction
of synalbumose is prevented by the presence of protalbumose, casein,
albumen, but not by heteralbumose despite its high protective action.

2 Acts similarly to synalbumose, only it coagulates colloidal gold
still more powerfully. 0.24-0.64 mg. of Witte's peptone have the
same effect as 0.64-2.24 mg. of synalbumose. (Zunz.)

8 Archives internat. de Physiologic, 1904, Vol. 1, p. 427.

COLLOIDAL SULPHIDS AND METALS

83

accuracy the gold figures of protalbumose and heter-
albumose, and made the very remarkable discovery that
synalbumose alone changed colloidal gold solutions blue,
just like the "impurity" before described.

Zunz has recently l tested a series of pure albumoses and pep-
tones as to their behavior with colloidal gold solutions. He shows
that, just like synalbumose, they almost all possess the property of
turning the colloidal gold violet without the addition of electrolytes,
but that the quantity necessary to do this varies from substance to
substance.

The chief results are given in the following table:

Substance.

Quantity of the Substance
which suffices to Change
let 10 cc. of the Colloidal

in mg
to Vio-
Gold.

Thioalbumose

2.60-4 00

Albumose, A II

2 24-3.20

" B I

08-0 32

" B III. a

0.20-0.80

" B III. 6

0.50-1.40

" B III. c

40-1.20

" B IV. b

0.80-1 60

" B IV. c

70-1 80

" B/C

80-2.80

" C I

1 60-3 20

Peptone, a

24-0 52

" pb

3 60-7 40

" ffc

4 4 -8 20

Zunz has further found 2 that no relation exists between the action
of albumoses and peptones upon colloidal gold, and the alteration by
these substances of the surface tension of water.

According to Traube and Bodlander, 3 a 0.1 per cent solution of
albumoses give 125.3 drops in the stalagmometer, while pure water
gives 100, and a 2 per cent solution of egg albumen only 106.3 drops.

1 E. Zunz, Bull. Soc. Roy. des sciences med. et nat., June 11, 1906.

2 E. Zunz, ibid.

* Ueber die Unterscheidung von Eiweiss, Berichte d. D. Chem.
Ges., 1886, Vol. XIX, p. 1871-6.

84

HISTORY OF THE IRREVERSIBLE COLLOIDS

Whereas, therefore, albumen but slightly influences the surface tension,
albumoses influence it very strongly, but to about the same extent
among themselves; on the other hand their behavior with gold solu-
tion varies greatly, as can be seen from the table.

TABLE III

Substance.

Gold Figure.

Number of Drops in
the Stalagmometer as
Compared with 100
Drops of Distilled
Water.

Heteroalbumose. . .
Protalbumose

0.01 to 1.075
0.6 to 3 36

114.4
113 6

Synalbumose. . .

Produces a chancre in color

113 7

(0 G4-2.24mg.)

In the additional paper 1 Z\ITM shows that there is no relation what-
ever between the behavior of turbid mastic solution with albumoses,
and that of colloidal gold with the latter. Thus hcteroalbumose and
synalbumose precipitate turbid mastic solution, whereas the other
albumoses do not possess this property. As may be seen from what
precedes, heteroalbumose does not exercise a precipitative action on
colloidal gold, but rather a protective action.

From these facts it is evident that the electric charges alone do
not determine the reactions in question, for mastic and gold particles
are both negatively charged. If the electric charges alone exercised
the main influence, gold and mastic should react the same with the
albumoses. But this is not the case, and we are therefore confronted
with specific actions, whose nature is not yet completely understood. 2

W. Biltz 3 determined the gold figures of several inor-
ganic colloids, and Biltz, Much, and Siebert, those of
several sera and antitoxins. 4 The authors use the gold

E. Zunz, Arch..internat. de Physiol., 1907, Vol. V., p. 245.

2 Neither is there any connection with the diffuse dispersion of light.
According to Zunz all proteoses (albumoses) exhibit an intense light-
cone, whereas the peptones show only a faint bluish cone, which
vanishes upon the insertion of a yellow disk.

8 Berichte, 1902, Vol. XXXV, p. 4437.

4 Experimentelle Beitrage zu einer Absorptions-theorie der Toxin-

COLLOIDAL SULPHIDS AND METALS 85

figure in order to determine approximately the loss of
albumen content upon shaking with iron oxid. It appeared
that albumen solutions containing salt gave up less albu-
men to the iron oxid gel than did dialyzed ones. The
physiological salt content, therefore, protects albumen to
a certain extent against absorption by hydrogels.

The authors further found that the gold figures of
nutritive bouillon and toxin solutions could not be
closely determined, because even without the addition
of electrolytes, they immediately produce a blue color
in colloidal gold solutions, a reaction which corresponds
with that of synalbumose and that of Witte's peptone
(see above). It was. pointed out by Biltz, Much, and
Siebert, that this reaction appears to be characteristic
for peptones. 1 The growth of bacteria changes the
nutritive bouillon, so that it regains the property of a
protective colloid.

As previously mentioned above, Wf Biltz observed
that oppositely charged colloids mutually precipitate
each other when they are mixed in certain proportions;
if an excess of one or the other colloid is used, then this
excess acts as a protective colloid. Similar facts have
been determined by Bechhold, 2 Neisser, and Friedmann, 3
in connection with the precipitation of mastic by posi-
tive hydrosols, dye solutions, and the like. Protective
action also appears in the mutual precipitation of acid
and basic dye-stuffs, if there is used an excess of either
dye-stuff over and above the quantity necessary for
precipitation.

neutralisierung. Behring's Beitrage zur experimentellen Therapie,
1904, No. 10.

l " Peptone" here refers to the commercial mixtures, not the pep-
tones of the physiological chemist.

2 Bechhold, Zeitschr. f . physik. Chem., 1904, Vol. XL VIII, p. 385.

s Neisser and Friedemann, Muchener med. Wochenschr., 1903, No. 11,

2

86 HISTORY OF THE IRREVERSIBLE COLLOIDS

These facts lend interest to some observations I made
several years ago, but which I have not been able as yet
to study more closely. I found that colloidal gold and
basic dyes like fuchsin (or methyl violet, methylene blue,
etc.), sometimes precipitate each other quantitatively,
but that the reaction between the two substances often
failed to occur or else took place very incompletely.
I then attributed this to the protective action of impuri-
ties, but it is very possible that in the unsuccessful
experiments, I exceeded the optimum of precipitative
action more accurately described by Biltz and other
investigators. From the precipitate the dye can be
extracted by alcohol (not, however, by water). There
remained on the filter a gold colored powder which was
not further investigated.

Other investigators have shown that the protective
action of some few colloids extends to a certain degree
even to coarser subdivisions. 1 Arthur Miiller showed
this in the case of a suspension of red phosphorus. 2 That
this is only superficially analogous to the protection in
the case of gold, is at once evident when the two kinds of
protective action are compared. While 0.025-0.05 mg.
of gelatin completely protect 50 ccm.^of colloidal gold,
on the other hand, 25 mg. of the same colloid (that is
500 to 1000 times as much) are insufficient to completely
prevent the flocculation of 50 ccm. of phosphorus sus-
pension by sodium chlorid. Furthermore, raw sugar
prevents the precipitation of phosphorus, but not the
color change of colloidal gold.

Recently Arthur Miiller and Paul Artmann 3 deter-

1 For instance Neisser, Friedemann, and Bechhold have determined
protective actions in the case of suspensions of bacteria.
2 Berichte, 1904, Vol. XXXVII, p. 11.
8 Oestrr. Chemiker-Ztg., 1904, Vol. VII, pp. 149-151.

COLLOIDAL SULPHIDS AND METALS

87

mined the smallest quantity of protective colloid neces-
sary to prevent the precipitati6n of several irreversible
hydrosols, As 2 S 3 , CdS, etc. I reproduce their table.

TABLE IV

Metal Sulphid Solution.

Smallest Quantity of Colloid in mg.

Substance.

Strength
in
Per Ct.

Quan-
tity
used cc.

Casein

Gum,
Arabic

Glue.

Isin-
glass.

Albu-
men.

Dex-
trin.

A 82 S, . . .

0.05

"5

0.16

0.32

0.40

55

2.50

50

CdS . . .

0.10

3

0.65

1.15

1.80

1.50

8.00

00

Ag,S . . .

0.05

5

0.06

13

0.18

0.18

0.60

00

From the table it will be seen that the protective col-
loids, gum, glue, dextrin, etc., exercise upon the colloidal
sulphids a protective action similar *to that exercised
upon colloidal gold, but that the quantitative expression
of the protective action varies with the colloid. As with
gold, raw sugar exercises no protective action on sul-
phids. Miiller and Artmann further observed that under
certain conditions glue solutions yield precipitates with
colloidal solutions of cadmium and arsenic sulphids.

Just as did Ebell in the case of the colloidal sulphids,
and Barus and Schneider with colloidal silver, so Stockl
and Vanino l have tried to prove that colloidal gold is
an instance of very fine suspension. As Bredig and
Coehn 2 have already pointed out, and as may be seen
from the above remarks, they have advanced no dis-
tinctly new idea. I myself had considered my fluids as
profound subdivisions of the metal, as may be seen from

1 Zeitschr. f. physik. Cliem , 1899, Vol. XXX, p. 98.

1 Zeitschr. f. physik. Chem., 1900, Vol. XXXII, pp. 129-132.

88 HISTORY OF THE IRREVERSIBLE COLLOIDS

my first paper on this subject. 1 I must, however, object
to the term suspension /for my fluids, because of their
permanence and homogeneity, should be classed as col-
loidal solutions, and not as " suspensions/' as at present
understood by chemists.

The discussion between Stockl and Vanino and myself
led Lobry de Bruyn to make some very noteworthy
remarks 2 regarding colloidal solutions. From the molec-
ular weight of soluble starch, Lobry de Bruyn calcu-
lated the molecular diameter (approximately 5 /*/*) and
called attention to the fact that, according to Lord Ray-
leigh, particles of this size are large enough to polarize
incident light.

In concluding this section reference must be made to
the valuable work of Schulze, Lottermoser and von
Meyer, Picton a^nd Linder, Coehn, Hardy, Spring, Bil-
litzer, Quincke, Freundlich, and others, whose object
was to determine the laws governing coagulation by
electrolytes, and who by preference chose colloidal sul-
phids and metals as subjects of investigation. Taken in
connection with the earlier work of Scheerer, F. Schulze
Schloesing, Barus, and Bodlander, oa the clearing of
clay turbidity, these researches lead to a sharp distinction
between flocculation and sedimentation, 3 and estab-

1 Zeitschr. f. Elektrochem.,* 1898, .Vol. IV, p. 546: "Dr. Bredig
exhibited to us yesterday a series of interesting properties of the
electric arc. For example, by atomizing metals under water he
obtained dark-colored fluids in which the metal was so finely sub-
divided that it could be regarded as dissolved; but they are not
solutions, because on long standing they lose part of their metal
content. // the metal is still further subdivided fluids are obtained
which no longer settle, that is, collodial metallic solutions. Up to the
present Carey Lea's silver solution was the only one of this class
known."

2 Rec. des trav. chim. Pays-Bas., 1900, Vol. XIX, pp. 251-258.

8 See for example, Spring, Bull. d. 1'Acad. roy. de Belgique, 1900,
No. 7, p. 468; see also note on p. 31.

CONCERNING THE NATURE OF HYDROSOLS 89

lished the dividing line between the flocculation of clay
turbidity and the coagulation of irreversible hydrosols
(see Chapter I). That the two processes last referred
to are not identical, has been shown in Chapter L There
is at present no unanimity of opinion as to the funda-
mental causes of coagulation, other than what follows
from comparing the views of Billitzer, Quincke, Freund-
lich, and Jordis.

In passing, it may be remarked that electrolytes not
only cause pectization or coagulation, but also pepti-
zation of colloids, and that a complete theory of col-
loids cannot consider only one of these processes. In
these very processes, moreover, the nature of the sub-
divided substance and the presence of traces of foreign
constituents (which may even arise from chemical
action) influence to the greatest degree the behavior of
the hydrosols, and as long as these conditions remain
unexplained, generalizations can easily lead to error.

3. Some Questions Concerning the Nature of Hydrosols

The conviction has gradually gained more" and more
ground that colloidal solutions are in reality fine sub-
divisions of an originally solid substance, and there is a
diversity of opinion as to whether the individual parti-
cles which polarize light are to be regarded as very large
molecules in the sense of Picton and Linder and Lobry de
Bruyn, 1 or as minute particles of an originally solid sub-
stance. 2

1 Compare also Nernst, Theoretische Chem., 3d. ed., p. 383.

3 Another question as to whether the particles in hydrosols are
solid or fluid, cannot at present be satisfactorily settled. Quincke
assumes the latter condition. Compare Chapter XXI, wherein is
briefly discussed the question propounded by Jordis, whether the
ame substance is present in hydrosols as in hydrogels.

90 HISTORY OF THE IRREVERSIBLE COLLOIDS

In the first instance, the hydrosols in question would
be considered as crystalloid solutions with very great
molecular weight. In the latter classification, which is
adopted by Bredig and others, they rank as heterogene-
ous according to the phase rule, but then it must be
borne in mind that even the homogeneous solutions of
the crystalloids are, as a matter of fact, only apparently
homogeneous (see Introduction, p. 7).

Without doubt the question above referred to is theo-
retically of high interest; but at present great difficulties
stand in the way of its satisfactory solution. As Bredig
has brought out, 1 in the case of colloidal solutions the
osmotic pressure (trennungsarbeit) can be referred to a
partial crystalloid solubility of the colloid, to crystalloid
impurities, to electric charge of the individual particles.
So long as these influences are not individually deter-
mined, the molecular weight of the dissolved colloid can-
not be safely deduced from the results of the osmotic
method.

To decide -the question whether some or all colloidal
solutions possess large molecules, would be of particular
importance for the kinetic theory; ^for it is just the
very large molecules, the presence of which some inves-
tigators assume in hydrosols, which would most facili-
tate a purely experimental confirmation of important
corallaries of that excellent theory.

From the results of numerous physio-chemical investi-
gations, and quite independent of any theory, it is evi-
dent that the osmotic volume-energy 2 which, with crys-
talloids, dominates part of the phenomena, becomes of

1 Anorganische Fermente, p. 11.

2 Regarding the expressons volume-energy, surface-energy, distance-
energy, see Ostwald, Grundriss der allgemeinen Chem., 3d ed., pp. 53,
247, 251.

CONCERNING THE NATURE OF HYDROSOLS 91

less importance as compared to the influence of the elec-
trical and chemical energy, or of "surface energy"
("Oberflachenenergie ") * and perhaps of still other kinds
of energy, as yet unknown. As the volume-energy 2 di-
minishes or disappears, the hydrosols correspond to real
suspensions, but there is nevertheless an evident differ-
ence between the two types of subdivision last referred
to. While with the former the electrical energy and
the surface energy (Oberflachenenergie) form an important
part of the total energy, and render possible reactions
which are deceptively similar to chemical reactions, with
the latter their influence yields to that of gravity. (See
Chapter I). The action of coarse, suspended powders
is determined rather by purely mechanical influences
and the speed with which their particles sink to the
bottom, can be calculated according to the laws of falling
bodies. 3

Before a comprehensive theory of colloidal solution can
be promulgated, it will be necessary to determine in
each special case by thorough experimental investiga-
tion, the influence of these forms of energy which are
mot important in the case of hydrosols; a good begin-
ning has already been made in this direction. I need
only recall here the work of van Bemmelen, Spring,
Paterno, Barus and Schneider, Picton and Linder,

1 This expression is set in quotation marks simply because it might
easily lead to the assumpton that only the surface of the particles
contained in hydrosols comes into play. In this case the particles
themselves act upon each other and the surrounding medium, and the
existence of this action can be demonstrated by numerous observa-
tions. Macroscopically considered, this same kind of energy comes
into evidence as surface-energy.

a Which may be considered in the sense of Ostwald, Bredig, as well
as in the sense of Lobry de Bruyn.

8 See Barus and Schneider, loc. cit., p. 287.

02 HISTORY 6F f liE tftftEVER&IBLfi COLLOICS

Lobry de Bruyn, Coehn, Bredig, Bruni and Pappad&,
Lottermoser, Biltz, Billitzer, Quincke, Friedemann, and
others, which has much advanced- our knowledge of
colloids and also suggested questions that deserve atten-
tion.

If we regard hydrosols as extremely fine subdivisions
of substances originally solid, certain other questions
immediately arise, some of which can be solved by the
aid of ultramicroscopy.

First. Are the particles contained in colloidal solu-
tions independent of each other, or does the colloidal
solution contain a network or honeycomb of threads or
thin walls connected together? One would be led to the
latter opinion by the assumption that the process of
solution of a colloid is simply a matter of swelling up.
We will see that this assumption is untenable, for as far
as particles in hydrosols can be seen, they are freely
movable.

Second. Do the particles float quietly in the fluid, or
fall to the bottom under the influence of gravity; or
have they 'a more or less rapid motion of their own? In
the latter case, what is the nature of th^ motion, oscilla-
tory or translatory? What are the dimensions of the
paths described relative to the size of the particles? Is
the motion a temporary one brought about in the removal
of differences of concentration, or is it a continuous one?

Third. What color are the individual particles; can
a relation be found between their size and color?

Fourth. What is the size of the individual particles?
Is it possible by careful preparation to produce colloidal
solutions so homogeneous that the bulk of the subdivided
substance cannot be recognized by the best optical
methods, or can their heterogeneity in all cases be made
evident by means of a beam of light?

CONCERNING THE NATURE OF HYDROSOLS 93

Questions of this kind will, in certain definite cases,
be answered in the experimental sections which follow,
in which will also be given the answer to a question
which led to this investigation:

Faraday, as we know, observed in all gold hydrosols
and ruby glasses which he investigated, the appearance
of a diffuse dispersion. To him this was evidence that
the gold in these fluids is not dissolved, but is present
in fine subdivision. Evidence accumulated quite inde-
pendently of Faraday, that the red colored substance
in my fluids is metallic gold, led me to the same opinion
reached by Faraday, namely, that we were dealing here
with finely divided gold; nevertheless, upon the basis
of my experience, I believed optical inhomogeneity to
be an incidental characteristic of hydrosols, brought
about by coarser particles in the otherwise homogene-
ously subdivided (that is, dissolved) 1 substances. Fara-
day's view was afterwards espoused by various investi-
gators, especially by Brcdig, 2 and extended to colloidal
solutions in general. T^is very opposition led me to a
searching investigation of the point in question. By
means of ultramicroscopy I was able to decide the ques-
tion in favor of the view held by Faraday, Bredig, and
others. It was apparent that the gold particles which
occasion the red color of the colloidal solution, are the
same ones that polarize light.

On the other hand, it also became evident that the
particles in the particularly clear colloidal gold solu-

l " Dissolved" in the sense of the definition of the word solution.
Formerly, in conformity with the quite generally accepted view, I
regarded solutions as subdivisions completely homogeneous, optically
speaking. The investigations of Spring and Lobry de Bruyn on this
subject (see Introduction) were then unknown.

9 Bredig, Zeitschr. f. angew. Chem,, 1898, No. 41.

94 HISTORY OF THE IRREVERSIBLE COLLOIDS

tions closely approach molecular dimensions, and the
optical inhomogeneity of these same solutions is hardly
greater than that of many crystalloid solutions, l in both
cases it is hardly observable.

1 See Lobry de Bruyn, loc. cit.

CHAPTER IV

FACTS POINTING TO THE HOMOGENEITY OF GOLD
HYDROSOLS. DEVELOPMENT OF ULTRAMICROSCOPY

As the opinion just stated was based upon a long
series of experiments which I never published in detail,
and do not now" need to publish, since the question has
been settled in another manner, I may here briefly repeat
the grounds upon which this opinion rests. 1

I do this for the particular purpose of showing the
insufficiency of grounds of a similar nature, which may
doubtless be urged in favor of the view that solutions in
general are homogeneous.

"To begin with, my view was supported by the obser-
vation that although the majority of the hydrosols of gold
(colloidal gold solutions)/ produced by me, for the most
part showed a noticeable faint and diffuse scattering
of incident light, they were sometimes very clear 2 and
sometimes very turbid.

"Despite their varying appearance, all these fluids
contained equal quantities of metallic gold, as I con-
vinced myself by repeated analyses; they were red
colored, and also showed approximately the same behav-
ior toward reagents. 3

1 Zsigmondy, Verhandl. d. Deutschen physik. Gesell., 1903, Vol. V,
p. 209.

2 So clear that the light-cone is scarcely more perceptible than in
distilled water.

8 They were precipitated by electrolytes, with the development of
a blue color. Upon quantitative test, however, they developed certain
differences in their behavior with reagents, but not in their gold content.

95

06 DEVELOPMENT OF ULTRAMICROSCOPY

"Since, then, these fluids showed in general the same
mutual properties, and could in practice be differen-
tiated from each other only by their greater or lesser
turbidity, I assumed that the turbidity is not an essen-
tial, but an incidental characteristic of colloidal gold
solutions, occasioned by the presence of larger particles.

"The turbidity always showed a more or less intense
polarization, an evidence of the minuteness of the haze-
producing particles even in very turbid fluids.

"The above view received further support from a
quantitative test of the sensitiveness of the recognition
of subdivided gold by means of the convergent beam of
light, which showed that in very cloudy fluids (upon
considerable dilution) even less than 10~~ 8 mg. of gold
could in this manner be recognized with the naked eye
by the visible path of the convergent beam, 1 that is, a
less quantity of substance than Kirchhoff and Bunsen
could detect by means of spectrum analysis. My cloudy
fluids containing 0.0005 per cent of gold were diluted
100 to 1000 fold; the colorless dilute fluids still con-
tinued to show a more intense light-cone than the
undiluted, unclouded solutions containing 0.005 per
cent. v

"The addition of a minute quantity of the very turbid
fluid to the perfectly clear one sufficed to endow the
latter with the polarized dispersion of the former, as
evidenced by means of the light-cone; and this made
evident of how small a quantity of coarse gold particles
is necessary to produce, in a gold solution assumed to
be homogeneous, the diffuse dispersion referred to.

1 Since in using sun or arc light the diffuse light-cone shown by
distilled water exercises a detrimental effect, these tests were made
with Weisbach light which produced no light-cone in the water I
used for dilution.

DEVELOPMENT OF ULTRAMICROSCOPY 97

"It must be acknowledged, however, that although
the facts above referred to were favorable to my view,
they gave no evidence of the existence of an optically
clear gold solution. I hoped to be able to find out the
truth by microscopical examination of the cone of
light."

Before I carry this discussion further it might be here
stated that microscopical investigation led to the solu-
tion of the question. This investigation showed that
all the red-colored gold particles take part in the forma-
tion of the polarized cone of light, 1 and the greater or
lesser cloudiness of the colloidal gold solution is due to
the fact that the gold particles which produce the red
color are larger in some cases and smaller in others.

This result, which contradicts the view earlier advanced
by me, is of great significance in 'judging colloidal solu-
tions in general.

By the observations of Fizeau 2 and of Ambronn 3 it
was shown that very narrow light slits can still be seen
even if their breadth is considerably less than the limit
of microscopic resolvability. The narrower the slit of
light the less the brightness of its microscopic image,
whose breadth, however, from a certain point down-
ward, no longer varies.

It seemed to me probable that the larger particles
assumed to be present in gold hydrosols, whose previ-
ously mentioned property of dispersing transmitted light
was known to me by the above experiments, could be
made individually perceptible by a microscopic examina-

1 But in the case of the almost homogeneous gold solutions, this
conclusion cannot with certainty be reached; however, in this case
too, the faint trace of a light-cone can be attributed to the gold present.

2 Pogg. Ann., 1862, Vol. CXVI, p. 478.
8 Wied. Ann., 1893, Vol. XLVIII, pp. 217-222.

98 DEVELOPMENT OF ULTRAMICROSC'OPY

tion of the light-cone; for if the small particles reflected
enough sunlight, even if their size was below the limit
of microscopic resolvability, they would, just as were
the narrow rays of light, be individually perceptible
under the microscope and act to a certain extent as frag-
ments of such light slits.

The microscopical investigation should also allow me
to determine if the space between the coarser individual
particles was optically clear or was filled with smaller
gold particles.

As a matter of fact, in two very turbid fluids (AuP 8
and AU54) 1 , by the use of sunlight and a magnification
of about one hundred diameters, I could recognize the
presence of thousands of shining gold particles, whose
size, as was shown by a rough calculation based upon
the distance of the particles from each other and the
amount of gold present, must have been less than the
wave-length of light. With ordinary illumination, 2 even
with the best objectives, they were not perceptible.

The arrangement of the apparatus assembled for this
purpose was the following:

The sun beams fell upon a mirror S (Fig. 1), were re-
flected from this into the lens L, which brought them to the
focus at 6, over which was arranged a low power micro-
scope (Objective A, Huygens eyepiece 2 and 4). 3

As early as April, 1900, solutions of glue, gelatin,
tragacanth, stannic acid, and also a silver hydrosol of

1 AuP 8 contained gold-glinting particles which settled in about
eight days. Au B4 was over half a year old and in this time had de-
posited but little gold. The particles were green in color, the fluid
bright purple-red.

2 And even by use of the dark field illumination then in use.

3 Quincke had used side illumination with lamp or sun-light to
observe the electric migration of starch grains, etc., unde^ a magnifi-
cation of 30 diameters. (Pogg. Ann., 1801, Vol. CX111, p. 568.)

DEVELOPMENT OF ULTRAMICROSCOPY

99

coarser subdivision, were examined by this method. In
all these fluids, besides a general illumination of light-
cone, individual particles could also be perceived; x but
with the silver hydrosol there was visible a brilliant light-
cone, which could be resolved into individual particles
only upon extreme dilution. Of colloidal gold solutions,
Au 60 and AuPs were also investigated.

FIG. 1.

The slightly turbid gold solution Au 60 (0.005 per cent
Au) showed a green light-cone which vanished upon
dilution without having revealed the presence of indi-
vidual particles; the perfectly clear solution AuP 3 (0.005
per cent Au) only showed the presence of isolated gilded
dust particles, while the space between them appeared
optically clear. That this space still contained gold in
an invisible form, was made evident by an experiment:
upon addition of ordinary table salt the colloidal gold
was coagulated, and then the fluid was thickly filled with
brightly shining particles.

1 Some of these individual particles polarize light, others on the
contrary do not.

100 DEVELOPMENT OF ULTRAMICROSCOPY

By these experiments were settled several questions of
importance. Thus it was shown

1. That gold of various degrees of subdivision can be
present in one fluid without settling;

2. That gold can be subdivided to the point of optical
homogeneity. 1

It was further shown experimentally that by the
microscopic examination of a solar image formed in a
fluid, particles can be discerned which are beyond the
range of ordinary microscopy. I was convinced that by
the use of better objectives the individual particles could
also be seen in Au 6 o and similar fluids, and I determined
to attempt, with the assistance of a specialist, to render
such particles visible also. An investigation of this sort
seemed all the more promising, as the preliminary experi-
ments had shown that the color, brightness, and polariza-
tion of the individual particles show a great variation,
which might serve more sharply to characterize them;
furthermore, the colloidal gold solutions represented
test objects which made it possible to ascertain approxi-
mately the average mass of the individual particles, from
the mean distance between them, and the known gold
content. ^

The perfected arrangement, bringing into use objec-
tives of wider aperture, was later discovered by H. Sie-
dentopf incidentally to our mutual work, on the basis of
the principles developed by him for making visible ultra-
microscopic particles. But first another problem had to

1 Subsequent ultramicroscopic examination of gold ruby-glasses has
shown, even in such homogeneous appearing subdivisions, the polarized
light-cone can be seen by focusing objectives of higher aperture on
the very outside layer of the preparation, and thus preventing, as
far as possible, the absorption of the transmitted and reflected light.
This precaution must be observed in the examination of apparently
homogeneous colored solutions.

DEVELOPMENT OF ULTRAMICROSCOPY 101

be solved. Because of the motion of their particles,
colloidal gold solutions were not suitable for an extremely
exact determination of the mass of the individual parti-
cles. For this purpose it was necessary to prepare a
graduated series of subdivisions of gold containing fixed, 1
and, so far as possible, changeless particles. Test objects
for this purpose I had made for me in the glass factory
of J. L. Schrieber, at Zombkowice, Russia, incidentally
to my attempt to work out a method for the manufac-
ture of gold ruby glass which can be molded. After
this preparation could be begun in collaboration with
H. Siedentopf, an attempt to render visible finer ultra-
microscopic particles, in which the work was so divided
that H. Siedentopf carried out the construction and
improvement of the apparatus for illumination, while I
took charge of testing the apparatus constructed with
my preparations, and of developing the method for the
determination of size. The introduction of the bilateral
slit, and the application of microscopic objectives for
illumination by H. fiicdentopf, were essential to render-
ing visible the smaller particles. Siedentopf also con-
tributed the theory of making ultramicroscopic particles
visible, and calculated the probable limits of their visi-
bility.

It must be here noted that the limits of microscopic
resolvability (limit of visible "separation ") determined
by Abbe and Helmholtz, is often confounded with the
limit of "visibility." That isolated particles, whose
diameter is a fraction of a wave-length of light, can still
be seen, is expressly stated by Abbe himself. He writes
in this connection: 2 u Such objects can be seen, no mat-

1 For the purpose of making an exact count when determining the
size of the gold particles.

2 E. Abbe, Gesammelte Abhandlungen, Vol. I, p. 362. Jena, 1904,
G. Fischer.

102 DEVELOPMENT OF ULTRAMICROSCOPY

ter how small they may be; it is only a question -of the
contrast of the light effect, good definition of the objec-
tive, and sensitiveness of the retina." As may be seen,
the results of our investigation are not at all, as has been
often thought, in conflict with the statements of Abbe.

We received splendid assistance in our work from the
numerous means placed at our disposal by the firm of
Zeiss. 1 We were thereby enabled to work with the best
objectives, and complete the mechanical development of
the apparatus in a short time. Notwithstanding this, a
year and a half of study was necessary to bring the appa-
ratus to its present state of perfection.

l F. Auerbach states in his monograph: Das Zeisswerk und die Carl
Zeiss-Stiftung in Jena 2d ed., Jena 1904. G. Fischer:

"Recently in the workshop of Carl Zeiss, their scientific collaborator,
Dr. Siedentopf, has at the instance of Zsigmondy worked out inde-
pendently an idea previously conceived by Abbe, into a method
which makes it possible to see minute particles by means of intense
illumination."

As this statement can easily lead to a misunderstanding that the
idea conceived of by Abbe was known to Siedentopf or myself, it
must be expressly stated that on the occasion of the conversation
which preceded the support of our work by the firm of Zeiss, Professor
Abbe made no mention of this idea, and that it is unknown to me when
and where Abbe has expressed such an idea. Some verbal expression
of opinion is probably referred to, which was not available to a wider
circle.

CHAPTER V

DESCRIPTTON OF THE APPARATUS FOR MAKING VISI-
BLE ULTRAMICROSCOPIC PARTICLES

IT is easy to see why, with the earlier microscopic
methods of observation, the individual particles in col-
loidal solutions or in ruby glass could not be seen. Trans-
mitted light l was mostly used for observations, and the
eye dazzled by the profusion of light, could not distin-
guish the slight differences of brilliancy caused by the
diffraction of the light due to very small particles, just
as it is impossible to see the stars by daylight. 2

To render visible very small particles, there are neces-
sary:

1. The most intense possible illumination of the parti-
cles, in such wise that no beam of the illumination enters
direct the eye of the observer.

2. A field of view as dark as possible.

These requirements were met by the construction of
an apparatus for illumination, the description of which
by H. Siedentopf, 3 I here repeat. Fig. 2 shows the appa-
ratus -5*0- natural size.

1 The so-called dark field illumination, then very incompletely
developed, has been relatively little used; I do not therefore need
to take it up further.

2 See the detailed discussion of the principles of making visible
ultramicroscopic particles: Sidentopf and Zsigmondy, loc. cit., pp. 2-5,
and Siedentopf, Berl. klin. Wochenschr., 1904, No. 32.

3 Siedentopf and Zsigmondy, loc. cit., p. 8. See also the detailed
description of the apparatus for making visible ultramicroscopic
particles, by H. Siedentopf, Druckschr.-Verz. der optischen Werkstatte
von C. Zeiss, Sign. M., 164, Jena, 1904.

103

104 DESCRIPTION OF THE APPARATUS, ETC.

"The solar rays reflected from a heliostat, enter the
darkened laboratory through an iris diaphragm. In the
room is an optical bench about 1.50 meters (5 ft.) long,
having a metal flange P, supported on an adjustable stand
G, all made by C. Zeiss, Jena.

"On this, by means of carefully adjusted brackets, are
mounted the individual parts of the apparatus. The

E Fa

i

FIG. 2.

light-rays first enter the telescope objective F, having a
focal length of about 10 mm., which throws an image
of the sun about 1 mm. in diameter on the finely adjust-
able slit head >S, which is modeled after Engelmann's
microspectral-objective. 1 By the horizontal bilateral slit,
this image can be reduced to 5-50 hundredths of a milli-

1 Th. W. Engelmann, Zeitschr. f. wissensch. Mikrosk., 1888, Vol. V,
p. 289. H. Siedentopf, Sitzungber, D. k. Akad. d. Wissensch., zu
Berlin, 1902, Vol. XXXII, p. 717.

DESCRIPTION OF THE APPARATUS, ETC. 105

meter, as desired. The width of the slit may be read off
from an index on the drum connected with the screw.
The edges limiting the height of the slit are movable
horizontally, and may be placed from Vir to 2 mm. apart.
A polarizer N may be placed behind the slit when desired*
The iris diaphragm / shuts off any side-light which may
be reflected from the edges of the slit. By means of the
chisel-shaped diaphragm 5, one-half of the beam of
light may be cut off; this is necessary when immersion
objectives are used, in order to prevent objectionable
reflection from "the mounting of the front lens, due to
the closeness of the objective. A second telescope objec-
tive F2 of 80 mm. focal length forms a quarter-size
image of the slit at the focal plane E of the condenser K.
By means of the microscope objective AA, 1 used as a
condenser, this picture E (reduced to one-ninth its size),
is projected into the preparation. It should be seen
that full use is made of the aperture of the condenser
system K, by controlling the illumination of its rear
focal plane. The Dipper half of the rays emerging from
the objective AA are cut out, when the picture of the
semi-diaphragm J5, thrown by the telescope objective F 2
into the after-focal plane of AA, darkens the upper half
of this plane. By means of two micrometer screws,
working in a horizontal plane and perpendicularly to
each other, the condenser-objective may be readily cen-
tered in the optical axis of the microscope proper."

In order to bring the desired part of a solid preparation
into the axis of the illuminating beam, Siedentopf devised
a metal prism having slides permitting the vertical micro-
metric motion of a little plane table.

For the examination of fluids I had made a small
washable apparatus illustrated in Fig. 3, which makes it
1 AA is the mark on the Zeiss objective referred to.

106

DESCRIPTION OF THE APPARATUS, ETC.

possible to examine numerous fluids, one after the other,
without being obliged to rearrange the microscope for
each case.

A cell with a quartz window C, which serves as obser-
vation chamber, is connected by tubes on the one side
with the thistle-tube T for filling, and on the other side
with the pinch-cock H for drawing off. In a few seconds
one fluid can be run off and replaced by another after
washing the apparatus, which remains ready in place; for
the next observation.

H. Sicdentopf has very cleverly attached this little

o

FIG. 3.

apparatus direct to the objective of the microscope, thus
obviating the necessity of repeated focusing on one object
plane; and the fine adjustment of the microscope tube
may be used to move the cell vertically.

Recently W. Biltz proposed a slight modification of
this apparatus. In order to avoid the rubber tube con-
nections Biltz fused a thistle-tube direct on to the cell.
The danger of increased fragility has been avoided by
H. Siedentopf , who made appropriate changes in the cell
and the method of fastening it to the microscope objec-
tive. The manner of handling the new apparatus is

DESCRIPTION OF THE APPARATUS, ETC.

107

shown in Fig. 4. The cell, as a whole, is placed in a
pocket under the microscope, and held there by the clips
dj h, and s.

FIG. 4,

H. Siedentopf has also constructed a microscope for
the examination of ultramicroscopic bacteria, which has
a new arrangement for illumination, entirely different
from the one here described. 1

1 H. Siedentopf, loc. cit., Chapter IV, and Journal of the Royal
Microscopical Society, 1903, pp. 573-578.

108 DESCRIPTION OF THE APPARATUS, ETC.

, Another arrangement preventing the rays in ultra-
microscopic work from entering the microscope proper
by total reflection from the cover-glass, has been described
by Cotton and Mouton. 1

1 Cotton and Mouton, Comp. rend., June 1903, Vol. CXXXVI,
pp. 1657-59.

A detailed description with directions for the use of
the ultramicroscope and its attachments, is issued in
English and in German by the makers of the instrument,
C. Zciss, Jena, and may be had by writing for the cata-
logs known as Micros 227, 228, 229.

The observance of two precautions will save much time
for beginners :

1. Be sure the substance examined contains particle?
that can be rendered visible by the illumination used.

2. Avoid vibration of the instrument, especially when
examining fluids.

3. Use solutions sufficiently dilute.

J. A.

Since writing the above note there^ have appeared on
the market several other types of ultramicroscopes which
are much more convenient to use than the original Zsig-
mondy-Siedentopf instrument; notably those made by
Reichert (Vienna), E. Leitz (Wetzlar), Cotton & Mouton
(Paris), and Bausch & Lomb Optical Co. of Rochester,
N. Y. (U. S. A.) who are producing a form of Siedentopf s
cardioid condenser which is very useful in ultramicroscopic
work. See also " TUltramicroscope dans le Diagnostic
Clinique et les Recherches de Laboratoire " by Dr. Paul
Gastou (J. B. Balliere et Fils, 19 rue Hautfeuille, Paris.)

October 21, 1918.

CHAPTER VI
CERTAIN TERMS OFTEN USED HEREIN

E. VON BEHRING has proposed to call the herein described
apparatus for rendering visible ultramicroscopic parti-
cles, " Ultraapparatus " for short, and to designate as
" Ultramicroscope " the microscope constructed by H.
Siedentopf for the examination of bacteria. 1 The pre-
liminary research was carried out exclusively with the
first apparatus referred to above; and the name pro-
posed by von Behring will be often used. I must observe,
however, that other investigators 2 use the name "Ultra-
microscope " for the instrument termed by von Behring
" Ultraapparatus." It is therefore advisable to keep the
expression Ultramicroscope for both kinds of apparatus,
and in case it is necessary to distinguish between them,
to call the former Ultraapparatus, and find some short
name for the latter.

H. Siedentopf has also proposed to establish a further"
classification of the so-called ultramicroscopic particles. 3
He terms ultramicroscopic a particle or dimension which
is below the limit of microscopic resolvability (in prac~
tice about \ jj). According as the ultramicroscopic par-
ticle can be rendered visible or not, it is designated as

1 H. Siedentopf, see note 3, p. 103.

2 Raehlmann, Zeitschr. f. arztl. Fortbildung, 1904, No. o. W. Biltz,
GSttinger Nachr., 1904, No. 4.

* H. Siedentopf, Berlin klin. Wochenschr., 1904, No. 32.

109

110 CERTAIN TERMS OFTEN USED HEREIN

"submicroscopic " or "amicroscopic." These terms will
often be used here.

In order to avoid continual use of the term " particles/'
I will introduce an additional simplification, and ampli-
fying Siedentopfs proposed nomenclature, I shall call
ultramicroscopic particles "ultramicrons," the submicro-
scopic particles "submicrons," or "hypomicrons," and
the amicroscopic particles "amicrons."

CHAPTER VII

PRINCIPLES OF THE ULTRAMICROSCOPIC INVESTIGA-
TION OF FLUIDS

The manipulation of the glass apparatus, for the exami-
nation of fluids, illustrated in Fig. 3, is very simple. The
microscope tube is taken out from the stand, the space
between the objective and the upper quartz surface of
the cell is covered with the immersion fluid (in the case
of objective D* with water, which may be squirted
between the objective and the cell from a wash bottle),
after which the quartz window in front is dried with a
clean linen cloth. The funnel is then filled with a turbid
fluid (diluted milk, a solution of gum gamboge, commer-
cial colloidal silvey, etc.), the pinch-cock H (Fig. 3) is
opened until all air is expelled from the apparatus, which
is sometimes effected by tapping or holding the tube
slanting. 1 Then the tube, together with the objective
and the washable apparatus attached to it, is again slid
into the stand; the microscope proper is moved so as to
bring within the field of view the apex of the illuminated
cone (which can be easily found in the turbid fluid), and
care is taken that the narrowest constriction of the cone
is brought to the center of the field. 2

1 Good cells fill up at once with the fluid and retain no air bubbles;
the turbid fluid must be greatly diluted.

2 Beginners will best learn the necessary manipulation by practice.
Before using the apparatus it is necessary to see that the glass tubes
connected by rubber tubing approach each other closely.

Ill

112 ULTRAMICROSCOPIC INVESTIGATION OF FLUIDS

When the apparatus is adjusted, the fluid used for this
purpose is run off by opening the pinch-cock // (Fig. 3),
and the cell rinsed out with water until all turbidity-
producing particles are removed; the apparatus is then
filled with the fluid to be examined, the first portion of
it, in quantitative determinations, being used to wash
out the water.

I will now take up the description of examinations of
a general character, and will begin with distilled water.

i. Distilled Water

As Spring has found, distilled water is not optically
clear; it can be freed from the last traces of suspended
or colloidally dissolved substances only by somewhat
complicated operations. It is evident, therefore, that
these impurities prejudice the use of ordinary distilled
water in ultramicroscopic investigations to such a degree
that it cannot be used to dissolve colloids, or to dilute col-
loidal solutions. From experience, however, it has been
learned that in the case of water which stood long enough
protected from dust, the dust particles are present in
relatively small number, so that only a few of them
come into the field of view, and the space actually under
observation is for the most part free of them. The use
of such water for many investigations, therefore, seems
permissible. 1 In between the suspended dust particles
which float perfectly quietly, the water either appears

1 Stirring with glass rods and taking water from squirt-bottles, should
both be avoided. , Water is best drawn from a fixed vessel whose
siphon tube does not reach the bottom, into a beaker previously rinsed
out, and from this it can be poured into the* thistle tube of the appa-
ratus. It is advisable to set up several fixed vessels. The siphon is
closed with a rubber tube and a pinch-cock. (See also the suggestions
of Biltz and Gahl, mentioned in Chapter XX of this book.)

COLLOIDAL SOLUTIONS 113

optically clear, or shows upon further opening the slit
(whereby a light-cone of greater depth is seen) a weak,
diffused shimmering of light, due to traces of colloidally
dissolved substance, 1 which do not disturb ordinary
investigations where a narrow opening of the slit is
necessary.

In further prefecting the method, and in the examina-
tion of fluids whose particles approach the limit of visi-
bility, there should of course be used only water which
shows no light-cone in the ultraapparatus. By double
distillation and prolonged standing water can be obtained
that is suitable for refined work; in which case dust
must be excluded as far as is possible.

Recently W. Biltz has published a very good method
for the production of clean water (see Chapter XX).

2. Colloidal Solutions

In colloidal solutions the microscopic form of the
light-cone is distinctly different from that of distilled
water. The distance between the particles in them is
usually less than \ /*, and in such cases there appears a
more or loss intense homogeneous polarized light-cone;
very often larger particles arc present which shine
brighter and give the light-cone a heterogeneous appear-
ance. In order to form an idea as to the individual
particles, extreme dilution (0.001-0.0001 per cent and
less) is almost always necessary; then only does the
distance between the particles become so great that its
microscopic resolution is possible.

If the light diffracted from the individual particles
is intense enough to create an impression on the retina,

1 Probably of silicates from the glass or of metal oxids from the
cooling tube.

114 ULTRAMICROSCOPIC INVESTIGATION OF FLUIDS

the microscopic pictures of the particles will appear as
little reflecting disks; in the case of submicrons, with
the most favorable illumination, this is always the case.
If the diffracted light is too weak, the light-cone will
gradually vanish with increasing dilution, and there is
no optical resolution, the solution in such cases contain-
ing amicrons.-

It is very important to distinguish the casual, sus-
pended particles which, especially in concentrated solu-
tions, are often present in large numbers, from the regular
constituents of colloidal solutions. This can be accom-
plished by extreme dilution; the spaces between the
suspended particles then become so large that the light-
cone which lies between them and is produced by the
smaller particles, can easily be recognized.' Upon still
further dilution there is re-solution into individual parti-
cles (submicrons), or else the light-cone vanishes (ami-
crons). In the latter case the presence of extremely
finely divided substance can be demonstrated by the
addition of optically clear precipitants. 1 By such addi-
tion the colloidally dissolved particles can be forced to
combine into larger ones, which frequently, then thickly
fill the field. * '

In this manner I demonstrated the presence of col-
loidal gold and of albumen in almost homogeneous
solutions.

Those who later enter on this field of work will do well
always to make such a test, for the uninitiated are very
apt to be deceived by large particles, especially in the
examination of concentrated solutions. 2

1 Sometimes by boiling the fluid, as for example with albumen.

1 In dilution experiments it is to be observed in addition, that dis-
tilled water sometimes precipitates individual colloids, globulin for
instance being precipitated by water. In such cases instead of water a

COLLOIDAL SOLUTIONS 115

Polarization. Tyndall has already shown that light
is polarized by particles which are very small as com-
pared with its wave-length, whereas by larger particles
this effect is produced only partially, or not at all. In
connection with Rayleigh's theory, these facts were
often investigated and TyndalFs observations confirmed. 1

I might here make mention of an observation which
may seem very strange with reference to the foregoing,
but which has been repeatedly confirmed in the case of
colloids; there arp particles floating in the fluid, which,
notwithstanding their relatively large size as compared
with the wave-length of light, totally polarize it, and
can therefore be rendered invisible by turning the Nicol.
Particles of this kind I have observed, for instance, upon
the precipitation of a dilute solution of CuSO* by Na2COa;
the light diffusely scattered by the precipitated hydrogel
of copper carbonate proved to be polarized, even if the
flocks were so large that they could be seen with the
naked eye.

The same is tile case with colloidal ferric oxid; its
solutions, as well as its flocculent precipitate, plane
polarize a beam of light in the same direction.

This phenomenon appears to be quite common, and
shows itself as soon as hydrogel particles float in the
fluid, no matter whether they are large or small as com-
pared with the wave-length of light. This is no excep-
tion to the rule of Tyndall, above mentioned, which

diluent must be chosen which exerts no precipitative action on the
ultramicrons, providing it is desired to obtain the particles in their
original condition. In other cases increasing dilution may work
disruptively upon the individual particles, by transformation into
crystalline solution, for example. Cases of this kind have not yet been
carefully investigated.

1 See for instance B. Pernter, Wiener Denkschriften der Akademie
der Wissenschaften, 1901, Vol. LXXIII.

116 ULTRAMICROSCOPIC INVESTIGATION OF FLUIDS

refers to little drops of fluid in steam-clouds, and also
to such emulsions as the turbid mastic solution, the
polarization of which was closely investigated by Pernter,
who among other things concluded "that the presence
of larger particles involves a decrease of the polarization,
in proportion to the size and number of such particles
present." l

The hydrogel particles must, however, be conceived as
being thoroughly permeated with fluid. According to
Biitschli, hydrogels possess a fine, web-like structure, or
according to Quincke, they consist of foam-cells, or foam-
chambers with invisible walls. Some such structure,
but much finer and perhaps otherwise constituted, must
be assumed in the case of each hydrogel particle, even if
it is smaller than a wave-length of light, that is so small
that the examination of its structure is no longer possible.

Returning to the ultramicroscopic examination of gel
particles, a striking difference is at once evident between
such an examination and that with the naked eye.

While the rays diffracted by the flocculated particles
of the Fe2Os hydrogel, when macroscopically observed,
do not appear much brighter than those of the light-
cone in the original hydrosol, and Qan be almost com-
pletely blotted out by turning the plane of polarization
of the Nicol prism, there can, however, upon ultramicro-
scopic examination of the same fluid, be seen in addition
to numerous small particles, dazzlingly bright larger
particles whose light, although weakened, cannot be
entirely blotted out by any position of the Nicol prism.
On the other hand the less brilliant particles partially or
completely disappear with crossed Nicols, sometimes
even if they have reached considerable size.

An explanation of this phenomenon I will not here
1 Loc. cit., p. 11 (p. 310).

COLLOIDAL SOLUTIONS 117

attempt, but will merely call attention to the fact that
practically no inference as to the size of hydrogel parti-
cles can be drawn from the degree of polarization. For
a determination of their size, or more properly the mass
of the colloid-substance contained in them, recourse
should be had to methods which will soon hereafter be
described.

Color. Ultramicroscopic particles are usually colored,
the color of their tiny reflecting disks, when the particles
are alike, being nearly complementary to the color of the
mass of the fluid by transmitted light, as has already
been mentioned. 1

So far as our present experience goes this relation is
general; it indicates in harmony with the behavior of
turbid media that the color of the fluid is produced by
the preferential separation from the impinging beam of
light, of those component rays which are scattered by
the individual particles. It would seem reasonable to
extend this view to the rays scattered by the molecules
of dye-stuff solutions, but such an hypothesis is palpably
incapable of demonstration because of the extreme weak-
ness of the diffraction in particles of such small mass, :

In order to get a correct idea of the color of the indi-
vidual particles, it is quite evidently necessary to dilute
the fluid until a disk one quarter centimeter 2 thick is
practically colorless.

3. Determination of the Size of Particles

In the paper 3 frequently before referred to, two methods
were described in detail for the determination of the size

1 Siedentopf and Zsigmondy, loc. cit.

2 The approximate length of the path of the light beam in the fluid.
1 Siedentopf and Zsigmondy, loc. cit., pp. 16-29.

118 ULTRAMICROSCOPIC INVESTIGATION OF FLUIDS

of ultramicroscopic particles. It was there pointed out
that these methods give only the average mass of the

A

individual particles, according to the formula , where A

n

is the mass of the substance subdivided, and n is the num-
ber of particles in it.

For the purpose of tabulation in the case of gold ruby-
glass, the average linear dimension I of the individual
particles was figured out on the assumption that the
gold particles are cubical 1 and are of solid gold, having a
specific gravity s = 20, 2 that is, according to the formula

Z=

First Method. According to this method a certain
illuminated volume of glass or of fluid (V) is blocked out,

AA

1 "

a

ii/i.ukn i

1 1 \

30 40 60

FIG. 5.

the size of this volume determined, and the particles
contained in it counted. By means of the eye-piece
micrometer a part of the cone of rays dd (see Fig. 5),
may be sharply defined from side to side, whereby the
length and breadth of the volume chosen may be known.

1 That this assumption is not quite accurate is brought out on p. 36
of the paper quoted. Regarding the detailed discussion of sources of
error, I refer to pp. 21-29 of the same paper.

1 Rose found the specific gravity of precipitated gold to be 19.55 to
20.7.

DETERMINATION OF THE SIZE OF PARTICLES 119

The depth of the illuminated volume V l thus defined
may be easily determined with the eye-piece micrometer
upon the quarter (90) rotation of the graduated slit
S (see Fig. 2, Chapter V).

TVj mass A of the subdivided substance contained in
the volume, is found directly from the concentration,
providing the total mass of the subdivided body is present
in the form of ultramicroscopic particles, as in many
colloidal gold solutions. When, however, one part of
the subdivided substance is in homogeneous solution (a),
and another part is present in the form of submicroscopic
particles (/?), then since /? is unknown, we take, in place
of the latter the sum (a+^), and thus obtain a major
limit for the size of the particles. 2

For full details for the determination of volume, as
well as for the sources of error in this method, I may
refer to the original article, merely remarking here that
in exact determinations of the size of the particles, the
error does not exceed twenty per cent of the linear dimen-
sion, and is usually considerably less.

Second Method. From the distance between the parti-
cles, and using the same assumption, the linear dimen-
sion I maybe obtained by means of the formula

where r is the mean distance between the particles.

1 For particulars see pp. 17, 21, and 22 of the paper referred to.
It will be seen that the depth of the illuminated volume thus deter-
mined can enter the calculation only if it is less than the depth of
view of the microscope.

2 Sometimes /? can be indirectly determined, as in the case of gold
ruby glass, where we can determine /? by colorimetry. The major
limit is not much different from the actual size of the particles, pro-
vided that a does not constitute the bulk of the substance present.

120 ULTRAMICROSCOPIC INVESTIGATION OF FLUIDS

In using this method it should be carefully noted that
the mean distance between the particles can be correctly
measured only when the depth of the layer under obser-
vation is at least as large as this mean distance. The
depth of the layer in question must therefore be exactly
determined; for if this be not done, or if it is impossible,
apparent distances are obtained which may sometimes
differ in no small degree from the actual distances.

As mentioned in the paper above referred to, 1 the
measurement of the distance between the particles in
the case of fluids is made very difficult by their motion;
numerous measurements must therefore be made on one
and the same fluid in order to obtain an average of prac-
tical value. The probable accuracy of the results
obtained is considerably increased, if various degrees of
dilution are examined.

In the case of colloidal gold solutions I have for the
most part worked as follows: Each fluid to be investi-
gated was first reduced to a content of 0.005 per cent of
metallic gold, and when possible, the first series of counts
made with this. Thereafter the hydrosol was diluted 8,
27, 125 times, or 10, 100, some times even 1000 times,
and at each dilution the measurement ctf the distance
between the particles again repeated. The mean of all
the figures was taken, and from this was calculated the
distance between the particles in the original concen-
tration; and as a control of the value thus obtained, in
many instances the number of particles contained in a
given volume was determined by the first method.

First * Method Applied to Fluids. As the particles are
in continuous motion, it is not possible to count off a
large number of them; it is therefore necessary to so
limit the volume of the fluid illuminated, or so dilute a

1 Siedentopf and Zsigmondy, loc. cit., p. 29.

DETERMINATION OF THE SIZE OF PARTICLES 121

given volume of the fluid, that on the average about
from 2-5 particles are contained therein, which number
may be determined at a glance.

By frequent repetition of this same procedure in dif-
ferent parts of the fluid, and with various degrees of
dilution, there may be obtained a pretty good average
value of the number of particles in a certain volume,
which thus serves to control the value obtained by the
other method. 1

The results w of neither method, of course, have any
great degree of accuracy, but they both satisfactorily
serve the purpose of this book to establish some kind
of order in a field hitherto inaccessible to direct observa-
tion. The brightness of the diffracting disks also fur-
nishes a good control for the size of the particles, because

1 With fluids which contain a variety of different sized particles,
it is evident that the number of particles in a given volume will not
be found proportional to the concentration of the colloidally dissolved
substance, but that in the more concentrated ones it is too small.
For example, wit^ a silver hydrosol (Bredig's method) I obtained, as
an average of about 20 separate measurements with a concentration,
C = 0.0037 per cent:

Concentration in 440 ^i 3 Calculated to concentration C

C 3 .50 particles 3 .50

C/2 2.28 " 4.56

C/4 1.95 " 7.80

C/8 1.62 " 12.90

Upon greater dilution many more particles are therefore counted
than with greater concentration, and the reason of this is chiefly because
the smaller particles become distinctly visible only when by great
dilution the larger particles are in part removed from the field of
view.

The average linear dimension, according to the first count was,
found to be too large in the proportion of f^l2.9 : "^3.5, that
is about 1.5 times greater than by the last count. In such cases
it will be better to report, not the average value, but the superior
limit determined by various degrees of dilution.

122 ULTRAMICROSCOPIC INVESTIGATION OF FLUIDS

in general those particles of a certain substance which
have been determined to be the largest, must naturally
diffract most light. The brightness and color of the
diffraction-pictures, as well as their motion, are such
characteristic features of the particles that it is easy, in
a mixture of two fluids of different properties, to deter-
mine by ultramicroscopic observation of the particles,
which belong to one fluid and which to the other.

The results of the series of investigations hereafter
given, thus mutually support each other.

In addition it may here be stated that gold particles
whose linear dimension was found to be 15 /*^ (15.10~ 6
mm.), lie at the limit of visibility with illumination
by the electric arc, 1 and that smaller particles can be
discerned only by the use of sunlight; while the smallest
that can as yet be rendered visible, which are about as
small as 5 /*/*, can be seen only with the brightest sun-
light on very clear days and with efficient means for
cutting off all extraneous light.

As the index of refraction of metallic gold is very
different from that of water, and this difference is favor-
able for rendering visible ultramicroscopic particles, it
stands to reason that particles of other substances which
are smaller than 15 ftp cannot be made visible by arc-
light with the present apparatus, excepting those of some
few metals and perhaps those of fluorescent dye-stuffs.

I am obliged to state this here, because many divergent
statements have been made by others about the limits
of visibility of ultramicroscopic particles, and such state-

1 This statement refers to our apparatus ordinarily used for examining
fluids (objective AA [num. ap. = l/3] for illumination, and objec-
tive D* [num. ap. = 3/4] for observation). According to Siedentopf
the brightness of the diffraction disks is proportionate to square 'of
the product of the numerical apertures of the objectives used for
illumination and observation.

DETERMINATION OF THE SIZE OF PARTICLES 123

ments might easily lead to an exaggerated idea of the
method described by Siedentopf and myself. Without
further remarks, it follows from what has been said that
when colored particles are observed with the ultra-
microscope in the solutions of dye-stuffs like acid fuchsin,
naphthol yellow, etc., which dissolve as electrolytes,
these particles cannot be molecules or ions of the dye-
stuff in question, that is, they cannot be its smallest
particles.

CHAPTER VIII
PREPARATION OF COLLOIDAL GOLD SOLUTIONS

THE gold hydrosols used for the experiments hereafter
described, were prepared by two different methods.

One I have described in Liebig's "Annallen," Vol. 301,
p. 30; the other will be given here.

First Method. A dilute solution of chlorid of gold is
mixed with potassium carbonate and reduced at the
boiling-point by formaldehyde. If bright red clear fluids
are desired, which can be used as colloid reagents, the
details described in the "Zeitschrift fur Analytische
Chemie " l must be exactly followed.

The chief essential is distilled water of sufficient purity.
The absence of electrolytes is not so essential as the
absence of colloids; traces of the former do no harm,
seeing that a certain quantity of them is introduced
into the water by the reagents themselves. On the other
hand traces of colloids almost always present in all com-
mercial distilled water completely prevent the formation
of bright red gold hydrosols. With such water there
are usually obtained blue, violet, purplish suspensions
which, in a few days or weeks, deposit the greater part
of their gold.

For this reason I gave directions to redistil the dis-
tilled water, using a silver worm for condensation, 2

I Zeitschr. f . analyt. Chem., 1901, Vol. XL, p. 710. .

I 1 often obtained bright red fluids by using frozen water, and once

124

PREPARATION OF COLLOIDAL GOLD SOLUTIONS 125

Some impurities which I have found particularly
injurious are : Phosphates of the alkaline earths, silicates
from the glass itself, colloidally dissolved substances
originating from the cooling worms ordinarily used,
organic substances in commercial distilled water, and
others of like kind.

It is not difficult to see how these impurities act. The
colloidally dissolved substances permeate the entire
water, and in spite of their very small quantity are so
thoroughly distributed that the distances between their
particles are for the most part beyond the resolving
power of the microscope. Therefore, as above described,
they appear upon ultramicroscopic observation as an
extremely faint homogeneous, polarized light-cone, which
encloses much larger dust particles. The finest dust of
Konowalow's * fluids, which according to the publica-
tions of this investigator play an important role in the
turbidity of critical solutions, may, in so far as aqueous
fluids are concerned, be the same as the colloidal impuri-
ties here described .

The mutual precipitation of colloids makes possible
the formation of large particles capable of producing
turbidity. As Picton and Linder, 2 Lottermoser, 3 and
recently W. Biltz 4 have shown, colloid solutions with
oppositely conducted particles mutually precipitate each
other. When therefore reduced gold and the colloidal

even by using water condensed in a tin cooler, but in any event only
after prolonged distillation. The same was observed by W. Biltz,
Gottinger Nachr. 1904, No. I.

1 Konowalow, Drude's Ann., 1903, Vol. X, pp. 360-392 and also
Vol. XII, pp. 1160-64. The term " finest dust" is perhaps not a
happy one, for ordinary dust particles are as a rule incomparably larger.

a Loc. cit.

* Ueber anorganische Kolloide, Stuttgart, 1901.

Berichte, 1905, Vol. XXXVII, pp. 1095-1116.

126 PREPARATION OF COLLOIDAL GOLD SOLUTIONS

impurities are oppositely conducted, there is a possi-
bility that a coarse turbidity may be found. 1

Other conditions may enter, but to discuss them would
lead us too far.

/ By the use of sufficiently pure water a quantitative
reduction of gold may be obtained with formaldehyde in
fluids which contain only 0.00005 per cent of gold, that
is, one hundred less times gold than the bright red ones,
and the fluids thus obtained may be evaporated without
decomposition to one-fiftieth (1-50) of their volume, and
still further. In such fluids the gold is amicroscopic,
that is, in particles so. small that they cannot be made
visible.

/ The second method for preparing almost homogeneous
gold solutions may be regarded as a combination of my
method with that of Faraday. This method has the
advantage of making it relatively easy to obtain the

4 Recently Jordis (Zeitschr. f. Electrochem., 1904, p. 510) has
advanced the opinion that the impurities dissolved in water are neces-
sary, as "sol-formers," to the formation of colloidal metals. Upon
the basis of his observations on silica and also the observations of
Lobry de Bruyn and Paal, Jordis infers that it is easier to obtain
more concentrated solutions of colloidal gold in the presence of certain
organic substances (protective colloids) than without them. It should
not be overlooked that the colloids of Lobry de Bruyn and Paal are
analogous to the purple of Cassius, showing a much different behavior
with reagents and upon desiccation than the colloidal gold made by
Bredig's method or my own, and being subject to different conditions
of formation. To this I shall later return, A protective colloid would
by all means facilitate the production of colloidal gold and act as a
11 sol-former"; the impurities contained in water, however, generally
have the opposite action, and are not "sol-formers," but sol-destroyers.
Therefore the advice to work with purest water possible. The addi-
tion of alkali or alkali carbonate produces, as Bredig has shown, a
decrease in the concentration of the hydrogen ions, which also militate
against the formation of metal hydrosols, or an increase in the concen-
tration of hydroxyl ions which are favorable to the formation of gold
hydrosols (see also pp. 64 and 77-79).

PREPARATION OF COLLOIDAL GOLD SOLUTIONS 127

finest subdivisions even in more concentrated solutions
(0.005 per cent Au).

I use in exactly the saino concentration the same mix-
ture as with reduction by formaldehyde, but instead of
heating with formal, reduce at room temperature with
a few drops of an ethereal solution of phosphorus. 1 (Pro-
portions 120 cc. water condensed in a silver cooler, 15 mg.
AuHCU . 3H 2 0, 37 mg. K 2 C0 3 ). Here, as in the other
method, the use of colloid-free water is essential. The
formation of the- hydrosol takes place slowly, the fluid
first becoming bright brownish red, and then gradually
bright red, often with a tinge of brown-red, without show-
ing the slightest turbidity to transmitted or reflected
light.

If a good solution has been prepared, 2 it can be seen,
upon testing with a lens and sunlight, that in it diffuse
dispersion is almost entirely lacking, and that it is almost
indistinguishable from a mixture of the reagents without
gold. Ultrarnicroscopically there may be seen in it a few
gilded dust particles, and among them an unresolvable,
extremely faint light-cone, which soon disappears upon
diluting the fluid with the purest kind of water, no indi-
vidual particles being discernible.

This fluid stands boiling without alteration, in contra-
distinction to that prepared by Faraday, which was
thereby rendered turbid, and deposited its gold.

The contamination of 120 cc. of the purest water (con-
densed in a silver cooler) with a few drops of water in
which had been stirred up some powdered glass, or the

1 It is best to take about one-half cubic centimeter of an ethereal
solution of phosphorus, made by diluting a concentrated solution with
five times its volume of ether.

2 From a larger number of separately prepared fluids the best must
be selected in order to obtain fluids of the greatest homogeneity.

128 PREPARATION OP COLLOIDAL GOLD SOLUTIONS

use of commercial distilled water for the preparation of
the gold solution, yielded sometimes very turbid fluids,
which, like Faraday's, become violet and settle upon
boiling, instead of being nearly homogeneous. 1

The influence upon the reduction of gold of the col-
loidal impurities in distilled water was not known to
Faraday; 2 he obtained, therefore, upon reduction with
phosphorus, a variety of fluids, mostly suspensions which
settled soon after their preparation, but sometimes stable
colloidal solutions, which, however, would not stand
boiling.

Third Method. Proceed as in the first method, but
shortly before the addition of the formaldehyde add to
the 120 cc. of water from 0.5 to 4 cc. of the nearly homo-
geneous gold solution produced according to the second
method. The tiny gold particles act as nuclei (see Chap-
ter XVII, 3), and grow in the reducing mixture. Accord-
ing to the quantity of gold fluid added, there are obtained
gold solutions with larger or smaller particles. 3

1 In these experiments no alkali carbonate was added.

2 He also knew nothing of the beneficial influence of the addition
of alkali carbonates on the formation of finer subdivisions (for the
purpose of neutralizing the acid formed?) Nevertheless very profound
subdivisions may be obtained with phosphorous without K 2 CO 8 ,
providing the other conditions are observed. The age of the phos-
phorous solution used and its degree of dilution also influence the
properties of the gold solution.

8 Zsigmondy, Z. phys. Chem., Vol. LVI, p. 65 (1906).

CHAPTER IX

ULTRAMICROSCOPIC EXAMINATION OF THE SOLUTIONS

OF GOLD

THE figures given in the following table are the result
of a very careful investigation. Apart from the fact that
the work was several times interrupted and taken up
again, it was further complicated because for the most
part I needed direct, bright sunlight, which is somewhat
rare in Jena; it often happened that just when all the
preparations for an examination had been made, a cloud
would pass before the sun.

The hydrosol AUQT was most thoroughly investigated,
an average value being obtained based upon more than
three hundred individual measurements made in collabora-
tion with Dr. F. Kirchner on three different concentra-
tions.

We found as the average value of the distance between
the particles:

Distance
Observed.

Distance Calculat-
ed for Undiluted
Fluid.

With undiluted fluid

0.972/i

Upon dilution, 1:8

2.33yi

1.160

Upon dilution, It 125 . .

5.67 ft

1.130

Average of combined observations 1.13 ft.
Size of particles calculated therefrom 15.25 /j/t.

The values for the sizes of particles in fluids which
were less carefully examined, will show considerably

129

130 EXAMINATION OF THE SOLUTIONS OF GOLD

greater variation from the true mean, the error in some
cases being perhaps thirty per cent. There is great
uncertainty in the values for fluids about the limit of
visibility. On the other hand the accuracy of the resuHs
are strongly supported by the practice which the eye
gets in estimating the size of the particles from the bright-
ness of the diffraction-images. 1

As often previously stated, our method yields only an
average value for the mass of the particles; to make com-
parison convenient, the linear dimensions were calcu-
lated therefrom as before, on the assumption that the
particles are cubical and completely filled with metallic
gold. I must, however, expressly caution against assum-
ing that this imaginary cubical or globular form cor-
responds with the actual fact; many observations indi-
cate that never, or only in isolated cases, do the metal
particles have a similar shape on all sides. To this sub-
ject I shall later return.

In the first column of the following table are given
the names of gold solutions, AuP designating the solu-
tion obtained with phosphorus by the second method,
while Aura, Auioi, etc., designate those obtained by the
first method. With the^exception of Au 7 3 and Au 73o ,
which were reduced from a 0.0005 per cent solution of
gold chlorid, all the other liquids when they were made
contained from 0.005 to 0.006 per cent of gold. For the
purposes of examination they were diluted to 0.005 per
cent Au, and fractions of this percentage.

In the second and third columns are given the colors
of the particles and the fluid, and when no individual
particles were visible, that of the light-cone.

The length of the path describe! by the submicro-

1 As has been brought out in Drude's Ann., Vol. X, p. 29, the differ-
ences in brilliancy are extremely great.

EXAMINATION OF THE SOLUTIONS OP GOLD 131

scopic particles, designated by A in column V, serves
merely for a preliminary orientation. The motion of
translation is made conspicuous, while the much smaller
motion of oscillation, to which every particle is subjected,
is not given. As a rule not without exception it may
be stated that the motion is more rapid the smaller the
particles and the greater the number contained in unit
volume. Upon dilution, therefore, the activity of the
motion decreases somewhat.

Very large particles of gold and gilded dust particles
float, for the most part, quietly in the diluted fluid; in
concentrated ones they are sometimes struck by the
smaller particles, thus acquiring motion.

In the table the fluids are arranged according to the
size of the particles.

Glancing at the table, there is seen at the top a very
interesting fluid: AuP 16 . When just made, this is bright
red, and completely clear both to reflected and trans-
mitted light. The gold in it is subdivided to a degree
approaching Jiomogeneity, for outside of coarse yellow
particles about 0.05 mm. apart (probably gilded dust),
hardly a trace can be seen of the balk of the gold, even
in the ultraapparatus.

The milliards of individual particles which fill every
drop of the other fluids (for instance AUQT or Auioi) are
here completely lacking, or more correctly, they escape
observation on account of their small size. Their ability
to diffract light has become so reduced that even in their
totality, they do not diffract more light than is absorbed
by the colored fluid, so that it is doubtful if any light-
cone due to gold particles in the solution can be seen at
all. That this is a coDoidal solution of metallic gold,
and not some red chemical compound of gold, can at once
be proven by the addition of electrolytes. As the experi-

132 EXAMINATION OF THE SOLUTIONS OF GOLD

ment in the second section of the table shows, the well-
known color-change to blue then takes place, and the
light-cone due to gold particles then becomes visible.
(For more in this connection see Chapter XII.) That
reduction with phosphorus yields the metal, Faraday 1
has already shown. I have repeatedly tested the fluid
AuP, and have reached the conclusion that, if my instruc-
tions are adhered to, almost homogeneous gold solutions
can be obtained. By reducing the gold with formaldehyde,
I obtained, as already mentioned, similar fine subdivi-
sions only when I had at my disposal water of especially
good quality, and diluted the gold chlorid solution before
reduction to from eighty to one hundred times its usual
concentration, that is down to about 0.000005 per cent
of gold.

Between the finest subdivisions of gold just described,
and those whose particles just become visible under most
favorable conditions (Au 7 3 a , line 7), there are a whole
series of transition forms which can at present be pro-
duced, but whose detailed description would carry us
too far.

Upon examining the fluids in order of the size of their
particles, there are seen in the ultraapparatus, with
increasing size, the following phenomena; the light-cone
is hardly discernible (line 1, AuPi 6 ); it becomes more
distinct and brighter (line 3), but vanishes upon dilution;
the color of the light-cone may be recognized (lines 4-6),
it becomes still brighter, and acquires a heterogeneous
appearance, but the individual particles are still arniero-
scopic (Au 73 ); upon sufficient dilution and with the
brightest sunlight the individual particles become visible

1 Phil. Trans, of the Roy. Soc. of London, 1857, Vol. CXLVII, p. 163
a Slight deviations from the directions may result in very turbid
fluids.

EXAMINATION OF THE SOLUTIONS OF GOLD 133

(smallest submicrons, in fluid Auysa, for example); the
individual particles then become visible even by arc-
light (line 10 and the following). The particles become
brighter and brighter, and shine with various colors
(lines 14-20), their activity decreases; finally, they become
so large that they sink to the bottom (lines 21, 22); they
must, however, be still larger in order to be seen in the
microscope with ordinary illumination.

CHAPTER X
MOTION OF THE GOLD PARTICLES

As but few have as yet seen the motion of the smallest
particles approaching the limit of visibility with sunlight,
a few remarks on this subject are not out of place. 1

I here repeat the description which I had written for
my Wiirzburg lecture under the inexpressible impression
of the first view of this rare phenomenon, a description
of which I had stricken from the manuscript because, on
re-reading it at that time, it seemed exaggerated. I had
pointed out that large suspended gold particles float
quietly in the fluid and sink slowly to the bottom, or
else show only an unimportant Brownian movement, and
that by conceiving colloidal solutions as suspensions, one
would naturally imagine that the gold particles in these
too would be as quiet as those in the real suspensions. I
then added the description above referred to:

"How entirely erroneous was this idea! The small
gold particles no longer float, they move and that with
astonishing rapidity. A swarm of dancing gnats in a
sunbeam will give one an idea of the motion of the gold
particles in the hydrosol of gold! They hop, dance,

1 For the demonstration of ultramicroscopic particles with arc light
on the occasion of the exhibition of our apparatus at Jena, Berlin,
Kassel, and Leipzig, Siedentopf and myself chose for the purpose of
distinctness, coarser subdivisions of metals having particles of 20-40 /*/*.

134

MOTION OF THE GOLD PARTICLES 135

jump, dash together, and fly away from each other, so
that it is difficult in the whirl to get one's bearings.

"This motion gives an indication of the continuous
mixing up of the fluid, and it lasts hours, weeks, months,
and, if the fluid is stable, even years.

"Sluggish and slow in comparison is the analogous
Brownian movement of the larger gold particles in the
fluid, which are the transition forms to ordinary gold
that settles.

"The smallesjb particles which can be seen in the hydro-
sol of gold, show a combined motion, consisting of a motion
of translation by which the particle moves from 100 to
1000 times its own diameter in one sixth to one eighth of a
second, and a motion of oscillation of a considerably
shorter period, because of which the possibility of the
presence of a motion of oscillation of a higher frequency
and smaller amplitude could not be determined, but is
probable."

In Table 5 (page 132), column V, lines 7 and 8, etc., are
given in PL the numerical values for the amplitude of the
motion of translation, designated by A. 1 At the time of
examination hydrosol Au 9 2 was over a month old, and
hydrosol Au?3 three quarters of a year old; notwith-
standing this they showed the most active motion. Even
in still older solutions similar motions were evident, for
instance in the fluid Au 50 (made October 12, 1898, exam-
ined April 26, 190 1). 2 Frequently especially in the case
of less stable fluids a decrease in the activity of the
motion may be seen in the course of time, probably in
connection with the commencement of the coagulation of
the solution in question.

1 See what is said on this subject in Chapter IX, p. 130.
1 A fluid which had lost the larger part of its gold by being frozen
in the winter of 1900.

136

MOTION OF THE GOLD PARTICLES

I must expressly call attention to the fact that this
motion of the little gold particles just described, differs
in many respects from the typical Brownian move-
ment. 1

In the following table are arranged some of the data
concerning the Brownian movement given by various
investigators for comparison with those of the gold
particles. 2

TABLE VI

Kind of Particles.

Diameter in /i.

Free Path in /.

Observer.

1. Gold (Au 73 )

006

over 10

Zsigmondy

2. " (Auoo)

01 \

3-4, some- 1
times and \

2a. " (Au 70 )

(

0.035

over J
1-7

tt

26. Dust particles in dis-
tilled water ....

less than 2

imperceptible

tt

3. Little spheres

0.5

2 5

Regnauld

4. Gum gamboge

1-2

Zsigmondy

4a. " "

1-2

Chr. Wiener

5.

1.1

1.4

Ramsay

6.

4

o

Exner

The free paths referred to in the table are traversed in
different times. The little gold particles, as far as may
be observed, cover the distance designated, in J to | of a

1 Compare this with gum gamboge, Chapter XIX. Atterberg
(see note 1, p. 17) found (in harmony with Exner) that sand grains
less than 2/i exhibit lively Brownian movement, while grains coarser
than 3/1 no longer move.

2 The more lively motion (up to 8/0 observed by Renard in iodin
particles and turbid solutions of rosin in water and alcohol, may be
explained according to Quincke's idea (Drude's Ann., 1902, Vol. VII,
p. 67), as the consequence of the fluid stretching out in the little
interstices to the surface of the particles.

MOTION OF THE GOLD PARTICLES 137

second, and even less; larger particles take somewhat
longer to traverse their shorter paths.

The speed of the motion of the particles cannot there-
fore be seen from the table; it is very difficult to observe,
although the difference in motion may, however, be
clearly recognized. Perhaps more accurate data may be
made possible by the cinematograph (instantaneous photo-
graphy); for the present we must be satisfied to obtain
only an approximate picture of the motion. In order to
make this idea clearer, there is illustrated in Fig. 1,
Plate II, the motion of a gold particle 0.01 p in
diameter, 1 and for comparison the paths and diameters
of the other particles referred to in the table, all
enlarged 5000 times. And finally, in Fig. 6 is given
(according to arbitrary scale), the typical Brownian
movement of the fat globules in milk, according to O.
Lehmann. 2

Concerning this motion, Lehmann writes: "Here we
may wait in vain for such a drop to come to rest,
for it mdves continuously, describing an irregular
zig-zag line to and fro, vibrating unsteadily about
a mean position seldom reached; only when by
chance it rests against the side of the vessel, or
when several unite to a larger conglomerate, does the
motion cease."

In contradistinction to this typical Brownian move-
ment about a mean position, the motion of the little
gold particles is continuous, a quick moving gold
particle for instance, after a series of speedily

1 From the great variety of paths observed I select one arbitrarily,
so that the diagram has no claim to accuracy.

2 Lehmann, Molekularphysik, Leipzig, 1888, Vol. I, p. 264. In
this book there is also discussed in detail the work of other investi-
gators (up to 1888), from which several dates are taken.

138 MOf ION OP THE GOLD PARTICLES

executed zig-zag moves, rushing across the illumi-
nated field of view, almost as if it were a living thing,
and vanishing; whereas larger particles may for the
most part be conveniently viewed for a long time before
the slow current of the fluid removes them from the field
of view.

In accord with the observation of Exner, that larger
particles show less active Brownian movement than
smaller ones, I can also state that in general the
motion of the gold particles becomes more sluggish
and of greater period with increasing size, and in
the case of quite large sized particles sometimes ceases.
This latter rule is subject to exceptions, for I some-
times found quite large submicroscopic particles in
active motion.

Furthermore, the kind of the subdivided substance (or
its surface properties?), appear to influence the motion;
thus the ultramicroscapic dust particles of distilled water
show no appreciable movement.

Regarding the Cause of the Motion. I must here oppose
the natural inference that the warming of the illuminated
layer of fluid or a one-sided illumination of the particles
might have considerable influence on the motion described.
The motion persists even if a water cell be interposed in
the path of the rays of light, 1 and it remains unchanged
whether a certain part of the fluid be illuminated for a long
or a short time.

With the help of e micrometer screw on our apparatus,
the light-cone can be moved horizontally. By suddenly
turning the screw, I have repeatedly moved the cone

1 With very bright particles which could be seen in the broader
portions of the light-cone, I could see that the motion there was no
less active than at the apex of the light cone, where existed the maxi-
mum heat and light effect.

TH I

MOTION OF THE GOLD PARTICLES 139

into a distant, hitherto unilluminated portion of the
field of view, and found the movement of the gold
particles just as lively there as elsewhere, and simi-
larly independent of the direction of the illuminating
rays.

In determining the motion of the particles it should
be seen:

1. That during the observation the particles are in a
space entirely confined, so that there can occur no change
of concentration by dilution, etc.;

2. The motion is independent of the direction of the
light rays, and independent of the time a given
portion of the fluid has been illuminated, and as far
as may yet be determined, of the intensity of the
illumination;

3. That in general the smaller gold particles exhibit a
much more active motion than the larger ones; that
sometimes larger particles are met with, having active
motion;

4. Thfit some of the fluids investigated which showed
lively motion, were several months, some even one and
one half years old.

5. That the particles appear to influence each other,
and that for the most part the activity of the motion is
somewhat decreased by the dilution of the gold solu-
tion.

In the case of very small gold particles there is a possi-
bility that the continuous motion may be due to a cer-
tain extent to the molecules contained in them; I con-
sider more probable, however, the assumption that the
electrically charged gold particles enter into an action of
interchange with the ions of the fluid and with each other. 1

1 Meanwhile The Svedberg has shown that the motion of the par-
ticles is dependent upon their electric charges.

140 MOTION OF THE GOLD PARTICLES

It will require very careful study to elucidate with
certainty the cause of this motion. Although the causes
of this remarkable phenomenon may be manifold, it is
the kinetic theory of fluids which appears to be of prime
importance in explaining the motion, which persists
uninterruptedly in the fluid for months and years. 1

1 These predictions have been most satisfactorily fulfilled. Einstein,
and v. Smoluchowsky (DrUde's Ann., 1906, Vol. 21, 756-780), worked
out the kinetic theory of the Brownian movement; the numerical
values of their calculations agree quite well for the most part with
the amplitudes of the motion of the particles as directly determined
by The Svedberg (Zeit. f. Elektrochem., 1906, pp. 853-860) and
Ehrenhaft.

CHAPTER XI
SIZE AND COLOR OF THE PARTICLES

THE table in Chapter IX confirms in agreement with
Faraday the statement l heretofore often made, that
there is no recognizable interdependence between the
color and size of the gold particles in ruby-glass and those
in gold hydrosols, and that therefore no inference as to
the size of the particles can be drawn from the nature of
the absorption curve of light rays. 2

AuPie is a bright red fluid whose gold is in almost
homogeneous subdivision, while Au 2 o8 is a fluid of the
same color; both have absorption bands in the green,
and contain bright green particles averaging 32/x/, some
larger, some smaller; and on the other hand Au 73 , 92 , 101,
etc., are various bright red fluids with green particles
about 6-20/*/.

Still larger green particles are doubtless contained in

'Zsigmondy, Zeitschr. f. Elektrochemie., 1902, No. 36; Siedentopf
and Zsigmondy, Drude's Ann., Vol. X, p. 35, and Verhandlungen d.
Deutschen physikal Ges., Vol. V, p. 212.

* Stoekl and Vanino, and recently Ehrenhaft (Berichte d. Kals.
Akad. d. Wiss*., Vienna, 1903, Vol. CXII, p. 181) have calculated from
the light absorption of gold sols, the size of the particles contained
in them. Their conflict with the facts supported by numerous experi-
ments, indicates that the suppositions made by these investigators
are not correct. See also the theoretical treatment of the question of
optical resonance of finely subdivided metals, by F. Pockels. Physikal.
Zeitschr., 1904, Vol. V, No. 6, pp. 152-156.

141

142 SIZE AND COLOR OF THE PARTICLES

gold suspension A, for the brightly shining green particles
sank to the bottom just like the yellow and red ones.

From the above may be seen, as has been previously
noted in the case of ruby-glasses*, 1 that green gold particles
may occur in various sizes. Every red fluid, no matter
what the size of its green particles, changes upon the
addition of electrolytes to a blue color, and then contains
no (or only a few) green particles, but has instead yel-
low or red ones, which are all larger than the green ones
formerly present; and from the finest subdivision AuPi 6
may be obtained extremely small yellow ones, approach-
ing the limit of visibility, whereas the red hydrosols with
large green particles yield quite large brightly shining
yellow or red particles. It is therefore within our power
to produce both red and yellow particles of various sizes.

From the preceding it necessarily follows that there
are gold particles of various size (mass), which for the
most part diffract green light, and which when spread
through a fluid,' endow it with a light absorption whose
maximum is in the green; and on the other hand, that
there are gold particles of various sizes which diffract
chiefly yellow or red light, and also communicate to the
fluid a corresponding light absorption. For this reason,
and also because of the fact* developed by the study of gold
ruby-glasses, we may regard as ill conceived any attempt
to calculate from the light absorption the size of the particles
in metal hydrosols. Considerable significance must be
attributed, without doubt, to a series of unknown factors
(form and shape of the particles, substance of the sub-
divided metal, and others besides).

That the substance of the subdivided metal is not
without influence upon the color of the diffracted light,
is quite evident from the fact that in the case of gold we
1 Siedentopf and Zsigmondy, p. 30.

SIZE AND COLOR OF THE PARTICLES 143

always find red, green, and yellow particles, with palla-
dium and platinum, mostly white or gray-white (with
slight shading); and with silver the most variously col-
ored particles with a brilliant play of colors, all of which
will be later referred to (Chapter XIX).

CHAPTER XII
THE COLOR CHANGE OF COLLOIDAL GOLD

IF an electrolyte is added to a red colloidal gold solu-
tion, the solution changes color from red to blue. This
color change indicates an irreversible change of the col-
loidal gold; if the gold is pure, under no circumstances
can it be made retrogressive. If, however, protective
colloids are present, the colcr change of gold may be
made reversible, Faraday had already observed that
"jelly/' when dried out with gold chlorid, etc., became
red or blue after drying, and that the blue jelly became
red again upon moistening. Preparations exhibiting a
similar striking color-play upon moistening or drying,
had been obtained by me several years ago by drying
bright red colloidal gold solutions with very little gelatin.
The thin skin of gold-gelatin remaining, became red when
moistened and blue again when dried. This change of
color may be repeated as* often as desired, and recalls
the color change of blue cobalt chlorid upon treatment
with water. As Kirchner 1 has shown, silver bromid-
gelatin plates, made and developed according to Lipp-
mann's method, in which colloidal silver is the color-
producing constituent, show a similar color change upon
moistening.

In collaboration with F. Kirchner, there was under-

1 F. Kirchner, Ber. d. math.-phys. Klasse d. Kgl. Sachs. Ges. d.
Wissensch. zu Leipzig, 1902. Further Inaug.-Piss., Leipzig, 1903.

144

THE COLOR CHANGE OF COLLOIDAL GOLD 145

taken an elaborate investigation of gold-gelatin mixtures. 1
The gold solution used for their preparation was the
fluid Au 97 (sec Table V,. Chapter IX), containing 0.005
per cent of metallic gold. The average mass of the indi-
vidual particles was 7.10~ 14 mg., corresponding to a
diameter of 15/*/i. The number of gold particles per
cubic centimeter was about 0.7 milliards. Each fifty
cubic centimeters of this gold solution was mixed with
varying quantities of gelatin (from 0.5 to 10 parts of gela-
tin to one part^of gold), and the mixtures, after being
dialyzed in a closed vessel, were poured out upon glass,
etc. Upon drying there remained a blue or dark violet
film, which when moistened showed the color change
referred to, and when examined by transmitted light with
microscope objectives of highest aperture, was resolved
into a mass of extremely small, intensely-colored granules,
imbedded in an apparently colorless matrix. Each indi-
vidual granule contained several hundred (larger granules,
even several thousand) submicroscopic particles. In them
the a-pfirticles (Chapter I) are not inseparably united;
they (the /^-particles) consist of a mass of gold and gelatin
very rich in gold '(both apparently equally distributed),
imbedded in colorless gelatin. On boiling with water the
original subdivision of the gold can be reproduced. Upon
drying out, therefore, numerous gold particles have
formed a little group, just as do the particles of a pure
hydrosol upon the additiorf of salt. In each case the
color changes from red to blue, and in each case the
cause of the color change is the same. The original a-
gold particles (see Chapter I), which we may consider as
resonators, 2 approach each other, and this approach

1 Kirchner and Zsigmondy, Drude's Ann., 1904, Vol. XV, pp. 573-
595.

1 Metal particles had been previously conceived of as resonators by

146 THE COLOR CHANGE OF COLLOIDAL GOLD

influences their time of vibration. The chief cause for
the formation of " flocks " in water and groups in gelatin,
appears to us to be the attractive forces (cohesion forces)
between mutually similar gold particles, which tend to
unite them. The great influence of cohesion upon optical
constants has been already pointed out by Wernicke. 1
M. Planck 2 advanced a dispersion theory for isotropic
dielectrics based upon the assumption that fixed resona-
tors are distributed in the ether, and comes to the con-
clusion that the crowding together of the resonators has
as a result an increase as well as a broadening in the
maximum of absorption, which takes place more quickly
toward the red than toward the blue. These very
changes in the maximum of absorption are clearly evi-
dent in Kirchner's silver preparations, as well as our
gold-gelatin preparations. The application of Planck's
theory to the latter showed quite a complete arrange-
ment between the calculated and empirically determined
absorption curves, confirming the correctness of the
assumption that the cause of the color change is to be
sought for in the change of the distance between the
particles. As to the differences between theory and experi-
ment, and for all details, I must refer to the paper cited. 3
It might be remarked U passing that these experiments
may have theoretical importance in regard to many
solid colors, ultramarine among others.

Wood, Kirchner, Kossonogow, Ehrenhaft. (Literature in the papers
quoted.)

1 W. Wernicke, Wied. Ann., 1894, Vol. LII, p. 515.

2 Planck, Drude's Ann., 1900, Vol. I, p. 92. Sitzungber. d. Kgl.
Akad. d. Wissensch. zu Berlin, May 1, 1902.

8 Regarding the optical behavior of colloidal metals, see further
Ehrenhaft, Ann. d. Phys , 1903, Vol. II, p. 489. Mention must also
be made here of the detailed theoretical investigations as to the origin
of the colors of metal hydrosols carried out by G. Mie, Drude's An-
nalen, 1908, Vol. 25, p. 377, and by Maxwell Garnet, Phil. Trans.,
1904, Vol. 203, p. 385, and 1906, Vol. 205, p. 237.

CHAPTER XIII

THE PRECIPITATION AND PROTECTION OF COLLOIDAL

GOLD

THE coagulation of colloidal gold by the addition of
electrolytes is not easy to follow ultramicroscopically.
It usually takes place so quickly that there can be seen
only the result of the union of the small particles, not
the actual process. Sometimes, however, it can be
observed.

I shall now describe Experiment No. 2 of Table V, page
132. The light-cone becomes visible; wavy yellow nebu-
losities are seen; the fog thickens still more, and then
appear tiny individual particles having an active Brownian
movement. The particles unite and turn about their
common Center of gravity. After their union, a " molec-
ular motion" begins again, but a slower one than before.

The process does not always proceed as just described:
here too a great multiplicity of phenomena appear. If
under the microscope the electrolyte solution is allowed
to run into the gold solution, there is such a violent whirl-
ing that the course of the coagulation cannot be seen;
when the fluid comes to rest, the union of the particles
has been completed*

If a gold solution with visible particles is very largely
diluted, the process of union is hindered, and there are
usually seen larger gold-glinting particles, together with
smaller green ones which attract each other, but for the
most part separate without uniting.

147

148 PRECIPITATION OF COLLOIDAL GOLD

By adding gelatin to the coagulating fluid, one can
interrupt the course of the union of the particles at any
desired moment. Nos. 18 and 19 of the table are two
experiments of this kind. In Experiment 18 the gold
particles of AUQO have united into smaller flocks than in
Experiment 19.

Colloidal Gold and Fuchsin. As I discovered several
years ago, colloidal gold and fuchsin (or instead of fuchsin
methyl violet, Bismark brown, rnethylene blue), mutually
precipitate each other, sometimes almost quantitatively
so that the fluid is decolorized (see Chapter III). By
means of alcohol, not water, however, a greater part of
the dye-stuff can be removed from the precipitate. Before
precipitation commences, the mixture of gold and fuchsin
first changes to a dirty violet-red; it then contains only
large golden yellow particles, in which sometimes the
process of union can be seen.

Protective Action. The protective action against the
.addition of electrolytes, 'exerted on colloidal gold solu-
tions by numerous colloids, has in many cases been quan-
titatively determined by me alone or in collaboration
with Fr. N. Schulz * (see Chapter III, page 79). This
protective action can be explained, as I have already
remarked, upon the assumption that a gold particle
unites with few or many particles of the protecting col-
loid, or upon the reverse assumption that one particle
of the protecting colloid unites with several gold particles. 2

Protective action can also be explained by assuming
that the "suspended" particles are encased in an ole-
aginous skin of the other colloid, according to the idea 3

1 Schulz and Zsigmondy, Beitrage z. chem. Phys. and Path., Vol. Ill,
p. 137 (1902); Zeitschr. f. analyt. Chem., Vol. XL. p. 697 (1901).

* Verh. Naturforscher-Vers., Hamburg, 1901, pp. 168-172.

1 Or by being surrounded by a skin of easily movable fluid, such as
Konowalow assumes to exist in turbid fluids in the critical state.

PRECIPITATION OF COLLOIDAL GOLD 149

advanced by Quinckc. 1 This view, supported as it is
by the analogy of the well-known action of surface energy,
can be advantageously applied towards explaining the
action exerted by protective colloids upon larger sus-
pended particles. 2

I must not refrain, however, from pointing out certain
considerations which are contrary to it, when the particles
to be protected are very small, as in the case of gold solu-
tions of the finest kind.

If we consider, the dimensions of the molecule in the
light of the kinetic theory, the assumption that the
smallest gold particles are homogeneously encased by the
relatively large gelatin or albumen molecules (providing
the amicrons contained in these hydrosols actually are
molecules), is untenable. 3

There is another objection: some gelatin and some
glycogen solutions, and many albumen solutions are
found to be permeated with ultramicroscopic particles
(see Chapters XIX and XX), whose presence must
receive consideration even if perhaps they are not the
smallest present in the solution arid do not contain the
real substance of the dissolved colloid. Particles of this
kind as well as the smallest particles of the protective
colloid can unite with the colloidal gold; even much
larger hydrogel particles can fix the gold, as we will see
in discussing the reaction between gold and alumina gel.

In the case of the explanation first advanced, there-
fore, we must bear in mind the possibility that several

1 Quincke in Drude's Ann., 1902, Vol. VII, p. 95.

'Neisser and Friedemann, Miinchner, med. Wochenschr., 1903,
No. 11.

8 Compare the linear dimension of the starch molecule (5 /t/0 calcu-
lated by Lobry de Bruyn, and the graphic representation of the
relative sizes of ultramicroscopic particles given further on (Chapter
XV, Plate IV).

150 PRECIPITATION OF COLLOIDAL GOLD

gold particles can unite with one particle of the protec-
tive colloid, as well as that one gold particle can unite
with several or many particles of the protective colloid.

Unfortunately the reaction between gold and protective
colloid for the most part escapes direct observation in
the ultraapparatus, at least so far as concerns the smallest
gold particles whose behavior would just be the most
interesting, for they are just as invisible as the amicrons
of the protective colloid. In some cases the reactions
between two colloids can be followed to a certain point
ultramicroscopically. Two of these cases will be men-
tioned here. A detailed discussion of the reactions con-
cerned and of the literature on this subject, I shall reserve
until later.

Colloidal Gold and Gelatin. A mixture of Au 9 o (0.0005
per cent Au) and gelatin (0.01 per cent), was placed in
the ultraapparatus. The activity of the gold particles
had somewhat diminished, everything else being appar-
ently unchanged (as compared with a pure gold solution
of the same concentration); in the presence of the gold,
the much less luminous gelatin particles could not be
recognized. With the fluid Au 9 o the particles continu-
ously formed groups and then flew away from each other;
the somewhat largo addition faf gelatin (about one hun-
dred times as much as was necessary to produce a pro-
tective effect), appeared to oppose group formation, with-
out, however, destroying the free mobility of the gold
particles. Even subsequent addition of NaN0 3 altered
the microscopic appearance but little. Larger additions
of gelatin produced a further decrease in mobility, but
by no means destroyed it.

If, however, there is added to the gold just enough
gelatin to produce the protective action, no change at
all is seen in the mobility of the gold particles in the

PRECIPITATION OF COLLOIDAL GOLD 151

ultraapparatus. This fact is of importance because
Lobry de Bruyn l has attributed the prevention of
many precipitative reactions in the presence of colloids
to decreased mobility (which was partially the case in
the jellies used by him; see Chapter III, p. 78); and
recently Miiller 2 has advanced the opinion that the
protection of the gold is due to the natural increase in
viscosity due to the addition of the colloid, which
prevents the " suspended " gold from settling. That
this latter view cannot be correct is evident from the
fact that the slime of quince kernels, * despite its vis-
cosity exhibits no appreciable protective action on
gold, while traces of glue (0.0001 per cent), which
suffice for this purpose, do not, as we have scon, influence
the mobility of the particles, and do not hinder the
deposition of suspended gold particles if they are large
enough to sink to the bottom in water.

From what has been said it follows that under the
conditions in question the gold particles remain iso-
lated; tihe addition of gelatin does not diminish the
number of their little reflecting disks, nor is their bright-
ness increased. The union between the gold and gelatin
which must on other grounds be assumed, takes place
in such wise fhat each gold particle is united with one
or several gelatin particles (which were present in large
excess), whereby the gold is protected against precipita-
tion by electrolytes.

Colloidal Gold and the Hydrogel of Alumina. As I
have already shown, 3 freshly precipitated alumina takes
the gold from a colloidal gold solution, and forms with
it a red colored lake, just as carmine lake is formed by

Ber. d. D. chem. Ges., Vol. XXXV, p. 3079 (1902).

'Ibid. Vol. XXXVII, p. 11 (1904).

1 Verh. d. Ges. D. Naturf ., Hamburg, 1901, pp. 172-198.

152 PRECIPITATION OF COLLOIDAL GOLD

shaking carmine solution with alumina gel. In order
to follow the process of union, levigated alumina gel
and colloidal gold were simultaneously introduced into
the ultraapparatus; the process is not easy to follow,
because it is almost completed before the fluid reaches
the observation cell; besides, the dazzling brilliancy of
the larger gold-laden hydrogel particles prevents one
from seeing the individual grains. Sometimes one may
see a bright shining gold particle attach itself to the
groups. More often the individual gold submicrons
unite with small gel particles which have already taken
up gold, to form stationary or moving systems.

Here, too, the union takes place in such a way that
several gold particles are taken up by one gel particle.
The gold united with the alumina is protected against
the addition of salt, just the same as that which is
combined with gelatin.

CHAPTER XIV
FILTRATION EXPERIMENTS

IN order to obtain some insight into the question so
important to bacteriology, as to the size of the filter
pores of clay fillers and filter cells, I made a number of
experiments in the filtration through such media, of
colloidal gold solutions having particles of known size.

For this purpose three different types of filters were
selected.

1. Filter cell, candle-shaped, according to Dr. Maassen.

2. Clay filter, bottle-shaped (ballon filter), according
to Pukall.

3. Chamberland filter cell.

As may be supposed from the speed with which water
filters fthrough, the first named has the largest pores,
while the Chamberland cell has very fine ones.

Without going into details, I may mention the important
factors on which such experiments are judged.

All three kinds of filters contain pores large enough
to allow the passage, of gold particles of about 30 /*// and
less. The pores of a cell are of very different sizes,
the Chamberland cell containing, for example, large
pores, which allow the gold particles to pass through,
and others which retain most of them. The size of the
pores is, however, not the sole criterion in filter experi-
ments. It is of especial importance in coarser filters,
whether the particles to be filtered are held to the

153

154 FILTRATION EXPERIMENTS

surface of the cell by adhesion or " adsorption " (A),
or not (B).

(A) In the first instance the substance to be filtered
gathers upon the outside surface (and to a certain
extent in the deeper pores), and prevents the other
particles from forcing their way through; first, because
the pores are made smaller; second, because the particles
held fast to the surface of the cell repel the freely moving
particles following the course of the current. 1

(JB) When adhesion or adsorption does not take place,
all colloidally dissolved substances pass freely through
the cell, providing the pores are large enough.

This latter case occurs, for instance, when a gold
hydrosol with particles 20-30 /*/*, and containing egg
albumen, is filtered through a Pukall cell or a Maassen
filter. All the gold particles pass quite smoothly
through the cell without appreciable change of concen-
tration. The experiments were made with colloidal
gold containing egg albumen. The first of this kind
were performed by Prof. Fr. N. Schulz at the Physio-
logical Institute at Jena, and I have repeated the
experiment several times with the same result.

If the egg albumen or similar protective colloid is
omitted, and the pure gold* hydrosol used, then matters
proceed as in case A, and very many gold particles are
attached to the exterior surface of the cell (even in
cells with large pores), and at once partially, and by
prolonged use, totally prevent the passage of the other
gold particles. Brodig first observed this in the Pukall
filter. I obtained at first a slightly diluted, but finally
a totally colorless filtrate; the outer fluid remained

1 This action may be due to the well-known negative electric charge
of the particles, which apparently also affects the adhering gold
particles.

FILTRATION EXPERIMENTS 155

unchanged at first, but when the surface of the cell
had accumulated sufficient gold, it became richer in
colloidal gold in proportion to the duration of the
experiment. 1

The experiment was made with all three filters,
yielding the same result, except that the very fine pored
Chamberland filter promptly gave a very diluted
filtrate.

In the ultraapparatus the filtrates showed the same
properties as ,the original fluid, but the dilution was
considerable, fewer particles being in the filtrate; the
concentrated exterior fluid contained, however, numer-
ous larger gold particles in addition to the original
ones.

The fact that protected gold particles of 30 /*/* and
over easily pass through Maassen and Pukall filters,
should be of interest to bacteriologists. The Chamber-
land filter, too, contains, besides the very small pores
chiefly present, others which permit the passage of
particles of the size mentioned.

In order to avoid misunderstanding I may repeat
that this experiment can give no direct measure of the
size of the pores, because the size of the gold particles
has been calculated on the assumption that they are
cubical, whereas we know nothing of their actual shape;
nevertheless a gold solution with protective colloid
added can be used as a relative measure of the size of
the pores of various cells.

Perhaps the above remarks may lead to the production
of better filters with uniform pores, for it does not

1 The course of filtration through Pukall cells observed by me, differs
somewhat from that described by Bredig, who obtained, upon filtra-
tion of his hydrosols, at first a colorless, then a colored, and finally,
colorless filtrate again.

166 FILTRATION EXPERIMENTS

appear impossible to fill up the larger pores with a
uniform fine-pored film permeable to fluids, by sucking
through coarser subdivisions of gold, platinum, or
silver.

CHAPTER XV

THE SIZE OF THE GOLD PARTICLES COMPARED WITH
THE SIZE OF OTHER BODIES

FROM the preceding sections it follows that the
particles in colloidal gold solutions are much smaller
than those of substances that have heretofore been
susceptible of direct observation; further, that the
size of the amicroscopic gold particles approximates the
hypothetical molecular dimensions. In order to show
this diagrammatically, I have prepared Plate III, com-
paring the approximate sizes of several microscopic
bodies l with those of the gold particles in colloidal
solutions all enlarged 10,000 diameters; in Plate IV
the latter are compared with the hypothetical dimensions
of tfie molecule, magnified 1,000,000 times. In Plate IV,
instead of the gold particles themselves, whose form is
not known, is represented, enlarged a million times,
the plane surface of the corresponding cube, which,
when filled with metallic gold of twenty specific gravity,
would contain the same mass as the gold particles in
the fluids Au 7 a, Au 9 a, etc., according to Table V, Chapter
IX. Even when enlarged 10,000 diameters, in Plate
III, the gold particles /, g, and h in the colloidal solution
appear as extremely minute points, while in the same

1 The figures in the table are meant to express pictorially the rela-
tive sizes, and by no means give a true illustration of the bodies in
question.

157

158 THE SIZE OF THE GOLD PARTICLES

magnification the particles of a blood, starch, or bacteria
suspension, assume quite considerable dimensions. 1

Unfortunately I can as yet give no information as
to the size of the particles in albumen and glue solutions,
and in the solutions of colloidal oxids and sulphids, for
although in many of these individual particles can be
made visible, we have as yet no satisfactory information
as to their size. As work with the ultraapparatus is
now being done in various quarters, it is to be hoped
that the desired goal in this direction will soon be
reached.

Of great interest are the observations already noted
by Raehlmann, 2 that the individual particles in glycogen
solutions can be made visible, and that their disappear-
ance upon the addition of diastase can be followed in
the ultraapparatus (see also Chapter XX); further, the
analogous observation of Romer 3 concerning solutions
of true proteins. That the particles made visible in
glycogen and protein solutions consist of the essential
constituents of the solution in question, appears to be
proven beyond reasonable doubt by their action toward
specific ferments. That we are dealing here with rela-
tively large particles is^ evident from their visibility by
arc light.

After this significant beginning we may watch with
interest the development of this newly-discovered field.
It would be a useful and important work for the scientists
who make discoveries in this field to determine closely
in each case the size of the particles they recognize.

1 The starch grains of rice are the smallest. Potato starch has
granules as large as 100 /z in diameter, and would therefore be one meter
in diameter on the scale of Plate III.

* Mttnchner medizin Wochenschr., 1903, No. 48.

* Berlin, klin. Wochenschr., 1904, No. 9.

THE SIZE OF THE GOLD PARTICLES 159

In many colloidal solutions the particles cannot be
made visible, which is the case with the colloidal solu-
tions of many metallic oxids. From this it might be
concluded that the particles in such solutions are smaller
than those in colloidal gold solutions having visible
particles. Such a conclusion would be very premature.
For the limit of visibility of ultramicroscopic particles
has been carefully determined experimentally only for
gold; it will vary from substance to substance, and with
most bodies be considerably higher than with gold.

The nearer the indices of refraction and dispersion
of the subdivided substance and the medium, the less
light the particles will diffract, and the larger the particles
must be in order to be ultramicroscopically visible.
The very appearance of the subdivision clearly shows
the difference between noble metals and oxids, etc.

Whereas many fluids appear almost clear, 1 despite the
relatively large particles contained in them, gold sus-
pensions with particles of 70-100 /*/* show an intense
turbidity. Gold and silver subdivided in water disperse
light more intensely than do most other substances of
the same particle-size. Subdivisions of both noble
metals are therefore especially suitable as test objects
in ultramicroscopy, because their individual particles
can be seen down to extremely small dimensions; but
even they, as they become smaller, gradually lose the
property of diffracting light this property almost vanishes
as they approach, or when they reach dimensions which
the molecular theory ascribes to molecules.

1 For example, combined suspensions of alumina and iron oxid
hydrogel, also of hydrogels of Prussian blue, Congo red, etc.

CHAPTER XVI

SUPERIOR AND INFERIOR LIMITS OF THE SIZE OF
THE PARTICLES

Superior Limit, The question as to the superior
limit of the size of particles in colloidal solutions should
be of some interest. I do not believe that this limit is
constant; it will depend upon the specific gravity of
the substance subdivided, upon the internal friction of
the fluid, and also with many hydrosols upon the electric
charge of the particles, and especially upon the manner
in which their space is filled with the mass of the sub-
divided substance. A densely filled particle will tend to
sink to the bottom quicker than one which is distended
in the filling. Bredig gives as the superior limit of the
size of the particles in the metal hydrosols made by
him, a linear dimension of 0.14 ^ that is 140 /z/*. 1

From Table V, Chapter IX, it appears, however,
that gold particles larger than 75 /*/* already begin to
settle, and up to the present I have not come across
any stable hydrosol whose average particle-size was
owr about 60 ////. On the other hand, in rare instances,
I could observe that even bright rod gold sols with
particles about 30 /JL/J. showed, upon standing perfectly
quiet in a room at 4-10 C. ; a discoloration of the super-
natant fluid.

Thus the superior limit for stable gold hydrosols can-

1 Bredig toorganische Fermente, p. 21.

160

SUPERIOR AND INFERIOR LIMITS 161

not be determined with accuracy; in any case it is
somewhat lower than Bredig has stated. The chance
observation above referred to, of the incipient separa-
tion of a red gold sol, was of great significance to me,
because it gave me the first indisputable proof that the
ultramicroscopic green gold particles are identical with
those which gave the fluid its red color; for the colorless
fluid was practically free from ultramicroscopic particles,
while the colored layer contained a large number.

The inferior limits of the size of the particles in colloidal
gold solutions cannot be experimentally determined
with certainty. Prom Table V (p. 132), Example 1-8,
it may be seen that there are a series of transition forms
between Au73 a having the smallest visible particles,
and the gold solution AuPie which, when freshly pre-
pared, is almost homogeneous; and it is evident that
the particles of this transition group are much smaller
than 6 pp. Therefore the size of the gold particles in
AuPi6 surely approach the dimensions of the crystalloid
molecule.

Additional questions are also worth discussing- Have
we, in gold solutions having the properties of the fluid
AuPie, reached the limit of divisibility of the metal,
or is there a still more complete solution of it? Color-
less, optically clear gold ruby-glass contains such a
solution. 1

1 This expression needs explanation. Spring had already demon-
strated the presence in colorless ruby glass of gas bubbles which destroy
its homogeneity. Besides there were frequently found ultramicro-
scopically large yellow-gold particles in the glass, which had separated
out during smelting if the glass was supersaturated. In some varieties,
little submicrons can be seen even in the colorless glass. Between
these incidental constituents, often scattered in widely separated sec-
tions, many kinds of ruby glass show mo further inhomogeneity. The
space between them appears clear, and, overlooking these incidental

162 SUPERIOR AND INFERIOR LIMITS

The investigations of Golfier, Bcsseyre, Knapp, Miiller,
Ebell, Spring, and others, have made evident, and Spring l
has also pointed out that in colorless gold ruby-glass
we have to do with an optically homogeneous solution
of metallic gold. To the objection which might perhaps
be raised by those unfamiliar with this field, that we
have here to do with the solution of an oxid or a silicate
of gold, I may, upon the basis of my entire experience
in the glass factory at Zombkowice, reply as follows:
Even in the presence of a considerable excess of energetic
reducing agents in glass, which certainly had removed
all oxygen from the melt and even reduced oxygen
present in the gold, upon sudden chilling colorless glasses
are always obtained, which when reheated become red.
I can therefore confirm the view held by Ebell, Spring,
and others, that metallic gold can be dissolved in glass
without either coloring it or defracting light. My efforts
to obtain a stable, colorless gold solution in water, remain
as yet fruitless; in glass, colorless gold solutions are
sometimes realized. I do not consider these as colloidal,
but essentially as supersaturated, crystalloid solutions
of the metal, which view is supported by the observations
on the solubility of metals in molten salts, made by
R. Lorenz and his pupils.

R. Lorenz 2 first discovered this phenomenon in the

constituents, we may speak of optically clear ruby glass. With other
kinds of ruby glass, between the incidental constituents there may
be seen an extremely faint whitish light-cone, which quite often is
visible only upon the most intense illumination with sunlight (in
June when the sun is in high altitude). We have here amicrons,
which must be considerably smaller than about 4-7 /*/*; such glasses
cannot of course be called optically clear.

J W. Spring, Bull, de I'Acad. roy. de Belgique (cl. d. sc.), 1900,
No. 12, p, 1017 and 1021 (3d section).

1 For details see R. Lorenz, Electrolysis of Molten Salts, Part II,
Halle a. S., W. Knapp.

SUPERIOR AND INFERIOR LIMITS 163

electrolysis of molten halogen salts of the heavy metals
(ZnCb, PbCb, CdQ 2 , etc.). In these cases there appears
about the molten metal which separates out at the
cathode, a cloud-effect which spreads through and
finally completely fills the molten electrolyte. This
phenomenon is easily produced by throwing into a
molten salt a piece of the easily fusible metal in question. 1

The formation of metal fog is dependent upon the
temperature, the fog appearing and disappearing in
the molten mass as the temperature falls and rises. At
higher temperatures it vanishes by solution, and the
melt assumes a most characteristic color; upon cooling,
on the contrary, it reappears. This solution and reap-
pearance of the fog may be repeated as often as desired
by alternately heating and cooling the melting pot.

In collaboration with- Helfenstein, 2 R. Lorenz has
attempted to find out the relative, so-called solubility
of metals in molten salts, and has obtained a series of
numerical results.

R. Lorenz 3 also determined, in collaboration with
G/Auerbach, 4 that a melt of cadmium chlorid "satu-
rated " with cadmium fog, after cooling and re-solution,
leaves metallic cadmium in magnificent little crystals.
According to R. Lorenz a far-reaching parallelism can
be traced between the formation of metal fogs and the
vapor pressure 5 of metals.

The greater the vapor pressure of the metal, the
greater should be its crystalloid solubility, and it may
well be assumed that at the fusing temperature at which

1 Zeitschr, f. Elektrochem., Vol. VII, p. 277 (1900).

2 Zeitschr. f. anorg. Chem., Vol. XXIII, p. 255 (1900).
8 Zeitschr. f . Elektrochem., Vol. II, p. 318 (1895).

4 Zeitschr. f . anorg. Chem., Vol. XXVIII, p. 42 (1901).
* Zeitschr. f . anorg. Chem., Vol. XXIII, p. 97 (1900).

164 SUPERIOR AND INFERIOR LIMITS

glass melts (over 1400 C.), a small quantity of gold
undergoes crystalloid solution. This solution can be
very much overdone, for it is supersaturated l even at
the working temperature of the glass and sometimes at
ordinary temperature after quick chilling.

That colorless ruby glass, too, is probably filled with
amicrocsopic gold particles which act as centers of
crystallization, will be brought out in the next chapter.

1 At this temperature the gold separates out in the form of ultra-
microscopic particles, often upon slow cooling, almost always upon
reheating, but not upon quick cooling.

CHAPTER XVII
^MICROSCOPIC NUCLEI IN COLORLESS RUBY GLASS

IF molten colorless gold ruby glass is allowed to cool
slowly, according to its quality, it either becomes red l
upon cooling, or else remains colorless. If quickly
chilled to a set, ruby glasses of all grades remain color-
less. Colorless ruby glass, as Spring has shown, may
be without effect upon light. The normal red color of
the ruby glass is almost always brought out by reheating
it to the softening point; the color-change thereby
produced is technically known as " coloring the ruby
glass " (Anlaufen des Rubinglases). After coloring, part
of the metallic gold which is homogeneously dissolved
in the colorless glass, separates out in the form of ultra-
njicroscppic particles which reflect green light. 2

i. Gold Particles in Ruby Glass 3

The gold particles in ruby glass, like those in colloidal
solutions, are cither submicroscopic or amicroscopic,
according to circumstances. Poorly made, spoiled ruby-
glass, upon coloring, yields a blue or violet shade instead
of red. Below are described the phenomena which may

1 Glasses very rich in lead, or containing certain impurities, are
thereby colored yellow or brown instead of red.

2 See Siedentopf and Zsigmondy, pp. 19 and 30.

1 For further information regarding the size, color, and polarization
ofc gold particles in ruby glasses, see Siedentopf and Zsigmondy, loc.
cit., pp. 30-38.

165

166 AMICROSCOPIC NUCLEI IN COLORLESS RUBY GLASS

be observed in good and in spoiled ruby glass, if both the
glasses are heated at only one spot. With reference to
the following observations it must be noted that the glasses
under investigation were slowly cooled, and that they were
heated up more quickly than they had previously been
cooled.

1. A piece of colorless ruby glass which had been
very slowly cooled, was heated at one spot, so that n
began to melt at a (see Fig. 6), and still remained cold
at d. At a the glass became intensely red, the red
color decreasing toward c\ at d the glass remained
colorless.

. . ' FIG. ft. : : . '

2. Spoiled ruby glass becomes blue at a, the color
decreasing toward c\ at 6 it is violet, at b' bright red,
and at d colorless.

In the ultraapparatus using homogeneous immersion
and brightest sunlight, the following was seen:

Sample 7, good ruby glass. At a and b there were
submicroscopic specks of green appearance and very
close together; at b' a homogeneous green light-cone,
becoming weaker as c is approached. 1 This light-cone
is produced by amicroscopic gold particles. In so far
as the individual particles can be traced, their average
distance from each other is the same as at a, only toward
V their brightness considerably diminishes.

1 With insufficient illumination the light-cone appears homogeneous
even at a. It is not alone sufficient that the distances bet ween 'the
particles be resolvable; individual particles must also be bright
enough to be seen.

ON SPONTANEOUS CRYSTALLIZATION 167

Sample //, spoiled ruby glass. The individual particles
are much brighter and much further apart than in
sample I; they are differently colored, being copper-
red at a, shading off to yellow, and they are green where
the glass is red to transmitted light. Individual parti-
cles can be recognized even beyond c, and they are, on
the average, about the same distance apart, that is,
they are just as far apart at a as at b' and c. The bright-
ness of the reflecting disks diminishes from a towards
&', c, and d. At d faint specks of indefinite color may
still be distinguished, but not at all points. 1

2. On Spontaneous Crystallization

All these phenomena are easily explained, if one recalls
the analogy existing between the formation of ruby
glass and the devitrification of amorphous substances.

Devitrification of a chilled melt is known to consist
of an interior crystallization of the isotropic mass. It
is f a general rule that in supersaturated solutions crystals
can be formed only if centers of crystallization (nuclei),
which may either be added or spontaneously formed,
are present in the solution. Lowitz, Gay-Lussac, Lowel,
Violette, Gernez, and especially Ostwald, deserve credit
for a series of excellent researches on this subject. 2

Among other things Ostwald determined the smallest
quantity of crystals necessary to produce the crystalliza-
tion of supersaturated solutions or supercooled melts,

1 The above figure is diagrammatic and gives only an approximate
idea of the entire process. In Example II the temperature gradient
upon warming was greater. Therefore the distance a d, as may be
imagined, is much smaller than in Example I. Certain imperfections
in glasses, such as air bubbles, cloudiness, etc., are not described in
order- to avoid prolixity.

2 Ostwald, Lehrb. d. allg. Chem., 2d ed., II, 2, pp. 704-784.

168 AMICROSCOPIC NUCLEI IN COLORLESS RUBY GLASS

and found it to be 10- 9 to 10 ~ 12 gms. in the cases he
investigated.

Recently Tammann 1 has made some very interesting
contributions to our knowledge of the process of devitri-
fication.

According to Tammann, the spontaneous crystalliza-
tion of a supercooled melt depends upon two factors:
First, its ability to crystallize spontaneously, as meas-
ured by the number of centers of crystallization formed

Melting Point

Decreasing Temperature -
FIG. 7.

in a unit mass of the fluid per unit of time; second,
upon the speed of crystallization.

Upon examining organic melts 'Tammann further
found that the number of centers of crystallization per
unit of time and weight increased at first as the super-
cooling increased, but quickly diminished again at
temperatures considerably below u the melting point.
But the number of centers of crystallization formed can
be determined only when they have grown to visible
size. In cases of excessive supercooling Tammann
accomplished this by subsequently raising the tempera-
ture high enough to increase the speed of crystallization.

The process for making red ruby pressed glass is

1 Zeitschr. f . Elektrochem., 1904, Vol. X, p. 532.

SPONTANEOUS CRYSTALLIZATION IN RUBY GLASS 169

quite similar. The glass is first cooled and then heated
to redness.

In addition to the power to crystallize spontaneously
(sp KV) and speed of crystallization (KG), Tammann
holds that the viscosity (>?) of the supercooled melt
also influences its behavior. I reproduce from his paper
a diagram (Fig. 7), illustrating the dependence of these
three properties on the temperature.

The relative position of the three curves is not always
the same as .here illustrated, for they differ with
different substances; the diagram shows, however, that
the speed of crystallization and the ability to crystallize
spontaneously, increase with diminishing temperature
and then decrease again, while the Viscosity increases
steadily.

3. Spontaneous Crystallization in Ruby Glass

The working temperature of colorless molten gold
ruby glass is several hundred degrees lower than the
lemperature at which it is melted. If we conceive it,
at the working temperature, to be a supersaturated
crystalloid solution of metallic gold, and the smallest
amicroscopic particles of gold to be centers of crystal-
lization, we can apply Tammann's results to the con-
ditions existing in gold ruby glass. It will at once be
seen why ruby glass sometimes remains colorless upon
simple cooling, and only becomes red when heated to
the softening point. In this case the optimum temperature
for spontaneous crystallization is so low that the glass is
very viscous and the speed of crystallization reduced to a
minimum. The nuclei formed by cooling can no longer
grow in the almost solidified glass and either remain
hidden or lie at the limit of visibility. If by heating
the glass acquires a certain mobility, the gold in solution

170 AMICROSCOPIC NUCLEI IN COLORLESS RUBY GLASS

separates out upon the nuclei present, which by growth
become submicrons visible in the ultraapparatus, the
glass turning red or becoming darker in color.

That none, or but few nuclei are formed at the
higher temperatures at which these centers rapidly
increase in size, is evident not only from the fact that
the glass examined remained colorless and optically
clear 1 upon slow cooling, but also from the fact thaat
the mean distance between the particles in the reheated
glass was just as great at a as at b 2 and c. If new
nuclei had been formed during reheating at temperatures
favorable to the development of the original nuclei
into submicrons, these new nuclei would have developed
into submicrons in the strongly heated portion of the
glass (at a) and there would be more particles per unit
volume at a than at b or c. But the number of particles
per unit volume was approximately the same in all the
variously heated parts of the glass.

If, as in the case in question, the ruby glass is very
slowly cooled and then quickly heated again, it is self-
evident that most of the gold nuclei must have been
already formed during the cooling. Colorless, slowly
cooled gold ruby glass contains one part of its gold,
therefore, in supersaturated solution, and the rest in
the form of nuclei so small that they affect the homo-
geneity of the glass only slightly or not at all. Upon

1 The higher temperatures at which the crystals grow quickly are
slowly passed by in cooling the molten glass. If nuclei had been
formed at these temperatures they would have certainly developed
into visible bodies. In the pieces examined this was only partially
the case. With other ruby glasses the temperature for spontaneous
formation of nuclei is much higher; such glasses color up red upon
slow cooling.

a In Sample I this was only indistinctly recognizable; in Sample
II, distinctly.

SPONTANEOUS CRYSTALLIZATION IN RUBY GLASS 171

subsequent reheating these nuclei serve as centers of
growth for separating out the gold dissolved in the glass.
The hotter the glass is kept, 1 the more speedily they
grow at the expense of the gold in (crystalloid) solution.
Therefore (the time of heating being the same) the
largest particles are to be found at the places most
strongly heated. 2 The gold will continue to separate
out until the supersaturation for that particular tem-
perature and glass composition has disapperaed.

Colorimetric tests have shown that in gold ruby glasses
under ordinary working conditions, only part of the
total gold (about half), separates out in the form of
ultramicroscopic particles and serves as a color-producing
constituent, the balance remaining in homogeneous
solution.

At still higher temperatures the crystalloid solubility
of the gold is considerably greater, and by melting at
1350-1400 C. it is therefore easy to convert red ruby
glass into the colorless form, which can again undergo
Ahe process of coloring.

Application to Technical Observations (made in actual
practice). The value of this view of the coloring of
ruby glasses is shown incidentally by the fact that it
simply and naturally explains certain heretofore incom-
prehensible observations made in the course of their
manufacture.

Thus it was observed that the rim of a piece of pressed
ruby glass remained colorless, while the middle became
red. In the press the rim was more quickly cooled
than the middle, and by subsequent reheating was
heated more quickly and to a higher temperature.

1 But only to a certain limit of temperature, as will soon be seen.
1 This heating should not be carried so far that the glass becomes
fluid, otherwise the separated gold will redissolve.

172 AMICROSCOPIC NUCLEI IN COLORLESS RUBY GLASS

Investigation showed that the rim contained a much
lesser number but considerably larger green gold particles
than the rest of the glass. In the light of the above
remarks these facts are easily explained as follows: The
rim o the glass had very quickly passed through the
optimum temperature for the formation of nuclei, so
that only few were formed; subsequent heating to a
higher temperature therefore led to a speedy growth
of these nuclei into particles of 110-145 /*/*. The middle
section of the same glass had time enough to form a
large number of nuclei, which, being less strongly
heated, grew more slowly. 1

In spoiled ruby glass the formation as well as the
growth of nuclei is disturbed; fewer nuclei are formed
and these grow more slowly than in good ruby glass.
In this case I think that instead of simple crystals either
crystal-druses or sphero-crystals are formed; if every
individual little crystal is conceived to act as a resonator
for light waves, such crystals would be able to influence
each other differently in the crystal-druse than in simple
crystal formation, and thus by displacing the maximum
of spectral resonance, produce a change of color in the
glass and the diffracting disks. (See Chapter XII 2 .)

Difference between Devitrification and Ruby-glass
Formation. From what is stated in the present chapter,
it is evident that the phenomena which appear during
the coloring of ruby glass can be explained by consider-
ing the analogy between the process of coloring and
that of devitrification, which was closely studied by

1 The relations are not always as here given ; we are considering
here only the case where the temperature for the maximum value of
*p KV. is lower than for the maximum value of KG.

* In many kinds of ruby glass the color change is due to a con-
glomeration of the little submicrons into larger ones , in such cases the
process is exactly the same as in the coagulation of colloidal gold.

SPONTANEOUS CRYSTALLIZATION IN RUBY GLASS 173

Tammann. But the coloring of ruby glass should not be
confounded with ordinary devitrification.

The difference between the two is that in the formation
of ruby glass several milliards of nuclei exist and grow
in a cubic millimeter, and that the hypothetical little
crystals, of whose form and structure we know nothing,
are so small that their presence changes only the visible
appearance and not the working properties of the glass;
whereas, in the case of devitrification, relatively few
nuclei are present and lead to the formation of quite
large crystals, which prevent the usual working up of
the glass.

Regarding the size of the smallest gold nuclei hardly
anything can at present be stated; for in order to apply
the method used for determining their size, an idea
must first be formed as to the mass per unit volume
of the gold separated out into nuclei. That only a
small fraction of the total gold can be present in this
form is evident from the fact that most of the gold has
the property of depositing upon the centers of growth;
but how large this fraction is cannot yet be exactly
determined. The next question i&, if colorimetry is
applicable here; its applicability must first be demon-
strated. 1

Perhaps the intensity of the diffracted light will allow
us to form an idea of the size of the particles, providing
some relation may be found between their size and their
diffracting power.

Preliminarily I might state that the superior limit
of the mass of the smallest submicroscopic gold particles
is from 1.2 to 6.8X10" 15 rng. (indicating a linear dimension

1 My recent work on amicroscopic gold nuclei give very promising
indications of the applicability of colorimetric methods (Nov. 1, 1908),

174 AMICROSCOPIC NUCLEI IN COLORLESS RUBY GLASS

of from 4-7 ju/0, 1 and that they were found in the medium
heated portion of a piece of glass which had been heated
at one end, as described in the case of good ruby glass
on page 166. In the less strongly heated portion of
this glass no individual particles could be recognized,
but the amicrons present give rise to a light-cone which
became weaker as the slightly heated portions of the
glass were approached, and finally vanished where the
glass was colorless when the opening of the slit was small.
Upon opening the slit wide enough and using the brightest
sunlight, even here there could be seen a faint but dis-
tinct whitish polarized light-cone.

In the sphere of amicrons, therefore, there exist
numerous transition forms between the smallest sub-
microns and the smallest nuclei. If the former consist
of several thousand atoms, 2 it is probable that the
latter contain only a small number of gold atoms.

Recapitulation. The preceding remarks lead to the
assumption that at ordinary temperatures colorless gold
ruby glass contains gold in two different forms; most
of it is present in supersaturated crystalloid solutions,
but part exists as nuclei which at higher temperatures
serve as centers of growth, and are so small that their
presence influences the homogeneity of the glass slightly
or not at all. 3

1 Siedentopf and Zsigmondy, loc. cit.

2 The mass of 1 .2 to 6.8 X 10"~ 15 mg. indicates 8 to 42 thousand atoms,
if the absolute atomic weight of gold is taken as 196X8.2XlO~ 22 .
This, however, is the major limit.

1 In very great thicknesses, which cannot be practically realized,
even colorless ruby glass would presumably show some color, just as
colorless air appears colored in thick layers. (See p. 7 and Spring,
loc. cit., note 1, p. 162).

CHAPTER XVIII
GENERAL REMARKS CONCERNING METAL HYDROSOLS

i. Protective Action of Water

NASCENT gold reduced in water exists in homogeneous
subdivision, just as vapor of gold does at first in a very
hot tube. Neither in the tube nor in a beaker of water
is this subdivision stable at ordinary temperatures;
in each case crystallized masses form if the metal is in
sufficient concentration.

While in the case of gold vapor we have no way of
maintaining the original state of subdivision, we do
know of methods which, although not always satisfac-
tory, produce approximately this effect if the gold is
educed in fluid media.

One of these methods consists in the very extreme
dilution l of the mixture employed for reduction.

From concentrated solutions of gold chlorid well-formed
macroscopic crystals can sometimes be obtained upon
the reduction of the metal. Bearing in mind this ten-
dency of gold towards crystallization, it is evident that
the more dilute the fluid and the more centers of cry-
stallization formed in the fluid, the smaller the crystals

1 As brought out in Chapter VIII, the colloidal impurities of water
usually tend to prevent the formation of metal solutions, and must
therefore be jemoved as carefully as possible. The above considera-
tions lead to the result that the formation of a metal hydrosol can
also take place without the presence of foreign colloids.

175

176 REMARKS CONCERNING METAL HYDROSOLS

will be. If the reduction t is effected in very extreme
dilution, so that the whole of the chlorid is almost
instantaneously transformed into metallic gold, such
centers would be simultaneously formed in enormous
numbers, and the entire metal content of the fluid
consumed either in their formation or in their growth;
in the former case there would be obtained an approxi-
mately homogeneous fluid with amicroscopic particles;
in the latter, a fluid containing amicrons or submicrons,
according to the size of the crystals formed.

In fact, every drop of a colloidal gold solution of the
latter variety contains milliards of individual particles,
indicating the presence of an extremely large number
of nuclei during reduction; still much larger must be
the number of gold particles and nuclei in the gold
solutions which are almost homogeneous. That these
particles cannot settle out is evident from the fact that
the commencement of sedimentation is observed where
the particles are 30 /*JM; suspensions that settle completely
contain particles 80-200 pp. and still larger in size. (Stje
Chapter XVI.)

We can conceive the solutions of colloidal metals to
be fluids teeming with such tiny hypothetical crystals. 1
But it would be a grave error to carry the comparison
too far and to apply ,to these extremely fine subdivisions
of the substance the results obtained with ordinary
suspensions of gold crystals (which do not influence
each other). I refer to Chapters I and II, wherein some
of the differences are mentioned; I fully believe that

1 1 must here expressly state that I lay no particular stress upon
this idea, which would be abandoned if it could be replaced by a better
one. It has, however, often been of use to me, and has enabled me
to regard things, otherwise difficult of comprehension, from a broad
point of view. (See Chapter XVII.)

PHOTECTIVE ACTION OF WATteH 177

a world of wonders would be opened up to us if we could
observe the actual structure of the particle itself, and
its action upon its immediate surroundings.

Graham's remark that in colloidal solutions there
resides activity/ is especially applicable to colloidal
metal solutions. More so than all others do they undergo
irreversible changes of condition, changes irreversible
in the fullest sense of the word, for all means which, with
other colloids, can be successfully used to fluidify the
hydrogel formed by coagulation, fail in the case of
coagulated pure metal hydrosols. The greatest tendency
of these tiny metal particles is to unite into larger
complexes, and a variety of trivial causes are sufficient
to bring about this union. Upon such union there
occurs a much more complete separation from the sur-
rounding medium than with other colloids. This com-
plete separation from the fluid and union of the particles
with each other involves a considerable liberation of
heat. 2 No actual hydrogel is formed, but ametal-
spouge or metallic powder. And even the relatively
large particles, of which these latter consist, have not
exhausted their tendency toward union. With gold,
slight pressure with a polishing iron is sufficient to
convert the dried powder into coherent metal, quite
in contradistinction to dried hydrogels, which for the
most part are friable.

We have characterized the colloidal metal solutions
as fluids which, among other things, differ from equally
concentrated suspensions of the same metal by their
larger energy content. 3

1 See Chapter III, p. 34.

2 See Prange's experiment, p. 15.

8 The development of heat upon coagulation is explained by the fact
that the living force with which the individual particles hurl themselves

178 REMARKS CONCERNING METAL HYDROSOLS

It should be remarked that coagulation (in the absence
of protective colloids) always begins if the individual
particles are brought near enough to each other. It also
commences upon the addition of electrolytes whether
such addition increases the electropotential difference
between the individual particles and the medium (accord-
ing to Billitzer) or not.

All these facts, together with the impossibility of
preparing metal hydrosols from powdered metal in a
purely mechanical way, are evidence that with con-
centrated colloidal metal solutions the process of coagu-
lation must be self-contained, involving the performance
of a definite quantity of work, and therefore being asso-
ciated with a decrease in the free energy.

I consider the attractive forces between the individual
particles to be the actuating cause of this process,
forces of whose intimate nature we know nothing with
certainty, but which are identical with those which oppose
the mechanical separation of the united particles (forces
of cohesion).

If this idea is correct, the next question is what causes
the relative permanence of the dilute metal hydrosols.

It is safe to regard three causes as of importance in
producing the relative permanence of such hydrosols:
First, the proportionately large distance between the
individual particles in dilute solutions, which is unfavor-
able to their meeting each other. Second, the films
of water which immediately surround the particles and
are probably held by them with more or less force. 1

upon each other, becomes heat upon their conglomeration. In the
absence of complications, we can calculate therefrom the mechanical
work supplied on the average by one particle upon coagulation.

1 Regarding the pressure of fluids which are absorbed by porous
substances, see Lagergren, Zeitschr. f. anorg. Chem., 1900, Vol. XXIII,
p. 323. Barus and Schneider also assume aqueous envelopes.

PROTECTIVE ACTION OF WATER 179

Third, the electric charge of the ultrarnicrons, which
Hardy already regarded as of importance in producing
the stability of the irreversible hydrosols. 1

Finally, the addition of protective colloids materially
increases the stability.

The influence of the first cause above referred to is
shown by the fact that the change in color of colloidal
gold solutions indicative of coagulation and produced
by the addition of salt, is retarded if the distances
between the particles is increased. 2 On the contrary,
if the distances be decreased, coagulation begins spon-
taneously.

That attractive forces exist between the particles
and water is evident from the fact that metals are

1 Billitzer has already explained this on the sound basis that either
anions are given up by the particles or cations are taken up by them.
(Bredig had previously expressed a similar view.) If one considers
that the formation of colloidal gold proceeds much more smoothly in
neutral or faintly alkaline solutions than in slightly acid ones, and
reflects that in the former class of solutions the number of hydroxyl
ions is vastly greater than in the latter, the idea is easily formed that
the charge of the particles is due to hydroxl ions being taken up by
adsorption. The assumption that the alkali or another electrolyte, or
else some trace of impurity in the water, is absolutely necessary for
the formation of a hydrosol, does not seem to me to be of much sig-
nificance; for the purest water contains in itself all the conditions
necessary to give the hydrosol permanence. True protective action
by electrolytes is unknown with colloidal metal solutions. On the
contrary, the smallest quantities almost always produce coagulation.
Furthermore the assumption of a "finest dust" for condensation
nuclei appears to me to be superfluous, for if gold is reduced (or
vaporized by the electric arc) in a medium in which, as a crystalloid,
it is so difficultly soluble as in water, extreme dilution itself will yield
all the conditions necessary to the formation of centers of crystalliza-
tion. But this does not mean that many foreign substances hi almost
molecular subdivision may not act as nuclei. Perhaps phosphorous
proposed by Faraday for reducing gold, or else some of the oxidation
products of phosphorous, assume such a function.

3 See Kirchner and Zsigmondy, Drude's Ann., 1904, Vol. XV, p. 591.

ISO REMARKS CONCERNING METAL HYDROSOLS

Betted by water and also that pieces of metal take up
ivater from the air, and thus increase in weight. The
view that the particles of the surrounding fluid medium
which are united with the metallic particles by attractive
forces, oppose the union of the latter, is, among other
things, supported by an experiment of Barus and Schneider,
showing that at critical temperature colloidal silver
precipitates from alcohol just as do clay particles from
superheated water (Wiedem. Ann., 1893, Vol. XL VIII,
p. 335). Thus when fluid and vapor become identical,
the protection exercised by the fluid on the metal
disappears.

Freezing, the addition of electrolytes, etc., produce
SL gathering of the particles which cannot be made
retrogressive. In short, any circumstance which removes
DF to a certain degree lessens one or several of the before-
mentioned influences that give the metal sols their
stability, will cause the coagulation of the metal hydrosol,
wid a complete theory of coagulation cannot therefore
stllow for only certain of these causes individually, but
must take them all collectively into consideration.

Regarding coagulation of hydrosols by electrolytes,
it should also be noted that the addition of salt produces
lot only a gathering but also an electric discharge of
the particles, so that according to Hardy the coagulum
is isoelectric with the surrounding medium; further-
more, according to Picton and Linder, Spring, Whitney
md Ober, Billitzer, and others, there are taken up by
the precipitated hydrogel a certain number of such ions
rf the precipitating salt as have an opposite charge to
the particles.

As Quincke has shown, 1 we must also take into con-

1 Quincke, Drude's Ann., 1902, Vol. VII, p. 95 (paragraph 5). Para
praphs 3 and 4, advanced by Quincke as a basis for explaining the

PROTECTIVE ACTION OF WATER 181

sideration the eddies produced in the fluid by the addition
of electrolytes, for they cause the particles to dash
together in 'a most powerful manner. 1 This influence
can be shown to be unimportant by the fact that alcohol
and other non-electrolytes produce just as strong eddies
and even increase or diminish the difference of electric
potential between the particles and the medium, but
effect no immediate coagulation of the metal hydrosol.

On the other hand, it must be pointed out that such
addition may .exercise a protective action, slight or
passing though it may be. Thus Blake 2 ascribes to
ether a protective action on colloidal gold.

According to what precedes a protective action 3 can
be also ascribed to pure fluid water, which prevents
the metal particles from forming larger complexes and
makes possible the production of colloidal metals. The for-
mation of a colloidal metal solution can be conceived of as
a condensation process interrupted in its very incipiency.

It should be noted that electrolytes in general favor
the separation of matter subdivided in water, not only
bringing about the precipitation of metal hydrosols
and of mechanical suspensions, but also diminishing
the solubility of non-electrolytes (such as ether, phenyl
thiocarbonate, 4 etc.). 5

turbid solutions of mastic, clay, and oleic acid, are not well applicable
to the case in question.

1 Freundlich has found that solutions of arsenic sulphid are much
more indifferent to a very slow addition of barium chlorid than to a
sudden addition. Zeitschr. f. phys. Chem., 1903, Vol. XLIV, p. 143.

2 Blake, Am. Jour, of Sci., 1903, IV (16), p. 435.

8 No definite idea should be ascribed to the term "protective action,"
which simply expresses the fact that a union of the ultramicrons is
prevented, which, without the presence of water, would doubtless take
place at ordinary temperatures.

<Rothmund, Zeitschr. f. phys. Chem., 1900, Vol. XXXIII, p. 401;
Biltz, Zeitschr. f. phys. Chem., 1903, Vol. XJLIII, p. 41.

6 This action of electrolytes, which extends to crystalloids as well

182 REMARKS CONCERNING METAL HYDROSOLS

2. The Finest Subdivisions of Gold

In AuPie we have become acquainted with a gold
solution with amicroscopic particles whose dimensions
cannot be much larger than those which the kinetic
theory of gases ascribes to the molecules of crystalloid
solutions; in fact it is still an open question whether
the discrete particles in gold solutions of the best kind
must be necessarily larger than those assumed to exist
in crystalloid solutions of substances having high molecular
weight.

That actual discrete particles are to be assumed in
the latter follows unequivocally from the research
referred to in the Introduction, especially that of Lobry
de Bruyn and Wolff, 1 and van Calcar and Lobry de
Bruyn. 2

Barus and Schneider have expressly stated that it is
not necessary to assume an allotropic modification of
silver in colloidal solutions of this metal,, and that it is
simpler to hold to the normal molecular weight (which
with metals is for the most part the same as the atomic
weight). 3

On page 288 of their article they state:

"The question therefore arises if, under favorable
conditions, there may not exist suspensions in whose
particles are united 1000, 100, 10, or even still fewer

as to colloids, appears to have its cause not so much in the action of
the ions in uniting the particles, but rather in a change in the rela-
tions of the medium to the substance subdivided in it, a fact pointed
out by Faraday. In the sense expressed by Donnan (Zeitschr. f.
phys. Chem., 1901, Vol. XXXVII, p. 741) this may perhaps be ex-
pressed by saying that r < 1.

l Notel, p. 6.

2 Note 3, p. 6.

1 Chapter III, p. 59.

THE FINEST SUBDIVISIONS OF GOLD 183

molecules. In extreme instances such mixtures would
be difficult to distinguish from a true solution, and in
colloidal solutions, we think, we have to do with such
extremely fine subdivided sediments. "

Apart from the expression "suspensions " and "sedi-
ments," which do not appear to be at all appropriate
terms for such fine subdivisions, 1 Barus and Schneider
have expressed about the same idea that I have formed
with regard to gold solutions with amicroscopic particles.

If we imagine a metal solution dissolved in water
consisting of an enormous number of metal particles,
each of which consists of but few atoms, it is an open
question whether, in their totality, these particles could
exert osmotic pressure. If we assume that this property
is lacking we would have before us an ideal case of col-
loidal solution. Since it would have no osmotic pressure
("the driving force ") 2 it would be incapable of diffusion
arid be devoid of the property of passing through mem-
branes. Because of the high tendency toward union
possessed by their particles, trivial causes would lead
to their coagulation; in short, they would have the
characteristic properties of typical, irreversible hydrosols.

Such substances would be regarded as heterogeneous
in the sense of the phase rule; their particles wouljl
grow in a supersaturated solution of the same metal,
just like centers of crystallization. (Chapter XVII.)
Furthermore, the particles in them would be hardly
larger than those in crystalloid solutions; in both, the
optical inhomogeneity would almost or entirely vanish.
There would therefore be possible an ideal colloidal solu-

1 Instead of the expression "true solution," used by Barus and
Schneider, it would be better to say crystalloid solution. See also
pp. 1-10.

1 See Nernst, Theoretische Chem., 3d ed., p. 384.

184 REMARKS CONCERNING METAL HYDROSOLS

tion combining with its own essential characteristics the
homogeneity of a crystalloid solution. It cannot with
certainty be stated whether this ideal limiting case has
been realized, but colloidal solutions of the type AuP 16
closely approach it, with the exception that the dimen-
sions ascribed to molecules actually lie between 0.1
and 1 ///.

The study of the finest subdivisions lying on the
border line between crystalloid and colloid solutions,
would doubtless be of great interest. It might be antici-
pated that upon the transition of the crystalloid solution
into a colloidal one containing the smallest amicrons,
there would be a sudden change in certain properties
(such as osmotic pressure, behavior with reagents, etc.),
whereas other properties (such as the optical homogeneity)
would change only imperceptibly or not. at all.

Biltz and Gahl (Chapter XX), have investigated two
analogous cases and observed a sudden change in optical
homogeneity upon the separation of sulphur and selenium
from their crystalloid solution.-

The cause of this, according to my experience, is that
there had spontaneously formed in the crystalloid
solutions of these elements, relatively fewer nuclei than
in the finest subdivisions of gold; in the very much
supersaturated solution these nuclei had grown with
extreme rapidity. With gold solutions it is within our
power to permit the simultaneous formation of an
extremely large number of nuclei, which soon exhaust
the supply of gold and remain extremely small. In such
cases the ultraapparatus is incapable of showing the
transition from crystalloid to colloid solution, because
the optical homogeneity of the fluid is not perceptibly
altered by this process.

ACTION OF PROTECTIVE COLLOIDS 185

3. Action of Protective Colloids

We must differentiate two kinds of action exercised
by protective colloids: First, protective action upon
finished metal hydrosols free from appreciable quantities
of foreign colloids; second, protective action in the
production of colloidal metals. Regarding the first
kind of protective action, the most important facts have
been stated in Chapter III and XIII; 1 in. addition it
might here be mentioned that even protective colloids
having the same electric charge as the metal to be pro-
tected, exercise a very efficient protection, and that
the origin of the protection of the gold can be most
simply explained by the assumption that specific attrac-
tive forces bring about a union of the ultramicrons of
metal and protective colloid, even if both carry like
electric charges. That a union actually occurs I have
been convinced of several years ago by experiments
which cannot be discussed until later.

It has also been stated that in observing certain
relations in the "gold figure/' we can obtain a relative
measure of the protective action upon a certain metal
hydrosol.

The second type of protective action, which is closely
related to but not identical with the former, comes into
play in the production of water or alkali-soluble col-
loidal metals. They make it possible to obtain very
concentrated, water-soluble colloidal metals, which,
because of their appreciable content of foreign colloids,
may be regarded as analogues of the purple of Cassius,
or else as colloid compounds. (Chapter III.)

Here belong the processes for the production of the
purple of Cassius, of colloidal metals according to Carey

1 Pages 81-84 and pp. 150-152.

186 REMARKS CONCERNING METAL HYDROSOLS

Lea, Lottermoscr and Paal, as well as Mohlau's process
for making colloidal indigo.

As to the action of protective colloids in the formation
of metal hydrosols, only hypotheses can at present be
advanced.

It seems to me not improbable that the amicrons
of the protective colloid take up by adsorption a part
of the dissolved metallic salt, and that the reduced
metal then "remains united with the amicrons in such
fine subdivision that the homogeneity of the fluid is
not affected notwithstanding the reduction of the metal.
(Some years back I was thus enabled, by reducing gold
chlorid in the presence of stannic acid, to obtain purple
of Cassius whose solution in ammonia diffracted just
as little light as the ammoniacal solution of stannic
acid by itself. A faint, polarized light-cone was dis-
cernible in both cases).

It can also be assumed that the metallic salt is not
adsorbed, but that the reduced metal is taken up by
the amicrons of the protective colloid. Finally it appears
to be a general property of protective colloids to pre-
vent or interfere with the growth of tiny little crystals.
Other less probable assumptions are also possible, but
it would take too long to recount them.

Only one thing must be referred to: The protective
action is not absolute; the protective colloid is incapable
of preventing the incipient condensation of the metal,
which would have to be the case in order to obtain
stable colorless metal solutions analogous to quickly
cooled ruby glass. 1 The protection is quite often so
perfect that the metal particles which separate out do

1 In individual cases temporarily colorless gold solutions may indeed
be formed; by indicative measurements the subsequent reduction
could be easily determined.

ACTION OF PEOTECTIVE COLLOIDS 187

not appreciably increase the inhomogeneity of the
solution.

The reactions of metal solutions made in the presence
of protective colloids, are chiefly determined by those
of the latter; if, however, the metal is present in large
excess, its tendency to combine into larger particles
comes into evidence, influencing the reaction accprdingly.

CHAPTER XIX

ULTRAMICROSCOPIC EXAMINATION OF CERTAIN
SOLUTIONS AND SUSPENSIONS

To supplement what has been said in the preceding
chapters, there are here given some of my observations
on various colloidal solutions and suspensions, and also
on solutions of dyestuffs.

Professor Bredig of Heidelberg was kind enough to
place at my disposal the following four hydrosols of
definite concentration prepared according to his method,
for which I herewith tender him my sincere thanks.

Bredig's Gold Hydrosol. The gold hydrosol examined
was a fairly clear fluid, purple or violet-red to transmitted
light, with a brownish diffuse dispersion. It contained
0.0053 grm. of gold in 100 grms., and T1) W normal
NaOH. The fluid contained red, yellow, and green
particles; the yellow ones present in the smallest num-
ber were usually the brightest, while the red and green
ones, present in about equal numbers, exhibited various
degrees of brilliancy. All of the particles, however,
were considerably brighter than those in my bright
red colloidal solutions, for instance Au 9 o, Au 95 , etc.,
and despite their active motion had smaller free paths
than those in the latter. The amplitude of the trans-
latory motion was generally smaller than 4 /*. Because
of the great difference between the particles, their mean
size (42 /*//), determined by measuring the spaces between
them, is naturally of little value. The very brightest

188

CERTAIN SOLUTIONS AND SUSPENSIONS 18$

particles could be poparated by filtration through very
heavy filter paper. By comparing their brightness with
that of gold particles in ruby glasses and in gold hydrosols
already examined, the size of the particles in Bredig's
preparation was placed at 20-80 ////.

Bredig's Colloidal Platinum. A yellow-brown fluid
exhibiting slight turbidity to transmitted light (platinum
content J 0.0034 grm. in 100 cc.). At least four different
grades in the sizes of the particles were determined. 2
In contradistinction to the gold particles the platinum
particles are not strongly colored, but are all white or
rather gray-white, . with tinges of yellow or blue. I
could recognize no particles with decided red or green
color. Although the smaller particles, far outnumbering
the larger ones, were much fainter than those in the
gold hydrosol previously described, a calculation based
upon the distance between them gave as their mean
size 44 /*//, which indicates that in fine subdivision plati-
num diffracts less light than gold, the size of the particles
being about the same. Feb. 9, 1904.

Bredig's Palladium Hydrosol. (0.0050 grm. Pd in 100
grms.) After long standing (which yielded no sediment),
this blackish-brown fluid appeared almost clear. By arc
light could be seen single bright white particles with
tinges of yellow, blue, etc., and a light-cone due to
invisible particles. With the use of sunlight the smaller
particles could also be seen; they were not in active
motion, and showed no decided color.

Bredig's Colloidal Silver Solution. (0.0038 grm. Ag in
100 grms.) A yellowish-brown fluid showing a gray
cloudiness to transmitted light; it can hardly be imagined

1 The content of this and other colloidal metal solutions were
kindly given me by Prof. Dr. Bredig.

2 By comparing the brightness of the diffraction disks.

390 ULTRAMICROSCOPIC EXAMINATION OF

what a beautiful color play this unsightly fluid offers
upon ultramicroscopic examination. Blue, violet, yellow,
green, and red particles of various shades and rare bril-
liancy of color, move in endless array. One particle
approaches another, circles it in a rapid zigzag motion,
and is then dashed away again; sometimes one follows
another almost in contact but without reaching it.
Sometimes several group together and dance like gnats
in a sunbeam, especially when, for the fraction of a
second, one particle approaches the other. The fluid
was examined in various degrees of dilution. Often the
influence of one particle on another could be seen even
at a distance of 2-4 /it. The linear 'dimensions of the
individual particles was about 50-77 /*/<.

Silver Hydrosol+NaCL. The introduction of sodium
chlorid solution to the silver sol in the cell immediately
produced a whirling together of the particles which
made it impossible to observe the process of coagulation.
Upon equalization of the differences of concentration by
diffusion streams, the considerably larger particles can
be seen quietly floating. Sometimes two of these
approach each other, but rarely can the process of the
union be observed.

Colloidal Silver made According to Carey Lea. Dr.
Lottermoser of Dresden was kind enough to send me
some colloidal silver made according to the method of
Carey Lea, a rather concentrated solution containing
1.276 per cent Ag. Upon dilution this dark brown fluid
showed the same splendidly colored picture as Bredig's
hydrosol; brought to the same concentration, both
fluids would present about the same appearance. 1

1 This statement refers to arc light. With sunlight there could be
seen in Lottermoser's preparation still much smaller particles in very
active motion.

CERTAIN SOLUTIONS AND SUSPENSIONS 191

A solution of Argentum Cred6 (solid colloidal silver),
obtained more than four years ago from the Heyden
Chemical Works, contained somewhat larger and less
variegated particles.

Hydrosol of Mercury, The hydrosol of mercury which
was kindly placed at my disposal by Dr. Lottermoser, 1
also contained numerous brightly shining, active particles.
Unfortunately I neglected to make an immediate thor-
ough examination of this preparation, which is interesting
in several respects. As I was about to take up the com-
plete examination after several weeks standing, I was
sorry to find that it had coagulated.

Colloidal Ferric Oxid. Two solutions of ferric oxid
of different origin were examined; in concentrated
solution (1-5 per cent) both showed a very intense
polarized bluish light-cone, which became fainter upon
further dilution, and in which (besides the diffused light)
slow moving individual particles could also be recognized.
One of the fluids (A) appeared heterogeneous in a con-
centration of 0.001-0.0005 per cent; the other (B)
remained apparently homogeneous until the light-cone
vanished.

According to this result it seems to me very probable
that the solution of ferric oxid, in addition to large
individual particles easily made visible, contain chiefly
amicrons whose size should be between those of the gold
solution AuPia 2 and those of a gold solution of medium
subdivision.

As Biltz 3 has found, colloidal ferric oxid and colloidal

1 A. Lottermoser, "On Colloidal Mercury," Jour. f. pr. Chem.,
N. F., 1898, Vol. LVII, p. 484.

2 In which the light cone was much less distinct than in the iron
solution of like concentration.

3 Biltz, Ber. d. chem. Ges., Vol. XXXVII, pp. 1095-1116.

192 TJLTRAMICROSCOPIC EXAMINATION OF

gold mutually precipitate each other. I therefore mixed
solution B with AuP*. Before mixing, individual parti-
cles were not to be seen in either, but a short time after
mixing there could be seen a large number of bluish-
green individual particles, having a brilliancy equal to
that of gold particles about 15 /* in diameter, but a much
less active motion.

Hydrosol of Silver lodid, A solution of silver nitrate
(0.2 mg. AgN0 3 in 100 cc.) was mixed with a very
dilute solution of KI. The immediate result was a
perfectly clear fluid in which could be seen a blue polar-
ized light-cone. At first no individual particles were
visible, but after a long time extremely small particles
having active motion could be seen, appearing as blue
diffraction-disks. April 7, 1902.

After ten days the particles became distinctly visible
by sunlight, their active motion persisting. Calculated
from the measurement of the distance between them,
the approximate mass of these individual particles was
10" 14 mg.

Lottermoser l has also succeeded in producing con-
centrated and quite stable solutions of silver ioclid. Dr.
Lottermoser was kind enough to send me some of his
preparation for examination; in this yellowish-white,
milky hydrosol, the particles have already grown to
considerable size. They are so closely crowded in the
concentrated fluid that it is a wonder that so many
particles can continue to exist alongside of each other.

The concentrated fluid, Lottermoser states, contained
0.98 grms. silver iodid in 100 cc. In it were seen num-
berless white particles, rushing past each other as in a
surging throng. One extremely important observation
should be noted: isolated dust particles and threads,
1 Lottermoser, Jour. f. pr. Chem , 1903, VoL LXVUI, p. 341.

CERTAIN SOLUTIONS AND SUSPENSIONS 193

sometimes stellated, were surrounded by a ring-shaped
(or spherical) space, optically transparent, into which
no particles of silver iodid could force their way.

Upon further dilution the color of the individual
particles became evident; they were mostly bluish-
white, some being yellow-white and some greenish-white.
Average size of particles about 60 /*/*. April 7, 1902.

lodin Suspension. An alcoholic solution of iodin was
poured into water, until a permanent turbidity was
produced. Extremely bright, shining particles could be
seen, some showing slight Brownian motion, others
not.

As soon as the particles reached the focus of the light-
oone, they were dashed back from it as from a racket,
in the direction of the light rays. 1 This phenomenon
may perhaps, like the negative photodromy of Quincke, 2
be attributed to the unilateral heating of the suspended
iodin. April 2, 1902.

Barium Sulphate Suspension. These were produced
svith dilute solutions of H 2 S0 4 and BaCl 2 in varying
proportions.

Only larger particles were visible, floating quietly and
shining brilliantly.

The crystalloid solubility of BaSO 4 appeared to be
too great to permit of the preparation of a colloidal
solution. Either larger crystals are formed at the very
outset, or else perhaps certain crystals grow at the
expense of the smaller ones incidentally formed, in analogy
with the rule stated by Lord Kelvin, 3 that in vapors the

1 Upon repeating this experiment two years later I could no longer
discover the conditions under which the iodin particles were dashed
away.

2 Quincke, Drude's Ann., 1902, Vol. VII, p. 86.

1 Lord Kelvin, Proc. Roy. Soc. Edinburgh, 1870, Vol. VII, p. 63.

194 ULTRAMICROSCOPIC EXAMINATION OF

large drops grow at the expense of the smaller ones. 1
April 4, 1902.

Silver Chlorid Suspension. Lottermoser, in his paper
on colloidal silver halids, 2 remarks that the hydrosol
of silver chlorid cannot be obtained according to the
method described by him for the preparation of silver
halids: even when very dilute solutions are used, the
resulting fluid is bluish colored to transmitted light, and
deposits AgCL This statement I can entirely confirm.
The precipitate yielded by very dilute sodium chlorid
with silver nitrate solution, showed a strongly polarized
bluish light-cone produced by larger particles having
slight Brownian movement. Several experiments made
in April, 1902, gave the same result. In one instance
the distance between the particles was about 10 /x, the
amplitude of the motion of translation about 1 /*. A
hydrosol can no- more be obtained in this case than in
the case of BaS04, apparently for the same reason.
April 7, 1902.

Turbid Solution of Gamboge. The very bright particles
exhibit the slow, trembling motion characteristic of the
Brownian movement; and to show that the motion of
the gold particles is of a quite different nature, it is best
to mix the two fluids. While the gamboge particles
have a free path of 1-2 //, 3 that of the gold particles is
10-20 fi and over. April 29, 1902.

Carmine. Levigated commercial carmine exhibited
bright red shining particles with bui little or no motion.

Solution of Carmine Dye in Ammonia (0,2 per
cent carmine). Much smaller particles 6-10 t u apart

1 According to Ostwald (Lehrbuch. d. allg. Chem., 2d ed., II, p. 757)
a solution may be supersaturated as to larger crystals, but hypersatu-
rated as to smaller ones.

3 Jour, prakt. Chem., N. F., 1903, Vol. LXVIII, p. 343.

* In freshly prepared solutions the motion is somewhat more active.

CERTAIN SOLUTIONS AND SUSPENSIONS 195

show Brownian movement: A = 1-2 // (in this and all
subsequent cases A denotes the amplitude ol the motion
in question). The light-cone between them cannot be
resolved. The smallest particles are not visible, as I
already stated in 1902. l April 2, 1902.

Gelatin Solution. A 0.2 per cent gelatin solution
two days old appeared heterogeneous. The fluid was
entirely filled with small whitish particles at the limit
of visibility; along with them were visible much larger
hydrogel particles. In a 0.01 per cent gelatin solution
the distance between these free moving particles was
less than 1 /r, upon further dilution the microscopic
picture became indistinct because of the predominance
of the diffuse light arising from still smaller particles. 2
March 2(5, 1902.

Soluble Starch. 3 A 0.01 per cent solution showed
by sunlight a distinct but faint diffuse polarized light-
cone, together wth few brighter particles. March 14,
1902.

Further remarks regarding soluble starch are to be
found in Chapter XXI.

Rice Starch. When levigated in water isolated
starch grains, illuminated by the light rays, shine bril-
liantly, but exhibit no appreciable Brownian movement.
April 2, 1902.

1 Zsigmondy, Zeitschr. f. Elektrochem., 1902, p. 686.

'Upon repeating this experiment in the spring of 1904, using a
gelatine 'solution several days old which had been prepared at boiling-
point, the light-cone appeared homogeneous, and the individual
particles could not be seen. The solution of March, 1902, was prob-
ably prepared at medium temperature, and not at boiling-point. As
I have already shown (Zeitschr. f. analyt. Chem., 1901, Vol. XL, p. 714),
under these conditions there is formed a less perfect colloidal solution
of gelatine.

8 This preparation was kindly given me by Prof. Lintner, of Munich,
to whom my best thanks are due.

196 ULTRAMICROSCOHC EXAMINATION OP

Starch Paste. In diluted starch paste were to be
seen: First, very large and extremely brilliant clumps.
Second, numerous smaller particles showing slight
Brownian movement. Third, finally upon extreme dilu-
tion a distinct light-cone which could be almost entirely
blotted out by turning the Nicol. This light-cone
became much more distinct after the large suspended
particles settled, and upon further dilution assumed a
heterogeneous appearance. lodin colored it blue with-
out facilitating the view of the individual particles.
April 2, 1902.

Cigarette Smoke. Gases containing floating parti-
cles can be examined in our apparatus just as well as
fluids. Cigarette smoke, which can easily be blown
into the apparatus through the funnel-tube, exhibits
an especially beautiful appearance. By closing the
pinch-cock the air enclosed is protected from the motion
of the outside currents. The smoke, even when it was
very thick, contained particles 10-20 p. apart, which
danced about in the elastic medium with an extraordi-
narily active motion, and as soon as they were struck
by the direct sunlight shined most brilliantly. Even
their oscillations had an amplitude of 10 /* and over;
their motion of translation was considerably more
extensive. April 29, 1902.

Fluorescent Dyestuffs. For the sake of complete-
ness I must here mention some experiments made in
collaboration with H. Siedentopf, having for their
object the examination of solutions of strongly fluor-
escent dyestuffs in the ultraapparatus.

Fluor esc ein. A solution of this dyestuff, diluted to
1 : 1,000,000, showed quite an intense light-cone. 1 It
was still visible when this solution was diluted a hundred

1 The depth of the image of the slit was 1-2 /*, its breadth 3-6 /*.

DERTAIN SOLUTIONS AND SUSPENSIONS 197

fold, and finally vanished gradually upon further dilu-
tion. (As a matter of course a Nicol prism was added
as an analyzer, whenever necessary, in order to observe
the fluorescent cone alone without interference with the
colloids in the water.) Sometimes a kind of cloud for-
mation could be seen, but so indistinct that no con-
clusion can be drawn from this phenomenon. Several
repetitions of the experiment led to the same result, even
when objectives made entirely of quartz and fluorspar
were used for illumination. April 10, 19, and 23, 1902.

The very strongly fluorescent aescorcein, which had
been kindly placed at our disposal by Prof. 0. Lieber-
mann l of Berlin, for which we here express to him our
sincere thanks, showed under the same conditions a
distinct light-cone with a content of 2.5. 10~ 7 mg. per
cubic centimeter. Upon further dilution the light-cone
disappeared.

Tetraiodofluorescein behaved in the same manner;
several other non-fluorescent dyestuffs were still less
^isible in solution.

There is, therefore, but little prospect, with the present
arrangement of apparatus (especially with the use of
arc light), of being able to see the molecules of ordinary
dyestuffs which are dissolved as electrolytes and diffuse.
particularly if they exhibit slight or no fluorescence.

On the other hand it is very likely that in the colloidal
solution of dyestuffs 2 the individual particles can be
seen, if they are large enough; and it is not impossible
that the molecules of fluorescent dyestuffs can be made
visible, providing they are very large and sufficiently
fluorescent.

1 Liebermann and Wiedermann, Ber. d. D. chem. Ges., 1901, Vol.
XXXIV, p. 2611.

2 Benzopurpurin, for example. (See note, p. 206).

CHAPTER XX

ULTRAMICROSCOPIC INVESTIGATIONS FROM THE
PUBLICATIONS OF OTHER SCIENTISTS

Some Investigations of Raehlmann. I have above
made mention of Raehlmann's * very interesting obser-
vation that in the ultraapparatus a large number of
Rubmicrons can be soon in a solution of glycogen, also
their disappearance under the influence of diastase.
Raehlmann was also able to identify albumin particles
in the urine of nephritic patients; in this connection
he has pointed out that the distance between the indi-
vidual particles in urine containing originally one per
cent of albumin can be measured only after very extreme
dilution (1 : 40,000), and that at a dilution of 1 : 500,000
individual particlos can still be seen in the solution.

Earlier publications of Raehlmann describe the ultra-
microscopic picture of certain dyestuff suspensions and
dyestuff solutions and their mixtures. 2 Raehlmann con-
siders the new method of illumination as of especial
advantage because it permits particles to be seen in
their own color, thereby facilitating the differentiation
between particles arising from different dyestuffs. He
examined incidentally suspensions of chrome yellow and
ultramarine, also the colloidal solutions of Prussian

1 Raehlmann, loc. cit. (Chap XV) and fieri, klin. Wochenschr.,
1904, No. 8.

2 Ophthalmolog. Klinik, 1903, No. 16; Ber. d. D. physik. Ges., 1903,
Vol. V, pp. 330-339; Physikal. Zeitschr., 1903, Vol. IV, pp. 884-890.

198

PUBLICATIONS OF OTHER SCIENTISTS 199

blue, carmine,- etc., and described in detail the ultra-
microscopic picture of these fluids and their mixtures.

Raehlmann also examined the solutions of the electro-
lytes, methyl violet and naphthol yellow, and discovered
ultramicroscopic particles even in them.

Reference must here be made to the ultramicroscopic
investigation of Raehlmann on the constituents of the
blood. 1

Investigations of Much, Romer, and Siebert. Inter-
esting ultramicroscopic investigations were conducted by
Much, Romer, "and Siebert at von Behring's Insti-
tute. 2 In order to obtain a comparative measure for
the number of ultramicroscopic particles, these investi-
gators diluted their fluids to such an extent that only
3-4 ultramicroscopic particles wore present; the figure
expressing the dilution thus obtained was termed the
ultra-value. In this manner were examined numerous
sera, albumin solutions and wheys.

Just as Raehlmann had been able to see a large number
of individual particles in the diluted urine of nephritic
patients, 3 these investigators also found individual par-
ticles in albuminous urine, their number varying with
the severity of the illness, ITore, as well as in the fol-
lowing experiments, the ultra-value served as a com-
parative measure of the number of submicrons. In
Table VII which follows, are arranged the results of the
examination of the urine of healthy persons and of
nephritic patients.

The coincidence between the ultra-value and the
results of the boiling test is in every case remarkable,

1 Deutsche media. Wochenschr., 1904, No. 29.

2 Much, Romer, and Siebert, Ultramicroscopic Investigations.
Zeitschr. f. diat. u. physik. Therapie, 1904, Vol. VIII, pp. 19 and 94.

* Raehlmann, loc. cit.

260 ULTRAMICROSCOPIC INVESTIGATIONS FftOM THE

and leads to the conclusion that the greater part, if
perhaps not all, of the albumin in urine is in the form
of ultramicroscopic particles.

TABLE VII

Sample.

Reaction on Boiling.

Ultra-value.

I .

o

1-25

II

1:60

Ill

coarsely floccul6iit ppt

1 5000

IV

(diabetic urine)

1 * 10 bright light-cone

v

1-25

VI

slight turbidity

1-400

VII
VIII
IX

coarsely flocoulent ppt.
(diabetic urine)
opalescence

1:10,000
1:30 bright light-cone
1-200

X

coarsely flocculent ppt.

1 ' 15,000 f After removal! 1

25

XI

* i

i- 7,000 \ e f ^ bumin Mi

5

XII

( i

' uvv ] boiling with ( A
1 : 10,000 I acetic acid j 1

35

XIII

1-50

(Normal urine, ex-
clusive of portion
voided in morning)
XIV

1:20

(Same; 4 o'clock,
P.M., after heavy
meal)

XV

o

1:20

(Same: 11 o'clock,
P.M., after light
supper)

With this new method of determining albumin care
must be taken, because other constituents of the urine,
for example the phosphates, which sometimes separate
out, may also be present in a similar form. 1

Much, Homer, and Siebert have, in addition, deter-
mined the ultra-value of different sera and colloidal
solutions, which are repeated in Table VIII.

Also of interest are the experiments on the digestion
of globulin from horse serum by pepsin and hydro-

1 The authors themselves have referred to the influence of the rnucin
content.

PUBLICATIONS OF OTHER SCIENTISTS

201

TABLE VIII

2-4 particles were counted in one field of view of the
ultra-apparatus when the fluid was diluted

? Times.

Proportion-
ate Figure.

1. Horse sera:

100000

1

Dora serum

80,000

4/5

Ida serum ....

60 000

^/^

old tetanus serum. Ballon la . .

350 000

3 5

2. Fresh antitoxic horse serum . .'

300,000

3

3. 10 per cent globulin solution from horse
serum

20 000

1/5

4. Milk serum

800000

g

5. 10 per cent solution of Witte's peptone . . .
6. 10 per cent atmidalbumose from horse
serum

2,000
2 000

1/50
1/50

7. Marburg nutritive bouillon

250

1/400

8. Urine in slight albuminaria

500

1/200

9. Urine in severe albuminaria

20000

1/5

10, 10 per Cent gelatin solution

4 000

1/25

11. 10 per cent agar solution

10000

1/10

12. 10 per cent solution of laundry soap . . .

200000

2

13. 10 per cent mucin solution . . .

300000

3

chloric acid/ and of fat-free milk (ultra-value 750,000),
by hydrochloric acid pepsin and pancreatin. A series
of remarkable experiments was inaugurated with milk
freed from fat, and milk freed from casein by filtration
(whey or lactoserum). The wheys of a cow highly
immunitized to tuberculosis exhibited bactericidal prop-
erties against bacterium coli. These wheys were sub-
jected to electrolysis and an examination made of the
anode, cathode, and intermediate wheys. It appeared
that the anode wheys showed very powerful bactericidal
and agglutinative properties and a high ultra-value,
whereas, as may be 'seen from the following table, 2 the

1 After incubating for one-half hour the ultra value decreased from
100,000 to 100.

2 The spore-destroying and agglutinative action referred to is indi-
cated by + + +, when less effective by + +, when very slight by +,
and when absent by 0.

202 ULTRAMICROSCOPIC INVESTIGATIONS FROM THE

cathode wheys wore almost free from ultra-particles
and entirely free of bactericidal properties.

TABLE IX

JVirterif iclnl
Properties

Agglutinative
P.operties.

Ultramicroscopio
Examination.

Anode wheys

+ 4- +

+ + +

1:4000

Cathode wheys

'

1:50

Intermediate wheys .
Non-elect rolyzed wheys . .

+ ~0

1:3400
1:200

Some Remarks on the Preceding. I cannot here
enter into details regarding the work, and return to the
method of determining the ultra-value only in view of
more recent work in this field. By determination of
the ultra- value the authors obtained mutually com-
parable figures which gave an approximate idea of the

relative number of submicrons contained l in the fluid.

Unfortunately in this work, which was certainly care-
fully carried out, there are lacking the facts necessary
to reach a conclusion as to the absolute number of
particles in a unit volume, from which could be obtained
an idea of the major limit of the size and the mass of
the particles. Such information would certainly have
been of general interest and would have made possible
comparison with the work of other investigators.

By "field of view " fr evidently meant only that part
of the field of view directly illuminated, . for only in this
do the ultramicroscopic particles become visible. The
dimensions of the illuminated space can, however, be
arbitrarily chosen, and vary according as the light-cone
is defined, from front to rear, 2 and according to its breadth
and its depth.

1 Or perhaps formed by dilution.

2 If the field of view of the eyepiece is taken as a front-to-rear limit,

PUBLICATIONS OF OTHER SCIENTISTS 203

The value of all future work in this field would be
considerably enhanced if the facts as to the number
of particles in a given concentration (or else as to the
ultra-value) were based upon a certain predetermined
illuminated volume of the fluid. This problem would
be much simplified for investigators if eye-pieces pro-
vided with a suitable Ehrlich diaphragm l were com-
mercially procurable; 2 using this, together with definite
tube-length and a definite objective, there could, to
begin with, be marked out of the light-cone a disk of
definite size, having a surface or area of 100 or 200/* 2
for instance. The operator has then only to take care
that this disk is completely illuminated, and that the
light-cone is correctly limited as to depth. By such an
arrangement thr errors in the determination of volume
would be reduced to the errors in the determination of
the depth, which are not important factors, because for
the most part they are smaller than the errors in the
determination of the number of particles.

If the investigators who use the ultraapparatus would
take a little care to determine the mean number of
particles in a volume of the fluid measured off as care-
fully as possible, not only would the results of the
different experimenters be mutually comparable, but
there would also be obtained data for reckoning the

which as a matter of fad; is very disadvantageous, the length of the
visible portion of the light-cone changes with the nature of the eyepiece
used.

1 Catalogue of C. Zeiss, Mikroskope, 1903, p. 97.

2 Or a diaphragm with several different large openings so arranged
that one can be easily substituted for the other. In fact, H. Siedentopf
has recently introduced an eyepiece with checkerboard divisions, which
is well adapted for the purpose of counting (Description of the Appa-
ratus for Rendering Visible Ultraniicroscopic Particles, Note 3, p. 97).
To count submicrons approaching the limit of visibility, I should
recommend, however, the eyepiece diaphragm referred to.

204 ULTRAMICHOSCOPTC INVESTIGATIONS FROM THE

approximate size of the particles or their major limit,
which information is of greatest importance in the
further investigation of colloids.

Furthermore, I consider it dangerous to dilute so far
that only 3-4 particles are visible in the whole field,
because then the dust particles incidently present in
the water used for dilution are also counted in. It is
more advantageous to dilute only to such an extent
that about 1-6 particles are present in a small, exactly-
defined volume (of about 400-1000// 3 ). (See remarks
on the determination of size.) For dilution, of course,
only such fluids must be chosen as produce no precipi-
tative action on the colloidally dissolved substance.

Investigations of Michaelis. From a preliminary
communication of Leonor Michaelis, 1 which has recently
appeared, we quote the following. Michaelis subdivides
dyestuff solutions according to their behavior in the
ultraapparatus into:

"(i) Dyes Totally Resolvable Optically. Their
aqueous solutions even upon extreme dilution, exhibit
ultramicroscopically numberless granules. Here belong:
First, the aqueous solutions of many heavy molecular
sulphoacid dyes, as indulin, violet-black, aniline blue;
second, certain pseudo-solutions whose suspension-like
nature is probable, even without the ultramicroscope,
though it cannot be demonstrate^; for example, (a)
many dyes which, although their aqueous solutions cer-
tainly do not belong to this class, exhibit the optical
phenomenon of complete granular resolvability, if they
are dissolved in aniline water even in a great degree of
dilution (fuchsin); (6) a dilute solution of. scarlet in
alcohol, which is then diluted with five to six parts of

1 Deutsche medizin. Wochenschrift, 1904, No. 42, and Virchows
Archiv., 1905, Vol. CLXXIX, pp. 195-208.

JWBLICATtONS OF OTHER SCIENTISTS. 205

water. This procedure precipitates the dye, which of
itself is not soluble in water, but it remains in ultra-
microscopic suspension; (c) a dilute solution of fuchsin
in hot, saturated NaCl solution, upon cooling becomes
violet or blue to transmitted light, without yielding a
precipitate. This solution can be totally resolved opti-
cally. After twenty-four hours, the dye generally sep-
arates out in flocks.

" (2) Dyes Partially Resolvable. These also show
granules in the ultramicroscope, but a certain concentra-
tion must be reached before a large number is seen.
The dye is contained in the solution in two phases, which
bear a functional relation to each other; the first
being a solution showing nothing in the ultramicroscope,
the second being in a form optically resolvable. Here
belong the aqueous solutions of fuchsin, methyl violet,
and others.

" (3) Dyes Completely Unresolvable, but fluorescent.
They exhibit an optical inhomogeneity which is macro-
scopically recoginzed as fluorescence. But with our
present methods this inhomogeneity can by no means
be resolved into . granules even ultrarnicroscopically.
Here belong the aqueous solutions of fluorescein (already
examined by Siedentopf and Zsigmondy), eosin, toluidin
blue, Nile blue, rnethylene blue."

The fluorescence observed by Michaelis in methylene
blue I can confirm from previous observation.

From the second paper of Michaelis, already cited, I
abstract the following remarks:

Bavarian blue also belongs to the completely resolvable
dyestuff solutions. Even upon extreme dilution, num-
berless granules can be recognized in its solution.
Although this dye consists of sulpho-acids, and it may
be supposed to enter into solution, dissociated into ions

206 ULTRAMICROSCOPIC INVESTIGATIONS FROM, THE

like an electrolyte, nevertheless its aqueous solutions
act as hydrosols.

In the second class, in addition to the solutions of
fuchsin and methyl violet before referred to, belong
the solutions of neutral red, Capri blue, and picric acid.
Michaelis is of the opinion that the granules visible in
them are not due to an impurity but to an integral
constituent of the solution. He bases this opinion on
the fact that particles microscopically visible are too
profusely and too regularly distributed in the fluid to
be regarded as an impurity. 1

1 Tliis view is supported by observations made on solutions of gly-
cogen, benzopurpurin, and others. The solutions of benzopurpurin,
for instance, show with sunlight sometimes a light-cone resolvable
into a large number of bluish-green particles, and at other times a
light-cone of the same color, due to amicrons. A freshly prepared
solution of this dye-stuff appears turbid because of large particles. In
a dilute solution of fuchsin which had stood long enough I could
see but few particles, and therefore considered the particles seen by
other investigators to be impurities. Solutions of glycogen at the
beginning show in the ultra-apparatus more ultramicrons than upon
standing. (Raehlmann, Gatin-Gruzewska, and Biltz.) J. Lemanis-
sier observed the same thing with haemoglobin solution. (L'dtude des
corps ultramicroscopiques, Paris, 1905, Jules Rousset). The solution
of haemoglobin at first shows a large number of submicrons, but after
forty-eight hours hardly a particle is left. According to Lemanissier
this change is not due to a chemical alteration of the haemoglobin.

These observations indicate that with many substances the course
of solution is a double one, crystalloidal solution occurring simul-
taneously with a colloidal solution, which for its part may consist
either of submicrons or amicrons. From this it is quite evident that
determinations of molecular weights with aqueous solutions of fuchsin,
methyl violet, etc., made by Krafft (Ber. d. D. chern. Ges., 1889,
Vol. XXXII, p. 1612), would lead to a higher molecular weight than
determinations with solutions of the same substances in alcohol.
With these substances the disappearance of ultramicrons may perhaps
be due to a gradual transition from colloidal into crystalloidal solution.
With other substance there seems to be a splitting up of submicrons
into amicrons, according to the idea of Donnan (Chap. XXI). As may
be seen, there are still numerous unsettled questions regarding the

PUBLICATIONS OF OTHER SCIENTISTS 207

It is known that the acid dyes in general have but
slight affinity for cell nuclei. An exception to this are
those entirely resolvable optically. These dyes can be
used as nucleus-stains, and in addition possess the
property of staining evenly. According to Michaelis
it is a rule that those dyes which in aqueous solution
have the tendency of separating out in granular condi-
tion, are the very ones which are adsorbed most readily
and in least specific manner by all kinds of organic
substrates.

Michaelis further points out that in albumen solutions
the granules do not form the whole mass of the albumen,
but only a part of it; further, that a different number
of particles are obtained according as water or physio-
logical salt solution is used for dilution. The insolubility
of globulin in pure water is very well known, as well as
the ready partial coagulability of albumen solutions
upon mechanical agitation.

That tho coagulated particles, before combining into
larger ones, first appear as submicrons, is the more
evident, because albumen, globulin, etc., are excellent
protective colloids, 1 which tend to prevent the formation
of visible precipitates. These circumstances suggest care
in judging facts ultramicroscopically observed, and,
together with the observation, made five years ago, that
outside of a few suspended dust particles and a light-
cone due to amicrons, crystallized albumen shows noth-
ing, they have led me to exclude albumen solutions
from my sphere of work; for until a scientific basis for
the method of examination was determined, complications

aqueous solutions of many crystalloids, which can be decided by ultra-
microscopy assisted by the application of physico-chemical methods,
(Note added while in press).
1 See Schulz and Zsigmondy, note 3, p. 82.

208 ULTRAMICROSCOPIC INVESTIGATIONS FROM THE

might easily arise. But this should by no means halt
further research. The investigations of Raehlmann,
Much, Romer, Siebert, and Michaelis have made evident
the fact that submicrons are to be met with more fre-
quently and in larger numbers than^had formerly been
thought, and they may play an important r61e in life
processes which arc involved in the mutual reaction
between colloidal solutions and hydrogels.

Investigations of W. Biltz. A research carried out
by W. Biltz in collaboration with W. Gahl, and reported
by Biltz, 1 illustrates how an insight may be obtained
into solutions with amicroscopic particles by means of
careful ultramicroscopic examination. First are given
some precautions regarding the preparation and handling
of distilled water to be used for solution and dilution.
With both investigators ordinary distilled water yielded
unsatisfactory results; for example, 1.6 particles in
0.00004 mm. 3 By combining Spring's precipitation
method with careful distillation, they were able, how-
ever, to obtain water almost optically clear. By care-
ful filtration through a Pukall filter, there was also
obtained water suitable for most uses.

I can certainly confirm the fact that water can be
rendered unsuitable for ultramicroscopic uses by stirring
it with ground-glass stoppers or filtering it through felt.
On the other hand, I have had no unsatisfactory experi-
ence with rubber tubes, if they are cleaned by constant
washing with water which must now and then be allowed
to run through them. 2 For closing flasks containing

1 Gdttinger Nachrichten, math-phys., Section 1904, No. 4. Con-
tributed by W. Biltz.

9 See p. 112. I have worked much by sunlight with fluids containing
almost altogether amicroscopic particles, and was disturbed only a
few times by the bad quality of the water. The reason why in my

PUBLICATIONS OP OTHER SCIENTISTS 209

solutions to be ultramicroscopically examined, Biltz
recommends cork stoppers covered with tin foil.

The main subject of the investigation are two reac-
tions: first, the decomposition of thiosulphuric acid;
second, the formation of selenium from selenium dioxid
and sulphurous acid, both reactions, involving the sep-
aration of a solid from a solution originally homogeneous.
They next undertake to elucidate the phenomena of
delay in the action of acids on thiosulphate investigated
by Landolt, Foussereau, Hollemann, von Oettingen, and
others. As we know, when thiosulphate solutions are
acidified, it is some time before the fluid becomes turbid.
Foussereau has shown that the decomposition of the
acid begins the moment it is formed, and the retardation
of the visible reaction is to be attributed to a delay in
the sulphur separating out.

It may be assumed that the sulphur is in crystalloidal
solution, or that it is colloidally dissolved, in which case
the increasing formation of sulphur must gradually
change the colloidal solution into a distinct suspension.
According to the former view, supported by Ostwald,
it is a case of a supersaturation phenomenon, which
within a short time is broken up; according to the latter
view there is at first formed a microscopically imperceptible
colloidal solution, whose particles in time grow large
enough to be seen with the naked eye. In the former
case the process must be continuous; in the latter,
discontinuous.

With the help of the ultramicroscope Biltz and Gahl
were able to prove definitely the discontinuity of the
process. For several minutes the microscopic picture

investigations especially pure water was needed only in exceptional
cases, is because in examining my fluids I generally used a volume
fifty to one hundred times smaller than Biltz and Gahl.

210 ULTRAMICROSCOPIC INVESTIGATIONS FROM THE

remained unchanged, then suddenly appeared an increas-
ing turbidity, with the formation of submicrons.

The ultramicroscopic investigation of the second
reaction referred to, illustrates how much difference
there is between macroscopic and ultramicroscopic ob-
servations. While in the case of the action of sul-
phurous acid upon selenious acid a turbidity can be
macroscopically recognized only after about thirty min-
utes, upon ultramicroscopic examination, submicrons can
be seen separating out in as soon as two minutes and
twenty seconds, and this separation considerably increases
within the next few seconds. Here too the discontinuity
of the process indicates that there is at first formed a
supersaturated crystalloid solution of selenium, whose
supersaturation is broken down with the formation of
submicrons.

An interesting research l of Mme. Z. Gatin-Gru2ewska
and W. Biltz deals with the test of the observation of
Raehlmann above referred to, that glycogen solutions
are filled with ultramicroscopic particles. A particularly
pure preparation was made by Z. Gatin-Gruzewska in
Pfliiger's laboratory, and with the observance of all
precautions was dissolved in distilled water, which had
been purified according to Biltz' method; the solution
was examined in the ultraapparatus. By the use of this
preparation, Raehlmann's results could also be confirmed,
for the 0.07 per cent solution of glycogen showed numer-
ous extremely small white particles having an oscillatory
motion. The 0.007 per cent solution also showed a dis-
tinct light-cone as well as submicrons. With a con-
centration of 1:300,000, contrary to Raehlmann's
observation, no more than 1-2 particles were to be seen

1 Archiv. ftir die ges. Physiologic, 1904, Vol. CV, pp. 115-120.

PUBLICATIONS OF OTHER SCIENTISTS 211

in the whole field of view; the light-cone was extremely
faint but still recognizable.

If, instead of water, alcohol in varying degrees of
concentration was used for dilution, the particles increased
proportionately with the increasing strength of the
alcohol (the final concentration being 0.07 per cent of
glycogen).

These experiments prove that in addition to easily
recognizable submicrons the aqueous solution of glycogen
contains amicrons which, recognizable at first as a
homogeneous light-cone, are clumped together into
submicrons by the precipitative action of the alcohol.
The separation of the dissolved glycogen is not discon-
tinuous, but increases continually with the concentration
of the precipitant.

CHAPTER XXI
ON THE FORMATION OF HYDROSOL, AINU HYDROGEL

i. Sol Formation

IT is vain to attempt to convert a practically insoluble
substance into a hydrosol by pulverization and levigation.
By these means there are always obtained mere sus-
pensions of the substance in question. In order to obtain
a hydrosol something else is necessary.

Cohesion very soon sets a fixed limit to mechanical
subdivision. It would be of interest to determine this
limit for different substances; it would vary from one
substance to another, and in the case of friable, soft
substances, should not be much lower than a diameter
of J p.. The difficulty of converting metals into fine
powder is very well known; special technical appliances
are necessary to do it. But the particles in such powders
are still very coarse and cannot be compared with the
hypomicrons in metal hydrosols.

There are several ways to obtain hydrosols: (a) By
the unaided subdivision of a water-soluble colloid in
water; (6) by peptization, starting with a true hydrogel;
(c) in some cases by electric atomization of metals
(Bredig's process); and finally, (d) starting with crystal-
loids, by the formation, by means of chemical reaction
or change of temperature, of a substance within a fluid
in which it has practically no crystalloid solubility;
this at first leads to the finest subdivisions.

212

SOL FORMATION 213

(a) Solution of Reversible Colloids. A start can be made
with the solid form of a reversible hydrosol (solid hydro-
sol or false hydrogel), which possesses the property of
dissolving in water of itself. Donnan l has advanced
some interesting theoretical considerations regarding
this process of subdivision.

Applying the basic principles of capillary theory to
the process of colloidal solution, Donnan shows that
under certain assumptions the mutual reactions of the
forces of cohesion and the forces of molecular attraction
can result in an auto-subdivision of the substance, which
ceases before molecular dimensions are reached. 2 It
should be carefully noted that such a process is possible
without any particular solution-tension, that it takes
place without the help of electrolytic dissociation and
can be explained on the basis of the theory of capil-
larity, and that it leads to fluids which of course possess
no appreciable osmotic pressure.

(6) Peptization and Pectization. Starting with true
hydrogels, 3 which already actually contain pre-formed
an extremely fine subdivision, a hydrosol can no more
be obtained by simple grinding up with water, than if
we had started with an insoluble powder. In order to
obtain a colloidal solution from hydrogels it is neces-
sary to add a very minute quantity of a peptisizing

1 Donnan, Zeitschr. f. phys. Chem., 1901, Vol. XXXVII, p. 7r5;
also 1903, Vol. XLVI, p. 197.

2 In passing it might be remarked that the process of subdivision
can cease when the particles are of medium size, if the bulk of the
reversible colloid consists of indivisible submicrons, which are pre-
vented from combining into an inseparable whole by traces of pro-
tective colloids, as is the case with Lea's colloidal silver and with
colloidal indigo.

1 A true hydrogel means a jelly which of itself is not soluble in pure
water; a false hydrogel or "solid hydrosol" (see Lottermoser) is a
water-soluble colloid in solid or semi-solid condition.

214 FORMATION OF HYDROSOL AND HYDROGEL

substance which need not have any actual chemical
action on the hydrosol or its main constituents. The
hydrogel then becomes fluid and can be diluted as desired.
In this respect many substances show a difference between
the true hydrogel 1 and the water-free substance; the
latter remain intact and under the saipe conditions yield
no colloidal solution. 2 Stannic acid may be mentioned
as an example of this kind. By dilution and careful
washing out, a hydrogel, sometimes acid-free, may be
obtained from stannous chlorid. As van Bemmelen
has shown, the hydrogel of stannic acid is not stannic
acid hydrate, as had formerly been assumed, but an
adsorption compound of SnC>2 with II 2 O. A drop of
ammonia can fluidify a large quantity of the gel, 3 but
upon being desiccated at ordinary temperature, the
stannic acid completely loses the power of being
fluidified.

The gel can again be precipitated from the hydrosol
of stannic acid by small quantities of acid. Both pro-
cesses, solution of the stannic acid by alkalis and pre-
cipitation by acids, remind one so strongly of chemical
reactions, that it seems as if this were a case of an ordinary
formation of a salt, and that the hydrosol of stannic
acid should be regarded as the solution of an easily
soluble salt of the practically insoluble stannic acid.
A similar view has recently been adopted by Jordis. 4

Jorclis regards all colloids as amphoteric substances
which form salts by acting both as bases and acids,

1 False hydrogels, such as albumen, dextrin, gum arable, etc., dis-
solve in water without the addition of foreign substances.

2 Therefore in analytical chemistry directions are given in the deter-
mination of silicic acid to evaporate to complete dryness.

8 Zsigmondy, Liebig's Ann., 1898, Vol. CCCI, p. 370.
4 Jordis, Zeitschr. f . Electrochem., Vol. X, p. 517, and Ber. d.
phys.-med. Soc., Erlangen, 1904.

SOL FORMATION 215

that is, they can act like cations and anions. The same
phenomena can be explained from another point of
view which can be hero only indicated *. If the gel
of stannic acid be treated with a less concentrated alkali,
part of the alkali is by adsorption 2 taken up by the
hydrogel; a certain quantity of stannate, K 2 Sn0 3 , may
thus be formed. The solution of the gel with the for-
mation of the sol takes place upon dilution with water.
Upon dilution, the adsorbed alkali (or the stannate
perhaps formed) undergoes electrolytic dissociation pre-
cisely as if it were dissolved in water, the dissociation
of course increasing with the dilution. In order to under-
stand the process of colloidal solution as well as the
behavior upon electrolysis and toward reagents, we
must assume in the case of this dissociation that the
anion remains adsorbed by the amicrons or submicrons,
of which the gel of stannic acid is composed. The
adsorbed OH 7 or Sn0 3 " anion gives the stannic acid

1 1 mention these observations only because I have made use of
them to advantage for several years past, in order to review the reac-
tions of the purple of Cassius and other colloids, and because their
publication may perhaps be of use to others. In fact, quite similar
assumptions have been made by Bredig (Anorg. Fermente, loc. cit.,
1901, p. 1C) and by Swigel Posternak (Ann. Inst. Pasteur, 1901, Vol.
XV, pp, 85, 169, 251, and 650), hi order to explain peptisation and
pectisation.

1 Or absorption, see van Bemmelen (Chap. III). That adsorption
or absorption is still somewhat "mysterious," as is the case with the
action of gravity, chemical affinity, and other fOrces ; cannot be doubted;
but its presence is indicated by a multitude of phenomena which are
neither to be explained by chemical affinity, nor to be referred to
other kinds of energy, such as electricity, magnetism, etc. The con-
densation of gases by charcoal, which was thoroughly investigated by
Saussure in 1814 belongs to this class of actions.' In Ostwald's Lehr-
buch. d. ailgem. Chem., 2d ed., I, p. 1084, is to be found a bibliog-
raphy and critical discussion of this subject. See also E. du Bois-
Revmond, Vorlesungen uber die Physik des Stoffwechsels.

216 FORMATION OF HYDROSOL AND HYDROGEL

particles the negative charge which is made evident upon
an electric migration test, and effects the separation
of the amicrons from each other, as well as their distribu-
tion in the fluid. The whole complex (particles of stannic
acid with anion attached) acts upon electrolysis just
like the complex anion of the alkali salt of a very weak
acid; it separates out at the anode, but in contra-
distinction to true complex ions, cannot pass through
the parchment membrane. The reactions of colloidal
stannic acid are likewise quite ' similar to t-hose of the
salt of a weak acid. While the behavior of colloidal
stannic acid can perhaps be in part explained upon the
assumption of the formation of a salt, according to
Jordis' idea, such a view cannot hold with many other
colloids, the purple of Cassias for instance. This acts
just the same as stannic acid, and in order to explain
its behavior upon the basis of salt formation, it would
be necessary to return to the assumption of Berzelius
that the purple is a chemical compound ; but this assump-
tion is untenable (see Chapter III, p. 67). Or recourse
might be had to the equally erroneous assumption that
the metallic gold particles themselves are the anions of
a complex salt, or else constituents of such anioris, just
like platinum in platinochlorids, or iron in potassium
ferrocyanid. This assumption contradicts all that we
know regarding it: the metallic nature of the individual
particles, their incapability of passing through parch-
ment membranes upon electrolysis, and, finally, the
fact that very large submicroscopic gold particles con-
sisting of millions of atoms, upon electrolysis, act just
the same as the smallest.

In order to give a preliminary sketch as to how these
relations may be imagined, a few illustrations are here
given.

SOL FORMATION

217

Designate an ultramicroscopic particle of stannic acid by
and an ultramicroscopic particle of gold by

In colloidal solution I believe them to be combined with
hydroxyl or with other anions, for example:

OH' SnO 3 " OH' .

oir

OH'

whereby they acquire their electric charge. In the
purple of Cassius the gold always remains with the
stannic acid, as long as the colloidal condition is main-
tained. This combination (by " adhesion " [Gay-Lussac],
"adsorption" or u affinity of constitution " l ) can be
expressed 2 by

SnO 2

Au

SnO 2

Au

SnO 2

These particles, too, are negatively charged just like
those of stannic acid; their charge can be pictorially
expressed in like manner, for example:

OH'

etc.

Au

SnO 2

1 This expression was proposed by W. Biltz. (Ber. d. Deutsch.
chem. Ges., Vol. XXXVII, pp. 1112, 1904).

2 Upon electrolysis the purple separates out at the anode as a red
homogenous looking mass, no separation of gold and stannic acid
being visible, which would be the case if both constituents were not
united with each other (otherwise it may be assumed that the gold and
stannic acid would separate the one from the other, according to the
intensity of their respective charges and their frictional resistance
in the fluid). In fact ydth an excess of SnO 2 a separation sometimes
takes place into red colored purple and colorless stannic acid.

218 FORMATION OF HYDROSOL AND HYDROGEL

Such signs may be used just the same as chemical
formulae. In contradistinction to the very expressive
chemical formulae, which incidentally express definite

proportions by weight, the sign indicates that we

are here dealing with a larger complex, with an amicro-
scopic particle that may perhaps contain as few as
several hundred molecules, or with a submicroscopic
particle in which may be contained thousands, or even
millions, of molecules. 1 But this sign is by no means
intended to give any idea of the shape of the amicron
or of the way in which its space is filled up.

Neither metastannic acid nor the purple obtained
from it dissolve in hydrochloric acid. But solution cer-
tainly takes place upon dilution. Just as in the case
of peptization with alkalis, we may assume that the gel
of metastannic acid absorbs HC1; or else that some
stannic chlorid or oxychlorid is formed arid thereupon
absorbed. Upon dilution, dissociation commences again,
and the hydrogen ions (or perhaps the cation of a salt
formed from stannic oxid and hydrochloric acid) which
are then present in greater concentration, are adsorbed
by the amicrons and give them a positive charge.

With the aid of these signs can be expressed in graphic
form the most important reactions of stannic acid and
the purple of Cassius, as well as those of many other
colloid reactions. This method is only indicated here;
I must at the same time state, however, that my con-

1 There is some indication that in this case numerous anions are
united by adsorption with one submicroscopic particle, and they then
resemble electrode rather than an ion, according to Billitzer's com-
parison. In opposition to Billitzer, I consider it probable that each
individual submicroscopic particle possesses a, charge which equals
or surpasses that of an ion.

SOL FORMATION 219

ception is based on Hardy's law of isoelectric state,
according to which the stability of irreversible hydrosols
depends upon their electric charge, 1 whereas if they
lose their potential difference against the medium, they
become unstable and coagulate. But Billitzer 2 has
shown that certain hydrosols are particularly stable
when in isoelectric state, and upon this built up a new
conception of coagulation, according to which the ions
which produce the coagulation of colloids act as con-
densation nuclei. Because of the consequences in-
volved in Bfllitzer's explanation, I will later on take
this up specifically, and also those points on which I
differ with him.

Whatever ^particular idea may be formed as to the
process of peptization and coagulation, the assumption
adopted by Bredig, and also by Billitzer and others,
that the particles in the hydrogel are the same as those
in the hydrosol and owe their electric charge to an
adsorption of ions from the surrounding fluid or to a
discharge of ions into it, is certainly capable of more
general application to the process in question than the
assumption of the formation of chemical compounds,
whose existence in some cases cannot be proved, and in
other cases must be denied on the basis of the facts.

In this section I must also mention that this xery
process of pectization and peptization, as well as the
precipitation of reversible colloids by NII 4 C1, etc., show
such manifold differences in phenomena with each indi-
vidual colloid, that a complete explanation of these

1 Other causes which increase stability are mentioned on p. 177.
In fact, pure metal hydrosols form a group by themselves, which
demands separate consideration. (See Chap. XXI, 2.)

'Billitzer, Sitzungsber. der. K. Akad. d. Wiss., Vienna; M. n. Kl.,
CXI, Ha., Nov. 1902; Zeitschr. f. physik. Chem., 1903, Vol. XLV, p.
107; ibid, 1905, Vol. LI, pp. 129-16G.

220 FORMATION OF HYDROSOL AND HYDROGEL

processes may be safely left to the future. I need but
recall the behavior of colloidal silicic acid, which Jordis l
is studying carefully; also that of globulin and albumen.

(c) The conditions under which metal hydrosols are
formed by electric atomization have been described in
detail by Bredig. 2

(d) Production of Colloidal solutions by the formation
within a fluid, by chemical means or decrease in tempera-
ture, of a substance possessing practically no crystalloid
solubility. We may, in general, assume that most sub-
stances are capable of forming crystals. It does not
seem improbable that colloids, even if they form jellies,
may perhaps consist of submicroscopic or amicroi^opic
little crystals. Such little crystals, with their water
envelopes,' must behave quite differently from ordinary
microscopic crystals (sec further on). The greater the
crystalloid solubility of a substance, the larger as a rule
will be the crystals separating out from its supersaturated
solution. 3 ' Very insoluble substances like barium sulphate
or silver chlorid yield precipitates whose crystal-like
nature can . hardly be recognized. Nevertheless their
solubility is so great that stable colloidal solutions cannot
be prepared from these substances, (See Chapter XIX,
p. 193.)

Jpi order to obtain stable colloidal solutions by chem-
ical interchange, it is necessary* to begin with substances
which as crystalloids are practically insoluble. Then

i Jordis, loc. cit., and Zeitschr. anorg. Chem., Vol. XLIV,pp, 200 208.

a Anorg. Fermente, loc. cit.

3 Compare Na 2 SO 4 with CuSO 4 and calcium oxalate. The rule stated
serves only for temporary orientation, the size of the crystals depending
upon numerous circumstances, important among which are tempera-
ture, concentration of the supersaturated solution, degree of super-
saturation, etc. But impurities and other influences exercise a con-
siderable effect.

SOL FORMATION 221

only will the little crystals finally formed remain so
small and be able to grow so slightly that there are
obtained subdivisions of the degree of homogeneity
demanded by colloidal solutions; or else the presence of
protective colloids is necessary, which powerfnlly retards
the growth of the little crystals, just as it does the union
of particles.

It must not pass unnoticed that Niigeli { had already
advanced the idea that vegetable fibers are formed of
micells, molecular complexes optically and structurally
anisotropic, that is, of ultramicroscopic structures, whose
chief properties are the same as those of tiny anisotropic
little crystals. According to Nageli these micells are
approximately ^ uniformly oriented in fibers, thus pro-
ducing double refraction in them.

H. Ambronn 2 has also frequently pointed out in
several papers, that a series of phenomena with colloids
(double refraction upon rotation of gelatin, gum solution,
etc., or upon stretching gelatin, cherry gum, etc.; also
dichroism of dyed fibers and doubly refracting gelatin)
may be explained upon the assumption of ultramicro-
scopic crystals or micells, according to Niigeli 's hypoth-
esis. Thus fibers dyed with salts of gold show a wonderful
red-greenish blue dichroism, the existence of which,
according to H. Ambronn, is most easily explained by
the oriented arrangement of the anisotropic little gold
crystals. Many phenomena therefore indicate tha.t sub-
microscopic crystals may have something to do with
colloids. 3

1 A brief and general statement of Nageli 's views are to be found in
Nageli and Schwendtener, Das Mikroskop, 2d ed. f Leipzig, 1877.

2 H. Ambronn, Ann. der Physik, 1888, Vol. XXXIV, p. 341 ; ibid,
1889, Vol. XXXVIII, p. 160; Ber. d. D. Botan. Ges., 1889, Vol. VII,
pp. 103-114; Ber. d. Kgl. Sachs. Ges. d. Wiss., Dec. 7, 1896.

'Dichroism may also be due to differences in the spaces between the

222 FORMATION OF HYDROSOL AND HYDROGEL

As W, Biltz has also pointed out, the formation of
supersaturated crystalloid solutions, even with practically
insoluble substances, is more frequent than had been
generally presupposed. It is therefore to be expected
that upon the formation of a substance having practically
no crystalloid solubility, there is at first formed a very
strongly supersaturated crystalloid solution. Such solu-
tions, however, are very unstable. In Chapter XVII and
XVIII it has already been brought out how amicrons and
submicrons can be formed in such supersaturated solu-
tions. 1

Ostwald has determined the smallest quantity of solid
matter which is just able to bring about the crystalliza-
tion of the supersaturated solution of an ^easily soluble
substance. He found that the limit w&s about 10~ 6 to
10~ 9 mg. 2 With substances difficulty soluble it should
be still less, as brought out in Chapter XVII; and the
smallest visible gold submicrons certainly have a mass
millions of times smaller than the smallest little crystals
extruded (thrown out) by supersaturations of easily
soluble substances. But still smaller yet are the ami-
crons which serve as centers of growth for them. 3 (Chap-
ter XVII.)

particles, that is to an anisotropic lattice-like structure (Ambronn,
Ber. d. Kgl. Sachs. Ges. d. Wise., Vol. XL VIII, p. 622, 1896; F. Braun,
Berl. Akad. Ber., 1904, p. 164; Drude's Ann., Vol. XVI; Kirchner
and Zsigmondy, Drude's Ann., 1904, Vol. XV, p. 587). As Mr. Ambronn
has stated to me that he has recently been able to prepare under
certain very simple conditions, very thin needle-shaped microscopic
crystals or even little plates united into druses, from gold and silver
salts; and these showed just as strong a double refraction with con-
commitant dichroism as is seen with fibers dyed with gold and silver
salts. This rather strongly supports Ambronn *s view, above given.

1 See also Chap. XX, pp. 208-211.

1 10~ 9 to 10~ 12 grams, Ostwald, Lehrb. d. allge. Chem., 2d ed., II,
2, p. 784.

* The reason for this enormous difference in size is easily compre-

SOL FORMATION 223

The conditions of colloid formation may be so chosen
that only amicroscopic particles are formed (for instance,
by extreme dilution of the reacting substances, or by
the addition of protective colloids which interfere with
growth), and a colloidal solution of great homogeneity may
be thus obtained; or they may be so chosen that larger
submicroscopic crystals are formed, as, for example, by
the use of higher concentrations and by the proper choice
of substances having greater speed of crystallization.
Under certain circumstances such a case may terminate
in a mechanical suspension. By such means can best
be studied the influence of the degree of subdivision
upon the properties of a substance; careful study along
this line promises to bring to light much valuable infor-
mation, towards which only an incomplete beginning
has here been made.

It might here be further stated that increasing the
temperature of the reacting mixture (which as a rule
increases the crystalloid solubility of slightly soluble
substances, besides increasing the speed of crystallization,
Chapter XVII) has the effect of causing the formation of

hensible with the aid of Ostwald 's classification. Ostwald distinguishes
two classes of supersaturated solutions, labile, and metastable. In the
former spontaneous crystallization occurs; in the latter it does not,
but their super saturation must be destroyed by a nucleus introduced
from without. The limits determined by Ostwald refer to solutions in
metastable condition. In the production of precipitates which are
difficult to dissolve and of irreversible hydrosols, we are dealing with
processes which belong to the labile group.

But the limits given by Ostwald on p. 757, depend upon the degree
of supersaturation. The greater the supersaturation in the metastable
group, tjie smaller the crystals needed to resolve it. This very import-
ant observation may be applied to the case in point. With increasing
supersaturation of the solution of a certain substance, it seems to me
that the limit referred to would be quickly carried over into the sphere
of small dimensions.

224 FORMATION OF HYDROSOL AND HYDROGEL

larger particles in the case of irreversible hydrosols. 1
Thus upon the reduction of gold and silver solutions at
boiling-point, larger particles are for the most part formed
than at ordinary temperatures. It seems to me not
improbable that the soluble metalumina and iron meta-
oxid which Graham considered to be allotropic modifica-
tions of colloidal alumina, are to be distinguished from
the ordinary hydrosols of these substances chiefly by the
size of the ultramicrons contained in them. The forma-
tion of the former at higher temperatures and its be-
havior, and the color and cloudy appearance of iron
motoxid, all point in this direction. Both modifications
should yield interesting objects for ultramicroscopic
examination.

A considerably different influence of temperature may
be observed with soluble starch, which is a reversible
colloid. At boiling-point it appears to give a crystalloid
solution with very high molecular weight. 2 A three per
cent solution prepared at boiling-point shows in the ultra-
apparatus even after cooling, a homogeneous polarized
light-cone due to amicrons. If this solution is allowed
to remain in an unhcated room (at about 3-10 C.), it
becomes opalescent after a few clays, and in three or four
weeks changes into a milk-white opaque fluid, which is
quite thickly filled with brightly shining particles of

1 If the solubility of a substance practically insoluble be increased,
then, according to the rule above-mentioned, conditions for the growth
of the little crystals are more favorable, and finally larger structures
can grow at the expense of the smaller ones.

2 Because of the size of molecules their general properties cling to
colloidal solutions. Lobry de Bruyn figures the linear dimension of
the starch molecule to be 5 ftp. (Rec. des. Trav. Chem. des Pays-Bas.,
1900, Vol. XIX, p. 253).

8 After half a month the hydrosol changed to a thin jelly; ultrami-

SOL FORMATION 225

By boiling up, clear solutions can be immediately
obtained again. The brightly shining appearance of the
particles in the ultraapparatus already indicated that
they were not actual hydrogel particles, but were granules
of starch heavily compacted. 1 An explanation of the
process might perhaps be that at boiling-point an unsatu-
rated crystalloid solution is formed, which becomes much
supersaturated at ordinary temperatures; 2 contrary to
the usual behavior of crystalloid solutions, the formation
of centers of crystallization as well as the growth of little
crystals take place with extraordinary slowness. At
the same time the amicrons apparently unite to a jelly
whose presence is indicated by various circumstances.

The development of subrnicrons in starch solutions at
3-10 C. recalls vividly the development of gold particles
in ruby glass (Chapter XVII), or the process in the pro-
duction of colloidal sulphur or selenium (Chapter XX).
In all these cases a supersaturated crystalloid solution
can first be assumed, in which grow the spontaneously
formed amicrons and submicrons. Whereas in the last
three cases growth takes place speedily at the proper
temperature, partially because of the smallness of the
crystalloid molecule, its very much more complete diffu-
sion and probably because of a great specific rapidity
of growth, that of the starch particles takes place with
extreme slowness. It may also be assumed that the
starch solution itself contains constituents which oppose
growth. With soluble starch, reversion to the original
molecular state of subdivision takes place at about

croscopically it could be seen in the diluted fluid that 2-5 brightly
shining particles were united together at resolvable distances.

1 Or else granules made up of smaller but solidly filled submicrons.

* Soluble starch swells up in water at ordinary temperatures, but
dissolves only to a very small extent.

226 FORMATION OP HYD&OSOL AND HYDROGEL

100 C., and with ruby glass at white heat. It is not
improbable that diluted sulphur or selenium solutions
can be temporarily changed again into crystalloid solu-
tion by heating in a sealed tube.

One remarkable point of difference is that with starch
both submicrons and amicrons are considerably larger
than those of gold or sulphur, etc., providing that starch
molecules have a linear dimension of 5 JJ./JL, as Lobry de
Bruyn has assumed.

Several cases have been considered in which the growth
of the individual particles was explained by assuming
that they grow like crystals in a crystalloid solution.

Essentially different conditions may obtain, however,
if the enlargement of the particles i^ consequent upon
the union of the tiniest particles, as is the case in coagu-
lation. This process is responsible for many irreversible
changes of the condition of colloids. It may, according
to the nature of the subdivided substance, lead some-
times to a very intimate, sometimes to a very super-
ficial union, as will be seen later on when we discuss
the hydrogels.

How the process of the particles uniting can affect
color, has been stated in Chapter XII.

We have described but two kinds of change of con-
dition in hydrosols: the first concerns the growth of
small hypothetical little crystals, and results in solid
ultramicrons, filled with the matter of the subdivided
substance; the other concerns union of the particles of
amicrons and submicrons themselves. Both must lead
to hydrosols with coarse particles which are different as
to quality. Thus the growth of gold particles does not
necessarily lead to a color change, whereas the union of
particles always does, if the original subdivision was
red colored. (See Chapters XII and XVII.)

A FEW REMARKS CONCERNING GEL FORMATION 227

This does not by any means exhaust the changes in
condition of hydrosols ; their further discussion at present
would not, however, be profitable.

2. A Few Remarks Concerning Gel Formation

We will now consider the final result of the union of
particles, as they occur in coagulated masses. From this
at once arises the necessity to divide the irreversible
hydrosols, which alone are considered in this section,
into two varieties, according to the nature and behavior
of the coagulum.

Two Classes of Irreversible Hydrosols. To the first
class, which may, perhaps, be appropriately called com-
pletely irreversible hydrosols, belong colloidal metals
which are free from foreign colloids; to the second class
belong most colloidal sulphids and many colloidal oxids,
which may be called imperfectly irreversible hydro-
sols.

In Chapter XIX the difference between both classes
has already been pointed out. This difference does not
as yet appear to be sufficiently appreciated, or it is incor-
rectly understood, for irreversible hydrosols of the first
class have improperly been considered as suspensions,
from which they differ not only in degree of subdivision,
but also in the nature of their irreversible change of
condition. 1 (Chapters I and II.)

1 There are also mechanical suspensions which undergo irreversible
change of condition upon desiccation; but these are not levigated
powders of insoluble substances, but levigated powders of hydrogels.
The changes of condition of these suspensions, however and therein
lies the essential difference are not changes of the relation of the
levigated hydrogel particles to each other, but changes in the hydrogel
itself. They are of exactly the same nature as those which under the
same conditions gel-lumps undergo under water.

228 FORMATION OF HYDROSOL AND HYDROGEL

Difference Between the Two Classes. The most im-
portant points of difference between these two classes
of irreversible hydrosols are the following:

The irreversible hydrosols of the first class undergo
upon coagulation a much more complete separation from
the surrounding water; the union of the ultramicrons
is much closer; the coagulation is usually accompanied
by a change in color. No true jelly is formed, but rather
a spongy powder or a metal sponge. All attempts to
peptisize the precipitate or change it directly into a
hydrosol, are in vain. To obtain a colloidal solution,
a salt of the metal must first be formed, and the metal
again reduced from this; or recourse must be had to
Bredig's method of atomization und^r water with the
electric arc.

The irreversible hydrosols of the second class can
easily be prepared a hundred to a thousand times more
concentrated than the completely irreversible; if, how-
ever, the individual particles are brought near enough to
each other (for example, in 1-10 per cent solution), coagu-
lation spontaneously occurs. In dilute solution they
are precipitated by traces of electrolytes, just like those
of the first group; but here they show a great diversity
of behavior; specific influences govern, and the pre-
cipitate has all the properties of a hydrogel. When
placed upon a filter the precipitate usually appears trans-
lucent and jelly-like, just like the hydrogel formed by
spontaneous coagulation. The water is so firmly held
by it that its complete removal in a desiccator is almost
impossible. These precipitates have often been considered
to be hydrates, but van Bemmelen has shown that this
view is untenable.

While the precipitate is still moist, it is easy, as con-
trasted with irreversible hydrosols of the first group,

,A FEW REMARKS CONCERNING GEL FORMATION 229

to convert it into a hydrosol again by electrolytes of
certain kinds. When air-dried, however, it can as a
rule no longer be peptisized. The resemblance and the
difference between both classes can at once be seen.
With metals there occurs a much closer union of ultra-
microns under water, and upon coagulation there is
immediately reached the end-point which, with imper-
perfectly irreversible hydrosols, is reached only upon
drying.

A clear idea of the differences between the two classes
may be formed by assuming that the water is held to
the surface .of the amicrons or submicrons of the second
class, with much greater force than is the case with
metals. 1 In hydrogels of the second class the individual
particles are probably separated at first only by an
aqueous envelope, but they nevertheless attract each
other just the same as do the metal particles, although
with proportionately less force because of the greater
distance. This attraction, in the absence of any pre-
ventative, results in the separation of the 'hydrosol from
the bulk of the water. As soon, however, as the parti-
cles receive an electric charge (peptization by electro-
lytes, Chapter XXI, 1 fr), they can be separated from
each other again because they are not as yet so closely
united as the metal particles. Upon further dehydration
(which sometimes occurs under water) the hydrogel
gradually solidifies; because of the decreasing distance
between the particles upon desiccation, the action of the

1 Quite a rough comparison may serve for an illustration: while a
powerful magnet covers itself with a thick coating of iron filings, a
weak magnet can hold but few. It requires some force to remove
them from the powerful magnet, but they can be easily rubbed off
the weaker. The powerful magnet would correspond to amicrons of
the second kind, the weak magnet, to those of the first kind, but of
course only in their behavior with water.

230 FORMATION OF HYDROSOL AND HYDROGEL

forces of cohesion speedily increases. They not only
visibly affect the solidity of the gel, but also militate
against the separation of the individual particles by
peptization. This does not mean that the process pro-
ceeds uniformly throughout the whole mass of the gel;
more frequently it goes on at different speeds in different
places. If, for example, an attempt is made to peptisize
a slightly dried hydrogel, there is sometimes obtained a
turbid hydrosol with numerous submicroscopic particles.
Van Bemmelen, 1 Biitschli and Quincke have published
notable work regarding the structure of hydrogels and
their behavior upon desiccation.

Formation of Jellies. According to Biitschli 2 and
van Bemmelen, 3 gel formation is to 9 be considered a
segregation process, whereby there occurs a separation
into two fluids, one of which subsequently solidifies.
According to Quincke 4 there exists at the bounding
surfaces of what he calls the oleaginous fluid A, a surface
tension against the solution B which is richer in water.

Considering the segregation of hydrosols, in accord-
ance with the idea above explained, as conditioned by
the mutual attractions of amicrons or submicrons (sur-
rounded by aqueous envelopes), it is at once evident
firstly that the precipitate will act like a fluid (even if
the individual particles are solid), and secondly, this

1 See note, Chap. Ill, p. 73.

2 O. Butschli, Untersuchungen fiber mikroskopische Schaume und
Protoplasma, Leipzig, 1892; Untersuchungen uber Strukturen, Leip-
zig, 1898; Untersuchungen uber die Mikrostructur kunstlicher und
natiirlicher Kieselsauregallerten, Heidelberg, 1900, p. 343, uber der
Bau quellbarer K6rper, Gdttingen, 1896.

8 van Bemmelen, Z. anorg. Chem., 1898, Vol. XVIII, pp. 14-38.

4 Quincke, Drude's Ann., 1902, Vol. VII, pp. 57, 631, 701; ebenda,
1903, Vol. IX, pp, 1, 796, 969, ebenda, 1903, Vol. X, pp. 478, 673;
ebenda, 1903, Vol. XI, pp. 54 and 449.

A 1 FEW REMARKS CONCERNING GEL FORMATION 231

fluid will exert an osmotic pressure against the surround-
ing medium. The fluid properties may be ascribed
to the slight friction of the aqueous envelopes against each
other, the 'surface tension to the attraction which the
amicrbns exercise upon each other through these envelopes.
Such attractive forces tend to contract the surface of
the precipitate, just as does ordinary surface tension
between crystalloid fluids. The fact that hydrogels
solidify under water or in the open air, can be considered
as consequent upon the co-operation of the attractive
forces. As before mentioned, segregation and dehydra-
tion do not proceed uniformly throughout the whole
mass, but with the formation of a large number of specks
which are much coarser than those of the original colloidal
solution. Biitschli's microscopic work in particular has
been of service in casting light upon this subject.
Biitschli examined a large number of hydrogels and
almost everywhere found a fine-webbed microstructure,
which is usually distinctly visible. By measuring, for
example, the diameter d of the little cavities in different
silica jellies and by calculating the thickness of their
walls m, Biitschli found:

TABLE X

With

d

m

Tabaschir

1.45/*

0. 152-0. 187/*

van Bemmelen's silica, gel

l.OOii

0.27ju

Biitschli 7 s silica gel

1.50/1

0.30

In a carefully prepared solution of silica I could find
ultramicroscopically, in harmony with the macroscopic

1 The figures in column marked m are to be considered the major
limits of the thickness of the walls. See Biitschli, Unters. ttber die
Mikrostrutur kiinst. und nat. Kicselsauregallerten, p. 313.

232 FORMATION OF HYDROSOL AND HYDROGEL

observation of Picton and Linder, only the slightest
suggestion of a light-cone; the silica therefore exists in
solution in the form of amicrons. of which size nothing
can be accurately stated; it is a substance particularly
unsuitable for exhibiting its individual particles. But
if we assume that its, particles are smaller than 0.005 jj.
or 0.01 /*, an assumption which very probably expresses
actual conditions, we can then form an idea of variations
in the inhomogeneity of colloidal solutions on the one hand
and of hydrogels on the other. In any event their
amicrons, being invisible upon intense side illumination,
are very much smaller than the walls or cavities in the
silica gels.

To determine the extent of the effect produced by
surface tension upon the formation of the gel skeleton,
Quincke conducted a series of interesting researches.
" Under the influence of surface tension thin-flowing,
oil-like lamellae form spheres, bubbles, spherical foam-
walls, and under some conditions spiral surfaces. Solid
thin lamellae curl up together into a hollow cylinder or
hollow sphere. Lamellae of very sticky, .oily fluid lie
between them, acting like thin solid lamellae and curling
up into a hollow cylinder or hollow sphere, which then
gradually subdivides into spherical bubbles like a fluid
tube, or else forms swellings and constrictions." (Drude's
Ann., Vol. XI, p. 1035 [8]).

Quincke furthermore could in many cases demonstrate
the existence of invisible layers of fluid consisting of
bubbles and foam walls; he gives eight methods by
which this proof can be established. (Drude's Ann.,
Vol. VII, p. 638.)

According to Quincke, surface tension plays a part in
the formation of membranes of precipitates, metal-
foliage, and stellate crystals, in modern photogra-

^A FEW REMARKS CONCERNING GEL FORMATION 233

phy, and in the double refraction of jellies and
colloids.

This is merely mentioned in passing, just as is the
important research of van Bernmelen 1 regarding the
physico-chemical behavior of hydrogels, for this book
deals chiefly with irreversible colloidal solutions.

In connection with the theory of hydrogcls one fact
appears to me to be of significance, that from hydrosols
of the first class, which of themselves form no true jellies,
there can be formed by the addition of small quantities
of other colloids, a hydrogel with its characteristic prop-
erties (transparency, ability to swell up and [in the case
of irreversible hydrosols] to be peptisized). (See " purple
of Cassius," etc., Chapter III, also Chapter XII). With
a hydrogel, there may therefore be incorporated particles
foreign to its nature in such a way that the foreign con-
stituents sometimes constitute the greater part, without
materially changing the character of the gel.

In Chapters XVIII and XXI an attempt was made
to assemble uniformly several important phenomena
concerning irreversible hydrosols, together with the
known views published by other investigators, mainly
upon the assumption that the ultramicrons in hydrosols
consist of the same substance as those in the hydrogels
obtained from them. 2 This assumption is by no means
new, for Graham, van Bemmelen, Hardy; Bredig, Billitzer,
and others assume essentially the same thing. And it

1 Loc. cit.

2 The fact supported by Jordis in particular, that irreversible
hydrosols have not as a whole the same chemical constitution as the
hydrogels obtained from them, is not affected by these statements,
but only explained from another point of view. I also agree with
Jordis that chemical reactions in cases where they are to be expected
must by all means be allowed for.

234 FORMATION OF HYDROSOL AND HYDROGEL

is also supported by the fact that such submicrons as
are concerned in gel formation, as, for example, gold in
the purple of Cassius, do not change their properties if
they are changed into a hydrogel and then back again
into a hydrosol. In the course of time other examples
will doubtless be found.

The second assumption made necessary to explain the
peptization and pectization of irreversible colloids, namely,
that the ultramicrons of colloids can, without being
chemically bound, unite with ions which give them an
electric charge, naturally follows from the application
of the view supported by Nernst in particular that ions
are an independent kind of molecules to the results of
Hardy's investigations. This is by nc\ means a ^new
proposition, for Bredig has already referred the electric
charge of colloid particles to the differences of the specific
particle coefficients of the ions present; and Billitzer
has directly maintained and established by experiments
that the electric charge of the individual particles is due
to the acquisition or loss of ions.

The third assumption necessary to explain plainly and
satisfactorily the course of colloidal solution, the coagu-
lation of metals, the existence of surface tension at the
boundary between gel and surrounding fluid, and espe-
cially numerous colloid reactions, viz., the assumption
of attractive forces between the ultramicrons themselves
and between them and the molecules of the surrounding
medium, is nothing new either, for the capillary theory
of La Place assumes molecular attractive forces between
similar and dissimilar substances and thus is able to
explain the great variety of capillary phenomena. Similar
attractive forces were assumed in colloids even by Gay-
Lussac, by Graham (to explain pectization), by Nageli,
and by Barus and Schneider. Donnan has pointed out

^L FEW REMARKS CONCERNING GEL FORMATION 235

the applicability of the fundamental ideas of the capillary
theory to what happens during colloidal solution, and
paved the way to show that a great number of phenomena
can be explained by a few special assumptions regarding
the forces in question, without the addition of any new
hypotheses. That these forces will vary from case to
case, can be seen from the fact that the cohesion of
different substances varies greatly in value (van der
Waals, Die Kontinuitat des gasformigen und fliissigen
Zustandes, 2d Edition, Vol. I, p. 175), and that the
surface tension at the boundary between "two media is
dependent upon it.

From the assumption made by Donnan, of attractive
forces between colloid and surrounding medium, to that
of attractive forces between colloid particles of different
kinds (which in addition to electric charge are connected
with colloid reactions), is only a short step. W. Biltz
also arrived at a similar conclusion and expressed it in
the same way by proposing the term "affinity of condi-
tion " (Zustandsaffinitat) for the basic causes of absorp-
tion processes.

The above explanation is of course not intended as
a theory of colloids, for the study of colloids will become
a great and extensive science, in the development of
which many must assist; it is only when the numerous
facts, developed from basic physico-chemical experi-
mental research, have been systematically arranged, that
the theory of colloids will be raised from the stage of a
generalization of special instances to the rank of an
exact science.

RECAPITULATION

THE starting point of this book is a fundamental ques-
tion of colloidal chemistry, viz., the question whether
or not the polarized dispersion of light recognizable in
hydrosols by the Tyndall test, is one of the essential
properties of- hydrosols.

By means of the method developed by Siedentopf and
myself, of rendering visible ultramicroscopic particles,
this question could in some cases be Decided. In the
case of the hydrosols examined, the diffuse dispersion
of light has proved to *be an essential characteristic; it
is caused by the same material particles which give to
those fluids their otherwise remarkable properties.

The newly developed method for making visible and
measuring^ ultramicroscopic particles, made it possible to
obtain a closer insight into the processes and relations in
colloidal solutions, hitherto inaccessible to direct observa-
tion.

Thus by a series of experiments, the size of the parti-
cles in colloidal metal solutions was determined. It
was shown that in coarser hydrosols the largest particles
have a mass corresponding to a linear dimension of about
60-80 fifi (Chapters IX and XIX), but that carefully
prepared solutions show an inhomogeneity which is
scarcely discernible and hardly to be distinguished from
that of many crystalloid solutipns; further, that numerous
intermediaries exist between these extreme cases (Chapter
IX).

It was shown that the smaller metal particles in hydro-

236

RECAPITU LATION 237

sols possess an extremely active oscillatory and translatory
motion, which in many respects differs from the typical
Brownian movement (Chapter X).

The statement made in conjunction with H. Sieden-
topf, that in the case of metal subdivisions (in ruby
glasses) there is no connection recognizable * between the
size of the particles and their color, was confirmed by the
examination of colloidal gold solutions (Chapter XI).

In collaboration with F. Kirchner a closer insight
could be had .into the course of events responsible for
the color changes of metal hydrosols (Chapter XII).

To obtain a preliminary insight into the question so
important to bacteriologists, as to the relative size of
the pores in the clay and other filters used, some experi-
ments-were conducted toward this end (Chapter XIV).

My own experiments and especially the experiments of
other investigators (Chapter XX) have shown that th(
individual particles in many reversible colloids and dye-
stuff solutions are also capable of direct observation.
The possibility of thus forming an idea as to the size of
these particles, renders it desirable to have in mind the
size of thp particles when classifying hydrosols. An
attempt in this direction is made in Chapter II. In
Chapter XV the sizes of the particles in some hydrosols,
experimentally determined, are compared with those in
ordinary suspensions.

Since this work makes frequent reference to earlier
publications concerning irreversible hydrosols, it seemed
desirable to give a brief resum of the papers "on this sub-
ject in an historical section, especially as abstracts of
them do not always give the authors' views with sufficient
accuracy. It seemed desirable to make readily accessible
to everyone interested therein the contents of the basic
work of Graham on colloids. In the historical portion

RECAPITULATION

his publications on this subject are reviewed at length
and in part quoted verbatim.

While actively engaged with the ultramicroscopic
examination of fluids, the author learned many facts of
general interest, and certain precautions which should
be observed; 'these are mentioned in Chapter VII.

Numerous observations made upon ruby glass have been
most simply explained by applying the results of Tam-
mann's work upon spontaneous crystallization of super-
cooled melts, to what happens in the formation of ruby
glass. This made it probable that the formation of red
ruby glass is due to the growth of amicroscopic crystals
in a crystalloid solution of metallic golcj.

It seemed desirable to apply the preliminary insight
thus obtained into actions only partially susceptible of
direct observation, to what occurs during the formation
of hydrosols and hydrogels, and to test upon what
premises the assumption of ultramicroscopic crystals
could be reconciled with the facts observed in connection
with colloids. These and some other questions concern-
ing the theory of colloids, are briefly discussed in Chapters
XVIII and XXI.

The numerous methods of chemistry, of physical
chemistry, and of physics, to which has recently been
added ultramicroscopy, will in proper combination make
it possible to settle certain questions the answers to which
are of importance for the further development of the
study of colloids. Even if the last-named method can-
not fulfil the exaggerated hopes sometimes based upon
it, present results have shown it is an important supple-
ment to other methods, and permits an insight into rela-
tions which, until now, were not accessible either to
direct microscopic observation, or to ordinary, physico-
chemical methods.

EXPLANATION OF THE COLORED PLATES

The gold solutions are arranged in order of the increasing size of
their particles; at A (Plate I) is illustrated their appearance in inci-
dent light, at C (Plate II), their appearance in transmitted light.
B (Plate I) shows the corresponding ultramicroscopic pictures, D
(Plate II) the macroscopic appearance of the light-cones. The num-
bers of the fluids are marked upon the cork stoppers.

In Plate I, under the bottles are given the linear dimensions of the
gold particles, as determined according to the method described and
conditions given in Drude's Annalen, Vol. X, pp. 16-30. For estimat-
ing approximately the size of the particles in amicroscopic hydrosols,
there was used a new method, the principle of which is given in the
Zeitschrift fur physikalische Chemie, Vol. LVI, p. 68, and in the
Zeitschrift fur Elektrochemie, 1906, p. 631.

The solutions shown each contain the same quantity of gold, i.e.,
0.005%. The ultramicroscopic images as well as the macroscopic
light-cones were observed and drawn after diluting each fluid ten
times. (In examining the ultramicrons of fluids, to 4, the isolated,
brighter, yellow particles should be disregarded. Because of dust
particles they form an incidental impurity of the fluid, difficult to
avoid; it must also be stated that it is impossible to reproduce, true
to nature, the brilliant light emitted by the gold particles in Fluid 5a*
and especially in Fluid 6. The gradations of brilliancy are naturally
considerably greater than appears from the illustrations. The red
colors of the fluids to 4 are more vivid than in the illustration. The
light-cone of Fluid 1 is distinctly green ; that of Fluid has no recog-
nizable color.)

At the right is a suspension that settles (No. 6), with particles of
100-150 pn, a turbid, faintly colored fluid whose ultramicroscopic
image consists of few very intensely luminous submicrons.

With decreasing size of the particles the brilliancy of the submicrons
considerably decreases ; from about 50 juju and under, the brilliancy of
the light-cone also decreases.

The almost-clear fluid marked No. 3, contains gold particles 20-25 ju/t
in diameter, all quite visible and mostly green. As is the case with

EXPLANATION OF THE COLORED PLATES

the subsequent numbers, and 1, it no longer settles, and furthermore
exhibits the properties of a colloidal solution, especially the irrever-
sible changes of condition characteristic of such solutions.

With further subdivision, the brilliancy of the ultramicrons speedily
diminishes, and we soon reach the amicroscopic field, where can be
seen the light-cone but no individual particles. The fluid marked O
(with particles of about 2-3 / ( u, not 0.2-3 as erroneously given on the
plate), is an example of this class. Tt appears completely clear both
in incident and in transmitted light, and its Faraday-Tyridall light-
cone (see 1), Plate II) is fainter than those of the other fluids.

Sometimes there can be obtained still finer subdivisions,, that show
in the ultraapparatus only individual dust particles, between which
the fluid appears optically clear. Fluids of this kind contain particles
still smaller than those previously described ; they resemble the latter
completely in superficial appearance and behavior, and their exist-
ence is a proof that gold itself can be subdivided until it is optically
homogeneous.

c

^q

o

INDEX

PAGE

Absorption 73^ 215

Absorption and adsorption , 73

Absorption compounds 68, 69

Adsorption 73, 215

^Escorcein, examination of 197

Albumen Grahams' work on 44

Albumen, protective action of 81, 82, 87

Albumoses, action of, on colloidal gold 82, 83

Alcogels 50

Alumina, soluble 38

Arnicrons, definition of 110

Aniline dyes, ultramicroscopic examination of 204

Antitoxins and toxins, mutual reactions of 73

Antitoxins, gold figures of 84

Argentous oxid 70

Argentum Cred6 191

Aurous sulphite 70

Barium sulphate suspension 193

Benzopurpurin 206

Bredig's hydrosols 188

Brownian motion , 137

Caesium subchlorid 69

Capillary theory as applied to colloidal solution 213

Caramel 43

Carey Lea's colloidal silver 190

Carmine.. 194

Casein, protective action of 81, 87

Chamberland filter, experiments with 153

Chemical compounds, simulation of, by colloidal mixtures 67

Chromium oxid, soluble* 41

Cigarette smoke, ultramicroscopic examination of 196

239

240 INDEX

Classification of colloids:

Graham's 22

Hardy's 19, 23

Muller's 19

Zsigmondy's 25

Cohesion, influence of, on optical constants 146

Colloid compounds 68, 69, 74

graphic representation of 217

simulation of chemical compounds of 70, 74

Colloid mixtures 68

Colloidal antimony sulphid 57

Colloidal arsenic sulphid 57-62

Colloidal condition 44

Colloidal copper sulphid 57

Colloidal ferric oxid 191

Colloidal gold 74

Colloidal gold and alumina 151

Colloidal gpld and gelatin 150

Colloidal gold, coagulation of 147

Colloidal gold, coagulation of by electrolytes 78

Colloidal gold, color changes of 144

Colloidal gold, color of particles of 141

Colloidal gold, color-producing particles in 161

Colloidal gold (Bredig's) examination of 188

Colloidal gold, finest subdivisions of 182

Colloidal gold, limits of size of particles in 160, 173

Colloidal gold, motion of particles of 134

Colloidal gold, preparation of 124

Colloidal gold, relative size of particles in 157

Colloidal gold, ultramicroscopic examination of 129

Colloidal mercury 191

Colloidal metals 63

Colloidal metastannic acid 54

Colloidal mixtures 67, 68

Colloidal molybdic acid 56

Colloidal palladium 189

Colloidal platinum 189

Colloidal selenium 210

Colloidal silicic acid 35-47

Colloidal silver 59

Colloidal silver, examination of 189, 190

Colloidal silver, heat developed by the coagulation of 15

Colloidal silver iodid 192

Colloidal solutions, characteristics of 11

INDEX 241

PAOB

Colloidal solutions, definition of 23

Colloidal solutions, distinguished from crystalloidal solutions 11

Colloidal solutions, distinguished from suspensions 12

Colloidal solutions, nature of 89

Colloidal solutions, ultramicroscopic examination of 113

Colloidal stannic acid 54

Colloidal sulphids 57

Colloidal titanic acid 54

Colloidal tungstic acid 55

Colloidal ultramarine 58

Colloids, Jordis' views on 126, 214

Colloids, preparation of by dialysis (Graham) 34

Colloids, protective action of 185

Color lakes, colloidal nature of 74

Crystallization, prevention of, by protective colloids 186

Devitrification 167

Dextrin, protective action of 81, 87

Diastase, appearance of action of, on glycogen 198

Dichroism in fibers 221

Distilled water, ultramicroscopic examination of 112

Dyeing, colloidal theory of 74

Dyes, examination of 198, 204

Energy of colloidal solutions 90, 91

Faraday-Tyndall method ; 3

Faraday-Tyndall test, sensitiveness of 96

Faraday's researches on colloidal gold 75

Ferric arsenite 73

Filtration experiments 153

Fluorescein, examination of 196

Fluorescent dyes, examination of 196

Gamboge solution 194

Gelatin, relative protective action of 80, 81

Gelatin solution, ultramicroscopic examination of 195

Gels, formation of 230

Globulin, digestion of 200

Globulin, protective action of 82

Glue, protective action of 81, 87

Glycogen, action of diastase on 158

Glycogen, disappearance of particles in 206

242 INDEX

FAOB

Glycogen, examination of 210

Gold, crystaloid solution of 162

Gold figure 80

Gold figure, determination of 80

Gold ruby glass 165

Graham's classification of Colloids 22

Graham's original nomenclature 30

Graham's original paper on diffusion 31

Graham's original paper on silicic acid 47

Gum arabic, protective action of 81, 87

Gum tragacanth, protective action of 80, 81

Haemoglobin 206

Hardy's classification of colloids 23

Heterogeneity of solutions 3

Heterogeneity of crystalloid solutions, demonstration of 6

Homogeneity of solutions 3

Horse sera, examination of 201

Hydrogel, definition of 24

Hydrogels, formation of 212

Hydrogels, remarks on. . .' 227

Hydrogels, structure uf 1 Hi

Hydrosols, formation of 212

Hydrosols, limits of size of particles in 100

Hydrosols, nature of 89

Hypomicrons, definition of 110

lodin suspension 193

Irish moss, protective action of 81

Irreversible hydrosols, definition of 25

Irreversible colloids', definition of 25

Irreversible hydrosols, effect of heat on the formation of 223

Irreversible hydrosols, two classes of 227

Isinglass, protective action of 81 , 87

Isoelectric state, Hardy's law of 219

Jellies, formation of 230

Maassen filter, experiments with 153

Matter, colloidal condition of 44

Mechanical subdivision, limitations of 212

Metal fog 70, 163

Metalutuina, soluble 40

INDEX 243

Micells, Nageli's views on 221

Microscopic resolvability 101, 109

Microscopic visibility 101

Milk, digestion of 201

Molten salts, solubility of metals in 1 ; 162

Mutual precipitation of colloids 72

Optical constants, influence of cohesion on 146

Osmotic method as applied to colloids 90

Osmotic method, uncertainty of 90

Pectization 213

Peptization ' 213

Peptones, action of, on colloidal gold 82, 83, 85

Polarization of light by small particles 115

Potassium subchlorid 69

Precipitation of colloids by each other 72

Protective action 25 148

Protective action of colloids:

relative values of 79

variation of 80

Protective action on suspensions 86

Protective colloids 77 f 7g j 79

Protective colloids, action of , 185

Pukall filters, experiments with 153

Purple of Cassius 65, 7^

Red phosphorus, suspension of 86

Reversible hydrosols, definition of .' 25

Reversible colloids, definition of 25

Reversible colloids, solution of 213

Rice starch suspension . 195

Rubidium subchlorid 69

Ruby glass 165

Ruby glass, gold particles in 165

Ruby glass, spontaneous crystallization in 169

Ruby glass, technology of 171

Selenium 209

Sera, examination of 201

Sera, gold figures of 84

Silica, ultramicroscopic appearance of 231

Silicic acid, soluble 35

244 INDEX

PAOB

Silicic acid, Graham's research on 47

Silver chlorid suspension 194

Silver purple 70

Silver suboxid 74

Smoke, examination of 196

Sodium subchlorid 74

"Sol-formers" '. 126

Soluble starch, effect of heat on solutions of 224

Soluble starch, examination of 195

Soluble starch, size of molecule of 88

Solution, definition of 8, 21

Solutions, heterogeneity of 3

Solutions, homogeneity of 3

Spontaneous crystallization 167

Stalagmometer, tests with 83, 84

Stannic acid, pectization and peptization of 214

Stannic acids 71

Starch paste, uitramicroscopic examination of 196

Subdivision, definition of 9

Submicrons, definition of 110

Subchlorids 69

Sulphagels 51

Sulphur, separation of from thiosulphates 209

Supercooled solutions 167

Supersaturated solutions, starting crystallization in 222

Surface tension, effect of, on gel formation 232

Suspension, definition of 9, 10, 22

Suspensions, contrasted with colloidal solution 8-17

Suspensions and colloidal solutions, intermediate forms between . . 16

Suspensions, flocculation of 13-15

Suspensions, uitramicroscopic examination of 193

Tetraiodofluorescein, examination of 197

Tin sesquioxid 71

Toxins and antitoxins, mutual actions of 73

Tragacanth, protective action of 80, 81

Ultramarine suspension 58

Ultramarine, color of 146

Ultramicrons, color of 117

Uitramicrons, determination of size of 117

Ultramicrons, limit of visibility of 122

Ultramicrons, definition of 110

INDEX 245

PAQB

Ultramicrons in gold hydrosols, relative size of 157

Ultramicrons, motion of, 134

Ultramieroscope, description of _ . . 103

Ultramicroscope, development of 95

Ultramieroscope, manipulation of 4 . . . , 106, 111

Ultramicroscopic examination of fluids Ill

Ultramicrosoopy, development of 95

Ultramicroscopy, questions for 92

Ultra-value, determination of 202

Ultra-value of solutions 199

Urine, ultramicroscopic examination of 199, 200

Vapor pressure of metals 163

Vegetable gum 42

Water for ultramicroscopic uses 208

Water, protective action of 175

Water, purification of 78

Weight, influence of, on suspended particles 20

Wheys, electrolysis of 202

Wheys, experiments with 201

Apparatus for Ultra-Microscopy and
Dark Ground Illumination

An illustrated and descriptive catalog is issued by Carl
Zeiss, Jena, copies of which, as well as the apparatus can be
obtained from us,

j &Iorab

NEW YORK

WASHINGTON

LONDON

CHICAGO

SAN FRANCISCO

FRANKFORT

ROCHESTER, N. Y.