501
PRACTICAL PROOFS
OF
CHEMICAL LAWS
VAUGHAN CORNISH
CUtnmr uf
(Culling
y? 47220
CHEMICAL LAWS
PRACTICAL PROOFS
OF
CHEMICAL LAWS
A COURSE OF EXPERIMENTS
UPON THE COMBINING PROPORTIONS OF
THE CHEMICAL ELEMENTS
VAUGHAN CORNISH, M.Sc.
ASSOCIATE OF THE OWENS COLLEGE, MANCHESTER
LONDON
LONGMANS, GREEN, AND CO.
AND NEW YORK
1895
All rights reserved
5b i
PREFACE
These experimental proofs (or more properly
verifications) of quantitative laws were undertaken
by pupils after the qualitative composition of the
principal substances employed had been carefully
dealt with in the accompanying lecture course.
Practical Physics went on side by side with the
practical chemistry course.
The pupils whose results are quoted in the
text were mostly between twelve and eighteen years
of age. A book of results was kept so that each
pupil could compare his results with others obtained
under similar conditions.
The pupils had hour at a time in the
laboratory, and attended twice a week.
vi Preface
I have not been satisfied with quantitative
experiments unless they yield good results in the
hands, not only of the teacher but of the pupils.
The results quoted in the text are those obtained
by the pupils.
I am not aware that a satisfactory standard as
to the accuracy required for such experiments has
yet been laid down. Within I per cent, is certainly
sufficient, but the standard may vary to some
extent according to the nature of the law or
problem investigated.
Perhaps the standard is best determined by
historical considerations, for the history of a science
is recapitulated by the learner. If the pupil can
verify a law to such a degree of approximation as
first served to convince the scientific world of its
truth, he may generally be satisfied with his work.
I have quoted in the text the results of early
historic experiments side by side with those ob-
tained by pupils. Although in these early experi-
ments the error is often large, yet there is less
difference than the learner may have supposed
Preface vii
between the accuracy of the first approximations
which have obtained the provisional assent of the
scientific world at the beginning and towards the
end of the nineteenth century, respectively.
As far as possible no numerical data, wThether
chemical or physical, were assumed. The density
of hydrogen and the proportion by weight in
which hydrogen and oxygen combine are neither
assumed nor determined in these experimental
verifications of the Laws of Combining Proportions.
It is not necessary that equivalent weights should
be referred to that of hydrogen, and the experi-
ments cannot be done with the same accuracy as
is attainable in the case of other elements.
The use of atomic and molecular formulae is
inadmissible in an examination of the facts upon
which the atomic and molecular theory is based.
Chemical equations and formulae have therefore
been excluded.
The course is, I believe, suitable for first-year's
students at colleges as well as for the upper forms
of schools.
viii Preface
I have to acknowledge valuable suggestions
received from other chemists while this course was
in preparation. My thanks are due more particu-
larly to Dr. J. B. Cohen, of the Yorkshire College,
Leeds, and to Mr. G. Stallard, of Rugby.
VAUGHAN CORNISH.
August 1895.
CONTENTS
CHAPTER I
PAGE
STATEMENT OF DALTON'S ATOMIC THEORY AND
OF THE LAWS OF COMBINING PROPORTIONS . I
CHAPTER II
THE LAW OF CONSERVATION OF MASS
Exercise I. — The Use of the Balance .... 6
Exercise IL — The Complete Synthesis of Silver Sulphide . 10
Calculation and Statement of Results . . . . .13
Note upon early 'Experiments relating to this Law, and upon
Stas' Determinations . . . . . 15
CHAPTER III
THE FIRST PART OF THE LAW OF DEFINITE
AND OF CONSTANT PROPORTIONS
Deductions, bearing upon this Law, from Exercise II. . . 17
Exercise III. — The Determination of the Proportion by
Weight in which Silver and Chlorine combine when
Chlorine Gas acts upon Silver . . . . 19
Exercise IV. — The Determination of the Proportion by
Weight in which Silver and Chlorine combine when
a solution of Hydrochloric Acid acts upon a solution of
Silver Nitrate ........ 22
X
Contents
Calculation and Statement of Results . . . . . 24
Note upon early Experiments relating to this Law, and upon
Stas' Determinations . . . . . . .25
Exercise IV. (a). — The Determination of the Quantities of
Hydrogen evolved on the Solution of Zinc in Sulphuric
Acid and in Hydrochloric Acid respectively . 28
CHAPTER IV
THE LAW OF EQUIVALENT PROPORTIONS
Scope of the Experiments in this Chapter . . . 31
Exercise V. — The Determination of the combining Pro-
portions of Silver and Bromine by the action of Hydro-
bromic Acid upon Silver Nitrate . . . . . 32
Exercise VI. — The Determination of the Percentage of Silver
in Silver Nitrate by the reduction of Silver Nitrate in
Hydrogen ......... 34
Exercise VII. — The Preparation of a Solution of Silver
Nitrate containing a known Weight of Silver per Cubic
Centimetre . . . . . . . . . 35
Exercise VIII. — The Determination of the Proportion in
which Potassium and Chlorine are combined in Potassium
Chloride 38
Exercise IX. — The Determination of the Proportion in which
Potassium and Bromine are combined in Potassium
Bromide. . . . . . . . 41
Calculation and Statement of Results obtained in Verification
of the Law of Equivalent Proportions . . . .41
Note upon early Experiments relating to this Law and upon
Stas' Work 43
CHAPTER V
THE SECOND PART OF THE LAW OF DEFINITE
AND OF CONSTANT PROPORTIONS
Exercise X. — The Determination of the constancy of the
Proportion between the Weights of Potassium and Chlo-
rine in Potassium Chloride and in Potassium Chlorate . 45
Note upon Stas' Determinations relating to the Second Part of
the Law of Definite and of Constant Proportions . . 48
Contents
XI
CHAPTER VI
THE LAW OF MULTIPLE PROPORTIONS
PAGE
Verification of the Law by Experiments upon the two
Chlorides of Copper 51
Exercise XI. — The Preparation of the two Chlorides of
Copper in a state of Purity . . . . 53
Exercise XII. —The Determination of the Copper in the two
Chlorides . . -55
Exercise XIII. — The Determination of the Chlorine in the
two Chlorides ......... 56
Calculation and Statement of Results obtained in Verification
of the Law of Multiple Proportions .... 62
A Second Method, alternative to that contained in the last
three Exercises, for Verifying the Law of Multiple
Proportions ......... 64
Exercise XIV. —The Determination of the Weights of Bro-
mine contained in known Weights of Mercurous Bromide
and of Mercuric Bromide ...... 65
Note upon early Experiments relating to the Law, and upon
Stas' Criticism of these Experiments . . . 68
CHAPTER VII
THE LAW OF SIMPLE VOLUMETRIC PROPORTIONS
IN THE CHEMICAL REACTION OF GASES
Upon the Molecules of Gases ...... 70
Exercise XV. — The Determination of the Volume of Nitrogen
obtained by the Decomposition of a known Volume of
Ammonia . . . . . . . ..72
Calculation and Statement of Results . . . . -77
Note upon early Experiments relating to the Law of Simple
Volumetric Proportions ...... 78
Upon the Relation of the (Physical) Molecule of Nitrogen Gas
to the (Chemical) Atom of the Element Nitrogen . . 79
Upon the relative Weights of the Chemical Atoms . . . 81
Upon the Choice of a Unit to which the Weights of the
Atoms are referred ....... 82
Upon Prout's Hypothesis . . . . . . . 84
xii
Contents
CHAPTER VIII
EXERCISES SUPPLEMENTARY TO THE COURSE ILLUS-
TRATING THE SCOPE OF THE TERM EQUIVALENCE
IN CHEMISTRY
l'AGE
Upon Diagrams illustrating Equivalence . . . . . 86
Supplementary Exercise I. — The Determination of the
Volume of Hydrogen evolved during the Solution of a
known Weight of Zinc in dilute Sulphuric Acid . . 87
Supplementary Exercise II. — The Determination of the
Weight of Copper deposited and of the Volume of
Hydrogen evolved during the Passage of an Electric
Current through a Solution of Copper Sulphate and
through dilute Sulphuric Acid . . . . 88
Supplementary Exercise III. — The Determination of the
Proportion by Weight in which Zinc and Oxygen com-
bine .......... 90
Supplementary Exercise IV. — The Determination of the
Proportion by Weight in which Copper and Oxygen
combine . . . . . . . . 91
Supplementary Exercise V.— The Determination of the
Proportion by Weight between the Zinc dissolved and
the Copper deposited when Metallic Zinc is placed in a
Solution of Copper wSalt, the Copper Salt being in excess 91
Supplementary Exercise VI. — The Determination of the
Proportion in which Hydrogen and Oxygen combine . 91
Errata.
Page 10 line 23 for 2 to 3 cm. read *2 to *3 cm.
35 ,, 28 ,, 50 cb.c. 500CD.C.
42 30 55 p. c. -55 p. c.
,, 42 ,, 32 ,, 60 p. c. '60 p. c.
,, 73 ,, 28 hypobromide ,, hypobromite.
PRACTICAL
PROOFS OF CHEMICAL LAWS
CHAPTER I
STATEMENT OF DALTON'S ATOMIC THEORY AND
OF THE LAWS OF COMBINING PROPORTIONS
DALTON'S atomic theory of chemical action may
be stated as follows : —
When a chemical action takes place, what we
observe on the large scale is the total effect of a
vast number of similar actions occurring between
ultimate particles, or atoms, of the substances. The
atom of each chemical element has its own specific
mass. These chemical atoms are beyond our
powers of vision, and we have no means of dealing
with them individually. Dalton's theory remains
therefore a theory only, and has not been raised to
the rank of a statement of observed facts. The
theory is, however, based upon observed facts
ascertained by experiment.
B
Practical Proofs of
The laws of chemical combination by weight
are the experimental basis of Dalton's theory.
The later discovery, by Gay-Lussac, of the
simple ratios between the reacting volumes of gases
(the 'Laws of Combination by Volume') led to
the development of the atomic theory in its present
form, in which we suppose the existence of two
orders of particles, the molecule and the chemical
atom. The modern development of theoretical
chemistry is due in great measure to the theory of
atoms and molecules, the theory itself being based,
as has been said, upon the laws of combining pro-
portions, gravimetric and volumetric. These are
both included in the term Laws of combining
proportions.
The methods of chemical analysis are founded
directly upon these experimental laws and upon
the law of conservation of mass, the results ob-
tained by analysis being independent of the atomic
theory.
These laws are therefore the foundation of the
greater part both of practical and of theoretical
chemistry.
We proceed to enunciate the laws before giving
the description of experiments by which each of
them may be verified.
i. The Law of Conservation of Mass. —
The law, in its bearing upon chemistry, may be
stated in a general form as follows : —
' The total mass of the substances taking part in
any chemical process remains constant' (Ostzva/d).
Chemical Laws
We will for the purpose of this course state the
law in a less general form, in which it can be readily
verified, as follows : —
When elements combine together chemically, the
mass of the compound formed is equal to the sum of
the masses of the elements before combination. — As
mass is almost always measured by weighing we
shall substitute the more familiar term weight in
the following statements of laws.
2. The Law of Definite and of Constant
Proportions. Part I, {definite proportions). —
When tzvo elements combine to form a particular
compound substance, they do so in a definite, fixed
proportion by weight, zvhich is independent of the
manner in zvhich tlieir combination zs brought about ;
and PART II. {constant proportions). This proportion
remains constant in compounds zvhich contain also
other elements.
3. The Law of Equivalent Proportions.
The zveights of tzvo elements zvhich are equivalent
(i.e. of equal value) in any chemical reaction are
equivalent in all
The meaning of this general statement of the
law may be illustrated by special statements ap-
plicable to particular cases which require different
experimental methods for their verification.
First special statement (applicable to the case
of elements each of which is capable of combining
with each of the others, e.g. silver, chlorine, and
sulphur) :
The zveights of tzvo elements (e.g. sulphur and
B 2
4
Practical Proofs of
chlorine) which combine zvith a certain fixed weight
of a third element (e.g. silver) are in the proportion
in which those two elements (sulphur and chlorine)
combine zvith one another. — These weights of sul-
phur and chlorine are said to be equivalent to one
another.
Second special statement (applicable to the case
of elements, some of which do not combine
together, e.g. silver, potassium, chlorine and
bromine) :
The weights of two elements (e.g. silver and
potassium) which combine with a given zveight of a
third element (e.g. chlorine) zvill also combine zvith
another fixed zveight of a fourth element (e.g.
bromine). — The experimental verification of this
statement of the law is given in Chapter IV.
4. The Law of Multiple Proportions. —
It sometimes happens that there are two (or more)
different substances formed from the same elements.
By different substances we mean materials which
differ in a marked degree in their physical characters
(e.g. in density, boiling point, melting point, and
so forth). To such cases the law of multiple pro-
portions applies. To simplify the wording of the
law, we will frame a statement suited to the case
of substances containing only two elements.
Statement of the Lazv of Multiple Proportions. —
If there be more than one substance formed by the
combination of two elements \ then, taking the zveight
of one element as fixed in each substance, the zveight
of the other element in the second compound bears a
Chemical Laws
5
simple proportion to the weight of that element in the
first. — When the student has fully grasped the
meaning of the word equivalence in chemistry,
which is generally not until he has had some ex-
perience of experimental work, he will find the
following statement useful as a summary of all the
laws of chemical combination by weight :
Elements combine together in the proportion [or
ratio) of their equivalent weights > or in the propor-
tion of ' ivhole multiples of their equivalent weights.
5. The Volumetric Law of Combination
OF Gases may be stated thus : — The volume of
an element in the gaseous state bears a simple pro-
portion to t)oe volume of the compound gas of zvhich
it is a constituent.
6
Practical Proofs of
CHAPTER II
THE LAW OF CONSERVATION OF MASS
Statement. — When elements combine together
chemically the mass of the compounds formed is
equal to the sum of the masses of the elements
before combination.
Mass is the one property of matter which re-
mains absolutely constant in every state of chemical
combination. The Balance is the instrument
employed in the comparison of masses, and a
knowledge of the systematic method of weighing
must be acquired before undertaking quantitative
chemical experiments.
Exercise I. — The Use of the Balance.
Apparatus required. — A chemical balance, the
pointer of which will move one division on the
scale when I mgrm. is placed on the empty pan,
and which will bear a load of 50 grams on each
pan. Becker's 50^. balance fulfils these conditions.
A set of weights from 50 grams to 1 mgrm. It is
well that each pupil should have a set of fractions
of a centigram. These can be obtained, in
platinum, for about 2s.
Chemical Laws
7
Determination of the Zero point. — Raise the
beam of the balance from its support by turning
the knob which is on the front of the balance
case. The beam now swings freely. The balance
is adjusted by the instructor, not by the pupil, so that
the pointer swings nearly evenly on either side of
the central line on the ivory scale. It is required
to determine exactly the point on the scale which
marks the position of equilibrium, or the Zero
point of the balance. This is best done, not by
waiting for the pointer to come to rest, but by
observing the swings on either side of the central
point. The amplitude of the swings steadily
diminishes owing to friction. Thus taking 10 as
the value of each division, calling the central point
100, and reckoning through from left to right, we
may observe the following extreme positions of
the pointer :
Left hand Right hand
• 85
IIO
the parts of a division being estimated by eye.
Now 85 is further from the Zero point and 87 is
nearer to the Zero point than no. We may
8 K 4" 87
assume that ^ —1=86 is as far on the left of
2
the Zero point as no is on the right. The Zero
point is found by taking the mean of the two
numbers 86 and 1 10 ; thus :
86 + I IO o ,rj . .v
= 98 (Zero point)
s
Practical Proofs of
Determination of Sensibility. — In weighing a
body it will generally be found after adjusting the
weights as nearly as possible that the pointer
swings nearly, but not quite, evenly about the
Zero point. The weights being placed on the
right-hand pan, and the body to be weighed on
the left-hand pan, then if the Zero-point is 98 and
the pointer is found to swing evenly about the
position 90, it is evident that the weights are some-
what too heavy. We require to know what extra
weight in one pan produces a deflexion of 8, i.e.
yo-ths of a division. To determine the sensibility
with empty pans, place one milligram on the right-
hand pan and observe three or five swings.
Suppose these to be
Right hand Left hand
126
52
122
the balancing-point is evidently
I24 + 5j = 88
2
The Zero point being 98, the sensibility with a
very small load is 10. Thus, in the example given,
if the weight of the body was small (say 5 grams)
we conclude that the weights on right-hand pan
were too heavy by T8Q-ths of a milligram. The true
weight of the body is therefore obtained by deduct-
ing *ooo8 gram from the amount of the weights on
the right-hand pan.
Chemical Laws
9
To determine the sensibility with a load of 20
grams in each pan. — The sensibility generally de-
creases with heavier loads. In the experiments
given in this course the loads are generally either
small (less than 5 grams) or about 20 to 25 grams in
each pan — e.g. in weighing porcelain crucibles.
To determine the sensibility in this second case,
place a 20-gram weight in each pan and determine
the balancing-point. This may not be exactly the
Zero point, since the two weights may not be
exactly equal. Suppose the balancing-point to be
96. Place *ooi gram (1 milligram) on the right-
hand pan and again determine the balancing-point.
Suppose this to be 88. The sensibility with a load
of 20 grams in each pan is 8.
Record of the results of the exercise. — The
observations of swings and the results for sensi-
bility are to be clearly stated in the pupil's note-
book. They should be taken down in the first
place in a small pocket note-book, in which all
weighings, as well as observations made during
the actual progress of experiments, should be
entered at the time, to be copied out afterwards
in a larger note-book. Weighings should on no
account be entered on loose sheets of paper ; the
results of experiments are often lost in this manner.
IO
Practical Proofs of
EXERCISE II. — Verification of the Laiv of Conser-
vation of Mass by the complete synthesis of
silver sulphide.
This experiment shows that when silver and
sulphur are heated together, they combine chemi-
cally, producing a new substance (silver sulphide),
and that the weight of the silver sulphide formed is
equal to the sum of the weights of the silver and
sulphur from which it zvas produced.
Apparatus and substances required. — Fine silver
wire on a reel — that used in surgery is the best.
Re-sublimed sulphur, as obtained from the dealers,
but freed from moisture by being kept on a clock
glass in a desiccator over strong sulphuric acid.
The clock-glass must be replaced in a desiccator
when a portion of the sulphur has been taken out
for the experiment. Glass tubing of soft glass,
not combustion tubing ; the most convenient width
of bore is i to rj cm. It should be sealed up at
one end and be of a length not less than 1 6 to 20 cm.
A cork to fit this tube, through which passes,
somewhat loosely, a piece of glass tubing about
2 to 3 cm. diameter. Carbonic acid apparatus. Foot-
blowpipe and bellows. An oven or gas-furnace in
which a tube of about 10 cm. length can be placed
and heated to about 5000 C. In default of this,
the tube may be covered with sand and heated on
a sand-tray. Weighing- tube , zvatch-glass, glazed
paper, camel-hair brush or, better, a stiff feather,
carbon bisulphide, sliarp three-cornered file, scissors.
Chemical Laws
Method of conducting the experiment. — Make
the glass tube thoroughly dry by moving it from
side to side and rotating in a luminous gas flame,
at the same time blowing air through from the
bellows. Measure off a length of the silver wire
which has been found to weigh about one gram.
Cut this up with scissors into short pieces, about
•5 cm. length, and weigh accurately. This sub-
stance may be weighed on the bare pan of the
balance. Transfer the silver wire to the tube. We
now have to weigh out about enough sulphur to
combine with the whole of the silver. For 1 gram
silver take *i 5 to -20 grams sulphur. The quantity
taken must be accurately weighed. The sulphur may
be placed in a small stoppered weighing tube which
must be dry. This is weighed with its contents,
and the sulphur is then shaken out carefully, little
by littie, into the tube which contains the silver.
Should any sulphur fall outside the tube it is
received on the glazed paper and can be brushed
into the tube. If the tube has not been properly
dried the sulphur will adhere to the sides. The
weighing-tube which contained the sulphur is
corked and weighed again, the difference of the
two weighings being the weight of sulphur taken.
If the tube were sealed up while containing air
some of the sulphur would burn on heating. In
order to avoid this complication the air is displaced
by carbonic acid before sealing. The gas is passed
in through the narrow glass tube carrying the cork,
the cork of course not being fitted into the wider
I 2
Practical Proofs of
tube. The current of gas is turned on slowly in
order to avoid the risk of scattering the sulphur
over the sides of the tube. The tube is full of
carbonic acid when a taper is extinguished at its
mouth. Now stop the current of carbonic acid
and fit in the cork ; draw the narrow tube through
the cork so that it projects not more than I to 2 cm.
into the wider tube. The wider tube must now be
sealed up. In the operation of sealing, begin by-
warming the tube above the luminous flame of the
blow-pipe ; then bring it into the luminous flame,
keeping it constantly turned. When the tube is
covered with soot the blast may be put on, but
very gently at first. When the tube softens draw
it out slightly, then lower the flame and heat
strongly, rotating the tube during the operation, so
that the sides fall together. When this has taken
place cool slowly in the luminous flame until the
tube is again coated with soot. If the pupil has
no previous practice in glass-working this part of
the exercise should be rehearsed with an empty
tube to master the manipulation. This practice of
rehearsing may with advantage be adopted when-
ever a quantitative experiment involves a new
piece of manipulation. Everything is now ready
for bringing about chemical combination by
heating. The method of doing this has already
been described. The time for completing the
reaction depends partly upon the thickness of the
wire. It is best to arrange matters so that the
weighings and sealing are completed at the end of
Chemical Laws
13
a lesson, so that the heating of the tube may be
left to go on for a few hours in order to ensure
completion. The remaining determinations can be
done in the next lesson.
After heating, it is found that the silver and
sulphur have been converted into the black, shining
crystalline silver sulphide. If an excess of sulphur
was taken this will be found collected on the part
of the tube which was coolest. We now have to
weigh the silver sulphide and the excess, if any, of
sulphur. With a sharp file make a deep transverse
cut at the middle of the tube. Touch this with a
fine point of glass which has been heated till quite
soft in the blow-pipe flame. In this way the tube
may be readily cut into halves. The sulphide is
readily detached from the tube and is brought on
to a weighed watch-glass and weighed. The tube
must be carefully brushed out, the watch-glass being
placed on the sheet of glazed paper. The accuracy
of the results obtained in this experiment depends
upon the attention given to the details of manipu-
lation, the chemical part of the experiment not
presenting any difficulty. If there is any excess
of sulphur, weigh the pieces of glass to which the
sulphur adheres, dissolve off the sulphur with
carbon bi-sulphide, in a draught chamber, and
weigh again. The difference gives the weight of
the sulphur.
Calculation and statement of results. — Examples
are given from the results obtained by two pupils.
The weights are in grams.
14 Practical Proofs of
The first pupil found
Before heating After heating
Silver . . '8301
Sulphur . . '1233 (no excess of sulphur)
Sum . . *9534 Sulphide . -9531
The weight of the sulphide is found to be less
than that of the elements from which it was formed
by 3 parts in 9534. Regarding the exercise as
intended to verify a law believed to be mathe-
matically exact, we may say that this represents
the experimental error. This should always be
expressed as a percentage. To find the percentage
in the above case divide 3 by 9534 and multiply
by 100. The result is, very nearly, '03. It must
be written with the minus sign, since the weight of
the compound formed is found to be less than it
should be ; therefore we write
Experimental error —'03 per cent.
The second pupil found
Before heating After hea ing
Silver . . .1 *oooo
Sulphur taken . . '2963 Excess sulphur . '1476
Excess sulphur left . '1476 Sulphide . . 1*1476
Therefore sulphur used
in combination . '1487
Sum of weights of elements before combination . 1*1487
Weight of compound formed ..... 1*1476
Difference ........ *oon
Therefore experimental error ..... — *iop. c.
The average error of thirteen experiments
recorded by ten pupils was -5 per -cent. One of
Chemical Laws
15
the experiments exceeded 2 per cent, in error, and
another exceeded 1 per cent. In no case was the
error +tive, i.e. the weight of the compound was
never found to be greater than the sum of the
weights of its constituents.
Note upon early experiments relating to the Law
of Conservation of Massy and upon St as determi-
nations.— W. Black (' Experiments upon Magnesia
Alba/ published 1782, pp. 66-68) converted 120
grains of chalk, by heating, into quicklime, of
which 68 grains were left. The lime was thrown
into a solution of carbonate of soda, by which
means chalk was again formed, which weighed 1 18
grains. The final weight was, therefore, found to
be less than the original weight by 2 parts in
1 20: experimental error — r6 per cent.
Lavoisier (' Elements of Chemistry,' translated
by Dr. Kerr, published 1792) decomposed mercuric
oxide by heat, weighed the mercury obtained, and
measured the volume of oxygen. He also deter-
mined the density of oxygen. The experimental
error appears to have been about — 1 per cent,
but the statement of results leaves some uncertainty.
Lavoisier stated (loc. eit.) the law of conservation
of mass in connection with his experiments upon
the fermentation of sugar in presence of water
and yeast. He found that the weight of the
products formed was equal to the weight of the
substances decomposed. The numbers given are
apparently not those actually determined, but
rather what he considered they ought to have been
1 6 Practical Proofs of
if experimental error were eliminated. The num-
bers, therefore, are not available for calculating the
approach to accuracy attained in the experiment.
For modern work upon this law, see Stas'
* Nouvelles Recherches sur les Lois des Proportions
Chimiques/ experiments upon the complete syn-
thesis of silver iodide, and complete analysis of
silver iodate, which confirmed the law within a very
small margin of experimental error.
Chemical Laws
17
CHAPTER III
THE LAW OF DEFINITE PROPORTIONS
Statement of the Law. — When two elements
combine to form a particular compound substance ,
they do so in a definite , fixed proportion by zv eighty
which is independent of the manner in which their
combination is brought about.
THIS is the first part of the law of definite and
of constant proportions ; for the second part^ see
Chapter V.
The significance of this law lies in the persis-
tence with which elements adhere to certain pro-
portions of combination in spite of variation in the
conditions under which combination occurs. The
conditions of combination may vary in the follow-
ing respects : —
1. Relative masses of the reacting substances.
2. Pressure under which the reaction occurs.
3. Temperature at which the reaction occurs.
4. Chemical composition of the substances by
the reaction of which the combination is brought
about.
The fact that the combining proportion does
not depend upon the relative masses present may
C
iS
Practical Proofs of
be verified by a comparison of the results obtained
by different pupils in the last exercise. Thus,
taking the two examples already quoted : —
Relative masses taken Combining proportion
First pupil . Sulphur I : Silver 6732 Sulphur 1 Silver 6732
Second pupil . I : „ 3-333 „ 1 „ 6777
the difference between the combining proportions
obtained by the two pupils is '66 per cent., the
combining proportions being constant within '66
per cent, when the proportion between the sub-
stances taken was varied from 2 to 1.
The second condition (variation in pressure) is
tested to some extent by the same experiment,
since the pressure in the tube depends, other things
being equal, on the excess of sulphur present.
The third and fourth variations of condition
(viz. of temperature, and of the chemical composi-
tion of the reacting substances) are tested by the
following experiments upon the proportion by
weight in which silver and chlorine combine
together. The last (fourth) mode of varying the
conditions is especially important as showing that
the combining proportions are independent not
only of physical but also of chemical conditions.
In the first experiment (Exercise III.), the ele-
mentary gas chlorine acts on metallic silver at a
temperature of about 4000 C. to 5000 C. In the
second experiment (Exercise IV.) the combination
of the two elements, chlorine and silver, is brought
about by the action of a solution of hydrochloric
acid upon silver nitrate at a temperature rather
Chemical Laws
19
below ioo°C. Exercises III. and IV. may be
worked side by side. Such an arrangement as the
following will save time. Weigh out the materials
and start the evaporation in Exercise IV. While
this is going on, weigh out for Exercise III. and
start the heating in chlorine. Then return to
Exercise IV., which can be finished. The chlorina-
tion in Exercise III. is, if possible, left going for
some hours, the weighing and calculation being
postponed till the next lesson.
EXERCISE III. — Determination of the proportion in
zvhich silver and chlorine combine zvhen chlorine
gas acts upon metallic silver.
The following are the apparatus and substances
required, and the method of conducting the experiment.
Precipitated silver is used, not less than *5 gram.
The heating arrangement to be adopted will depend
partly upon the equipment of the laboratory. Two
sources of error must be provided against : — First,
portions of silver may become surrounded by fused
chloride, and thus be kept from the action of the
gas. To avoid this as much as possible, the metal
should present a large surface, the stream of gas
should be slow, and the temperature should be
regulated so that the chloride only just fuses.
Secondly, chlorine gas is absorbed by the fused
chloride, and on cooling the escape of the gas is
apt to cause spirting. This is to some extent
guarded against by the precautions already men-
20
Practical Proofs of
tioned, and also by cooling slowly at the end of
the operation. In order to ensure that no loss
should take place even if spirting did occur, either
of the following arrangements will do. First
arrangement of apparatus : — The silver and the
resulting chloride are weighed in a glass tube,
closed at one end, provided with a loose plug of
glass wool about half-way up, and fitted with a
two-way stopper, the chlorine entering through one
tube, and the excess of the gas passing out of the
other tube to the draught. The closed end of the
tube where the charge of silver lies can be heated
by means of a Bunsen burner. This arrangement
can only be adopted conveniently if the balance is
capable of taking a tube of the necessary length.
Second arrangement of apparatus : — If a short com-
bustion furnace be available, the silver may be
weighed in a porcelain boat, which is placed inside
a glass tube lying in the bed of the furnace. If
any spirting take place the particles of chloride
may be detached from the glass tube and returned
to the boat after the operation is at an end. The
exit end of the tube is connected to the draught.
The clilorine apparatus should supply a regular
stream of the gas for some hours without requiring
attention. It should hot require replenishing
during use ; in fact, it should be of such a form
that it needs replenishing only at long intervals.
The Kipp apparatus, with balls of compressed
bleaching powder, is readily set working, and has
the advantage of not requiring heat. It has, how-
Chemical Laws
21
ever, the disadvantage that when left standing the
joints are subjected to pressure, and any leak
results in the liquid running down. It also needs
replenishing more often than the porcelain chlorine
still. The chlorine still, made by the Berlin
Porcelain Company, is filled to a depth of about
4 cm. with pebbles or glass stoppers, and on these
are placed lumps of manganese dioxide, 1 — 1*5 ch.c.
in diameter. The hydrochloric acid used is of the
strength of equal parts of the strong acid and of
water. The joint between the cover and the vessel
is made tight by a number of filter papers soaked
in oil. The porcelain vessel stands in a water-
bath, which is supplied with it, and when a current
of gas is required a burner is lighted below the
water-bath. The water is soon warmed, as the
surface of the metal is large and the volume of
water is relatively small. It is best not to allow
the water to boil, as the steam is apt to cause the
filter-paper joint to leak. A glass stop-cock con-
trols the flow of gas from the still, and a screw-tap
or nipper-tap should be used to control the entrance
of the acid from the upper vessel. The glass stop-
cock having been partly turned on, matters are
adjusted before leaving the apparatus to itself, so
that the level of the acid in the upper vessel re-
mains stationary. When it is desired to stop the
current the exit tap is closed. The amount of
liquid in the still can be judged by the level of the
acid in the upper vessel. If most of the acid is in
the upper vessel, turn out the burner under the
22 Practical Proofs of
water-bath and close the inlet tap. If, on the other
hand, most of the acid has run down into the still,
continue heating after turning off the exit tap until
most of the acid is driven into the upper vessel,
when the inlet tap may be closed. When the acid
requires renewal, syphon from the upper vessel,
the inlet-tap being closed. A large supply of
manganese dioxide can be put into the still,
sufficient to last without renewal for a term's
laboratory work. The still can then be replenished
when the laboratory is no longer in use by the
pupils, and the apparatus can be got ready again
(air expelled, acid driven back, and joints tested)
immediately before the commencement of the next
term's laboratory classes.
For calculation and statement of results of the
exercise see post, at the end of Exercise IV.
EXERCISE IV. — Determination of the proportion in
which silver and chlorine combine when a solution
of hydrochloric acid acts upon a solution of silver
nitrate.
The following are the apparatus and substances
required : — Granulated silver. Nitric acid \ the ' pure,
strong ' should be used, but a little hydrochloric acid
as impurity does not matter. Hydrochloric acid,
pure, strong. Porcelain crucible and lid of the size
generally used in quantitative analysis, weighing
about 22 grams. Pipette delivering 10 cb.c. Iron
tripod, pipe-clay triangle, crucible tongs, preferably
Chemical Laws
23
of gun-metal, desiccator \ wash-bottle, and water-bath.
Water-baths are somewhat expensive, and economy
may be effected in working this course of experi-
ments, in which the evaporations are done in
crucibles, by using water-baths such as those made
for Wanklyn's process of milk analysis, which are
provided with a number of holes of a size required
for crucibles. As it is desirable that the evapora-
tion should not require watching, the bath should
be furnished with an arrangement for keeping the
level of the water constant.
Method of conducting tlie experiment. — Weigh a
porcelain crucible and lid. Weigh out in the
crucible '$-'6 grams pure granulated silver. Just
cover with hot water and add strong nitric acid
from a pipette slowly \ so as to keep up a fairly rapid
evolution of gas without such effervescence as
might result' in loss. Towards the end the process
may be hastened by warming on the water-bath,
but the contents of the crucible must not be
evaporated to dryness while any silver remains
undissolved. When the whole of the silver has
dissolved, evaporate to dryness in order to expel
excess of nitric acid, the crucible being uncovered
and placed in a draught chamber during the
process. Dissolve the silver nitrate in a very little
hot water, add 6-10 cb.c. strong pure hydrochloric
acid, and evaporate to complete dryness on the
water-bath. When dry, replace the lid and heat
gently on the pipe-clay triangle, preferably with a
rose on the Bunsen burner. When there is no
24
Practical Proofs of
longer danger of spirting, remove the lid and heat
the crucible carefully with a small flame of the
Bunsen burner, applying the heat round the sides
of the crucible rather than at the bottom. As soon
as the chloride begins to fuse round the edges,
cease heating, replace the lid, and transfer to the
desiccator. Weigh when quite cold.
Calculation and Statement of Results of
Exercises III and IV.
A pupil found that
•500 grams silver by second method gave
and that
'497 grams silver by first method gave
therefore
•500 grams silver by first method would give '663 of chloride.
Therefore, assuming the truth of the law of con-
servation of mass, which was verified in Exercise
II., the proportions in which chlorine combines
with a fixed weight of silver under the different
conditions of Exercises III. and IV. were found to
be as
163 : 164
The difference is *6 per cent., which, on the assump-
tion that the law of definite proportions holds
exactly, is the experimental error.
For a supplementary experiment (Exercise
IV. A.), illustrating the application of the rule of
•664 of chloride
•659 of chloride ;
Chemical Laws
definite proportions to the case of elements which
do not combine together, see end of this chapter.
For an experiment verifying the second part of
the law (viz. that the definite fixed proportion
between the weights of two elements combining to
form a particular compound substance is a constant
proportion between the weights of those two
elements present in chemical compounds con-
taining also other elements) see Chapter V.
Note upon early experiments relating to the Law
of Definite Proportions, and upon Stas* determina-
tions.— The accumulation of the results of chemical
analysis in the later part of the eighteenth century
gradually established the fact that a number of
well-known substances, e.g. certain salts and
minerals, had a definite fixed composition. Some
chemists were disposed to regard the fixed com-
position of these substances as evidence of a general
law that elements can only combine chemically in
certain constant proportions which are independent
of the conditions of combination. The growth of
this view was strongly combated by Berthollet in
the first years of the nineteenth century. He con-
tended that there was no such restriction upon the
combining proportions of the elements. The fixity
of composition of characteristic chemical substances
he considered to be determined mainly by con-
ditions, such as insolubility, volatility, &c. Ana-
lyses were undertaken by Proust, designed to
test the view that the combining proportions of
chemical elements are independent of conditions of
26
Practical Proofs of
temperature, pressure, solubility, and so forth.
The issue of the controversy confirmed the views
advocated by Proust in opposition to Berthollet.
We quote some of Proust's results. He writes
(' Journal de Physique,' vol. lv., A.D. 1802, p. 326) :
' One hundred parts of antimony, and the same
amount of sulphur, heated in a glass retort till the
whole is completely fused and the excess of sulphur
has been driven off, leave 135 parts by weight of
the sulphide.
4 This experiment, however often repeated,
always yields the same result. One hundred parts
of antimony heated with 300 parts of cinnabar (the
native sulphide of mercury) give from 135 to 136
parts of the sulphide. These sulphides heated with
an equal weight of sulphur did not increase in
weight. It follows, that antimony conforms to the
same law as all the metals which are capable of
combining with sulphur. They take up a constant
quantity fixed by Nature, and Man has no power
to increase or diminish this quantity.' This result
may be taken as verifying the Law of Definite Pro-
portions within about *5 per cent.
In dealing with the more difficult case of iron
pyrites (' Journal de Physique/ vols. lii. and liv.),
Proust's results are less satisfactory from a
numerical point of view, although they are more
interesting in so far as they afford an early example
of the artificial reproduction of a natural mineral,
and a rough confirmation of the fact that the amount
of sulphur with which iron can combine is the same,
Chemical Laws
27
whether the sulphide be formed in the laboratory
or in, e.g. a mineral vein. Proust found that 400
parts of the natural iron pyrites were reduced by
heating sufficiently to 318 parts. He then mixed
318 parts of the common black sulphide of iron
with sulphur, and after heating, not too strongly,
found that a substance was formed having the
principal properties of the natural iron pyrites and
which weighed 378, instead of 400. This confirms
the law to within about 5^- per cent.
In 1805 Gay-Lussac and Humboldt found that
the proportions in which hydrogen and oxygen
combine to form water is unaffected by differences
of temperature and pressure.
For modern work showing to a high degree of
accuracy that the proportions of combination in
the formation of a particular compound are un-
affected by circumstances of temperature and
pressure, or by the source from which the substances
are derived, see Stas' analysis of ammonium
chloride (*' Nouvelles Recherches sur les Lois des
Proportions Chimiques ' in the ' Memoires de
TAcademie Royale de Belgique/ vol. xxxv. pp„
48-57).
Application of the Law of Definite
Proportions to the case of elements which
REACT, BUT DO NOT UNITE TOGETHER.— To meet
the case of the chemical displacement oi one element
by another we may adopt the following statement
of the Law of Definite Proportions : —
Chemical elements react together in a definite fixed
28
Practical Proofs of
proportion. — The displacement or replacement in a
definite fixed proportion may be verified by an
experiment upon the action of magnesium (or zinc)
upon dilute sulphuric and hydrochloric acids, when
the metal dissolves in the acid and hydrogen gas
is evolved.
EXERCISE IV. (a). Determination of the quantity
of hydrogen evolved on the solution of a given
iveight of magnesium (or zinc) in dilute sulphuric
acid and dilute hydrochloric acid respectively.
The following are the apparatus and materials
required : — Either magnesium ribbon, or, if the ex-
periment be made with zinc instead of magnesium,
pure granulated zinc should be used. Glass tube
about 2 cm. long and I cm. wide closed at one
end. Glass wool Pad of caoutchouc (a slice from
a large rubber stopper will do) with a hole cut to
fit the closed end of the tube, and a nick or chan-
nel in the upper surface. Glass-mortar or small
pudding-basin, or deep evaporating dish. Pure
dilute sulphuric and hydrochloric aeids. Wide
glass cylinder, or other deep vessel, thermometer,
and a graduated gas-measuring tube which can be
closed with the thumb (see also Chapter VII.,
Exercise XV.). Retort stand and clamp,
Metliod of conducting the expej'iment. — Weigh
out with the greatest care about *o6 gram of
magnesium in short pieces (or about '15 gram
granulated zinc). With a balance such as has
been referred to in Chapter II., and weighing by
Chemical Laws
29
vibrations, the error of weighing need not be
more than *5 per cent, of the weight of zinc,
but may be as much as 1 per cent, on the weight
of magnesium. Place the pieces of metal in the
short glass tube. Plug the open end loosely with
glass-wool. Place the closed end of the tube in
the hole cut in the piece of caoutchouc provided
for this purpose. Fill up the short tube with water.
Half fill the dish or mortar with weak sulphuric
(or hydrochloric) acid. The ordinary dilute sul-
phuric acid used in the laboratory (1 of strong
acid to 5 of water) may for this purpose be diluted
with an equal bulk of water. Similarly, ordinary
dilute hydrochloric acid (1 of strong acid to 3 of
water) may be further diluted with an equal bulk
of water. Fill the graduated tube with the same
acid as that in the basin. Close the open end of
the tube with the thumb and invert it in the dish,
the dish being placed upon the foot of the retort
stand. Hold the graduated tube in a slanting
position, and quickly bring the open end of the
short tube, which contains the metal, under the
open end of the graduated tube. At once bring
the graduated tube into the vertical position and
clamp it so that it rests firmly upon the pad of
caoutchouc. The metal dissolves in the acid with
evolution of hydrogen gas. If the plug of glass-
wool is too tight the operation is delayed, but it
must not be so loose as to be carried away by the
rush of gas, otherwise pieces of metal may float to
the surface of the liquid and be left adhering to
30
Practical Proofs of
the sides of the measuring tube above the level
of the acid. In this case it would be necessary to
incline the tube so that the acid flows on to the
metal. When the metal is all dissolved transfer
the graduated tube to a tall cylinder filled with
water. Bring the water to the same temperature,
say IS°C, in both experiments. Allow the tube
to remain in the water at this temperature for five
or ten minutes, and then, holding the tube in a
paper holder, or by means of a clamp, to prevent
warming by the hand, read off the volume of the
gas, the level of the water being made the same
inside and outside of the tube. The level of the
gas is determined from the level of the bottom of
the water meniscus in the tube.
Calculation and statement of results. — Calculate
the volume of gas evolved for the same weight of
metal, (a) when sulphuric acid and {b) when hydro-
chloric acid is used, and compare the results.
Example : — A pupil found that the volumes of
hydrogen evolved by the same weight of zinc from
hydrochloric acid and from sulphuric acid re-
spectively differed by 7 per cent.
Chemical Laws
3*
CHAPTER IV
THE LAW OF EQUIVALENT PROPORTIONS
THE experiments described in this chapter upon
the verification of the above law show that those
weights of silver and potassium which combine with
a certain weight of chlorine combine also with another
fixed weight of bromine. For a general statement
of the law, see Chapter I. We assume, throughout
the course, the truth of the Law of Conservation of
Mass which was verified in Exercise II.
It is desirable that the pupil, before conducting
the experiments described in this Chapter, should
be acquainted with the evidence showing that
potassium chloride (or bromide) is a compound of
potassium and chlorine (or bromine) only. Davy's
proof of the composition of potassium chloride may
be found in his ' Collected Works/ vol. v. p. 58 et
sea., Cavendish Society's Publications.
The first experiment needed for our verification
of the law is that of the synthesis of silver chloride,
which has already been performed as an exercise
on the Law of Definite Proportions. The second
is the synthesis of silver bromide, which is shortly
described in the next paragraph. The analyses of
32
Practical Proofs of
the chloride and bromide of potassium involves the
application of methods not yet explained in this
course, and a more detailed description of them is
therefore given. If desired, iodine may be substi-
tuted for bromine in the following exercises.
EXERCISE V. — Determination of the combining
pTOpoi'tion of silver and bromine \ by the action
of liydrobromic acid on silver nitrate.
The following are the apparatus and substances
required: — Pure granulated silver, solution of hydro-
bromic acid (the unsaturated acid containing free
bromine will do), pure strong nitric acid, which
must be free from hydrochloric acid. To test this
point, dilute and add a drop of solution of silver
nitrate. If a precipitate of silver chloride appears
the acid must be purified or a pure sample obtained.
To purify, add some silver nitrate solution to the
nitric acid and distil from a small retort placed on
a sand-tray. The beak of the retort passes into
the neck of a glass flask which is kept cooled. The
acid which distils over will be free from hydro-
chloric acid. The apparatus required in Exercise
V. is the same as in Exercise IV.
Method of conducting tlie experiment. — Weigh a
porcelain crucible and lid. In the crucible weigh
out *2 5 — *3 gram of pure granulated silver. Dis-
solve in the pure nitric acid, evaporate to dryness
and re-dissolve in water. Add hydrobromic acid
cautiously. If the reaction is violent the lid must
Chemical Laws
33
be placed upon the crucible to prevent loss by
spirting. The evaporation should not be com-
menced at the full heat of the water-bath. When
dry, add a little hot water and more hydrobromic
acid, and take down to dryness again. Without
this second treatment some of the silver nitrate
would escape conversion to bromide. Finally
heat carefully over the bare flame till the substance
begins to fuse.
Calculation and statement of results. — Calcu-
late from the mean value obtained from Exercises
III. and IV. what weight of silver combines with
i of chlorine. From Exercise V. calculate what
weight of bromine combines with this weight of
silver.
For mode of statement of results and examples
of results obtained, see end of Exercise IX.
Before proceeding to the analysis of potassium
chloride (Exercise VIII.), two preliminary exercises
must be performed.
The analysis of potassium chloride is effected
by determining the chlorine in a given weight of
the salt and subtracting the weight of the chlorine
from that of the salt taken in order to find the
weight of the potassium. The quantity of the
chlorine needed is determined from the weight
of the silver needed to combine zvith it> using
the mean value obtained from Exercises III.
and IV. The method we shall employ is a
volumetric one, in which we estimate the chlorine
by determining the volume of a silver-nitrate
D
34
Practical Proofs of
solution of known strength which is just sufficient
to provide silver to combine with the chlorine con-
tained in the weight of potassium chloride taken.
For carrying out this method we require to know
the percentage of silver contained in a silver nitrate
which forms the subject of the next exercise.
Exercise VI. — Determination of the percentage of
silver in silver nitrate by the redtiction of the
silver nitrate in hydrogen.
The following are the apparatus and substances
required : — Crystals of silver nitrate. An arrange-
7nent for heating the substance in a current of a gas \
as described in Exercise III. A current of a re-
ducing gas, either coal-gas or hydrogen. If coal-gas
be used it is well to let it bubble through a liquid,
in order that the rate of supply may be observed
and controlled. The materials for generating
hydrogen are zinc (the distilled zinc is the purest,
but it is expensive) and dilute pure sulphuric acid
(i of strong acid to about 4 of wTatcr). The gas
may be purified by passing it through a solution
of potassium permanganate.
Method of conducting the experiment. — The re-
duction must not proceed too rapidly, as loss of
material might occur. The current of gas must
therefore be slow, and the reduction should be
carried out at as low a temperature as possible.
When no more brown fumes of oxides of nitrogen
Chemical Laivs
35
are formed the reduction is finished. The reaction
is quickly effected.
Calculation and statement of results. — From the
weight of silver nitrate taken and the weight of
silver left, calculate how many parts of silver there
are in ioo parts of silver nitrate. The following
calculation will also be found useful. From
Exercises III. and IV. we know the weight of
silver which combines with weight I of chlorine ;
calculate from Exercise VI. what weight of
silver nitrate contains this weight of silver. The
following is an example of results obtained. A
pupil found that '4765 silver nitrate gave '3044
silver. The percentage of silver is therefore
63*88 per cent, found by the pupil, as against
63-50 per cent, given by standard determinations.
Assuming the standard determination as absolutely
correct, the experimental error of the pupil's de-
termination is +'38 per cent.
EXERCISE VII. — Preparation of a solution of silver
nitrate containing a knozvn weight of the element
silver per cb.c.
The following are the apparatus and substances
required : —Crystals of silver niU'ate, distilled
water, solution of litmus, vaseline, a 500-cb.c.
measuring- flask, small beaker, small funnel, 50-cb.c.
pipette, two glass-stoppered bottles, capable of holding
at least 50 cb.c. each, gummed labels.
d 2
36
Practical Proofs of
Method of preparing the solution. — Wash out
the 500-cb.c. flask carefully with distilled water ; it
need not be dried afterwards. Weigh out accurately
about 8*5 grams of silver nitrate. The distilled
water for dissolving the silver nitrate should be
neutral, and must not contain more than a very
small trace of chloride. To test the latter point
add a drop of silver nitrate to some of the water.
It should give at most a very faint turbidity. To
test if neutral, pour one or two drops of blue (or,
better, claret-coloured) solution of litmus into a
porcelain dish and add about 25 cb.c. of the water.
The colour should not change. The distilled water
being such as is required, dissolve the silver nitrate
in a small beaker, preferably one which has a lip
for pouring. Place a small funnel in the neck of
the 500-cb.c. measuring- flask and pour in the
solution. A trace of vaseline put on the under
side of the lip of the beaker will prevent the
liquid from running down the outside. Rinse out
the beaker several times into the flask. Make up
the level in the flask so that the lower level of the
meniscus touches the mark on the neck. Place
the glass stopper, which must fit accurately, in the
flask, and, holding this in its place, thoroughly
mix the contents by inverting the flask once or
twice. We have now a stock of silver nitrate of
known strength, i.e. which contains a known
weight of silver per cb.c. Transfer the solution to
a clean and perfectly dry stoppered bottle. The
drying must be effected by heating and blowing
Chemical Laws
37
air through, as described in Exercise II. Label
the bottle thus :
Standard Silver Nitrate Solution
I cb.c. contains . . . silver = . chlorine
= . bromine
Name of pupil
Date .....
In the actual titrations a more dilute solution
made from this will be used. It can be made up
from time to time as required by withdrawing
50 cb.c. by means of a 50 cb.c. pipette, delivering
into a 500 cb.c. flask and making up to the mark on
the neck. The pipette used must be clean and dry.
To dry it, blow air through from the bellows while
warming not too strongly in the luminous gas-
flame. The pipette is held in a sloping position
with the delivery end downwards. The delivery
end, in which the water collects, should not be
allowed to come near the flame. In delivering the
solution from the pipette allow it to drain for about
a minute. Then, when there is a length of liquid
in the narrow portion which does not form a drop
and fall, touch the side of the vessel. About halt
the liquid comes out ; the remainder of the liquid
should not be blown out, as the pipette is made to
deliver 50 cb.c, its contents being slightly greater.
Transfer the dilute silver-nitrate solution to a
clean, dry bottle, and label.
It generally happens that some pupils fall
behind others in a course of laboratory work. If
it is desired that all should, as far as possible, work
38
Practical Proofs of
the same experiments side by side, those who have
fallen behind might omit the preparation of the
standard silver nitrate solution, making up the
dilute solution for themselves from some of the
stock prepared by others.
Exercise VIII. — Determination of the ratio {or
proportion) in which potassium and chlorine are
combined in potassium chloride.
The following are the apparatus and substances
required : — The dilute silver nitrate solution pre-
pared in Exercise VII., distilled zvater, solution of po-
tassium chromate free from chloride. To test if free
from chloride acidify with pure dilute nitric acid and
add a drop of silver nitrate solution. The liquid
should remain clear. If a turbidity is produced, a
purer specimen must be obtained, or the salt may
be purified by re-crystallisation. To effect this,
dissolve in the smallest quantity of water, with the
aid of heat, in a small flask of about 1 50 cb.c. Cool
under the tap, pour off the liquor from the deposited
crystals, allow the crystals to drain thoroughly,
dry them between folds of white blotting paper or
filter paper, dissolve in water, and label ' Potassium
Chromate — indicator.' We also require a 500 cb.c.
measuring flask, small funnel, small beaker, watch-
glass, a 25 cb.c. pipette, a burette and burette-stand,
two or three porcelain dishes to hold 70 or 80 cb.c,
one or two glass stirring rods, which are best made
by sealing up the ends of glass tubing ; solid glass
rods are apt to break the bottom of a beaker.
Chemical Laws
39
Method of conducting the experiment. — Weigh
out accurately upon a watch-glass about '35 gram
of potassium chloride, and dissolve in 500 cb.c. of
distilled water, which must be free from chloride
and neutral to litmus. Wash out a burette with
distilled water and allow it to drain. Then wash it
out with from 1 cb.c. to 2 cb.c. of the dilute silver
nitrate solution. Pour the dilute silver nitrate
solution into the burette and see that it is filled to
the end of the delivery jet. Take out 25 cb.c. of the
potassium chloride solution with the 25 cb.c. pipette
and deliver into a porcelain dish. If the distilled
water was even slightly acid, add a few drops of
pure carbonate of soda solution, free from chloride.
Add a few drops of the potassium chromate solu-
tion. The same number of drops should be added
in each titration. Read the lower level of the
meniscus in' the burette. If a float is used the
reading of the burette is rendered much easier.
Run the silver nitrate solution into the dish, keep-
ing the liquid stirred by means of a glass rod.
The silver solution will soon begin to produce a
faint reddish tinge, due to the formation of silver
chromate. This, however, is decomposed with the
formation of silver chloride as long as any potas-
sium chloride remains in the solution. When the
amount of potassium chloride left is small the
colour disappears slowly, an indication that the
reaction is nearly completed. At length the addi-
tion of a drop of the silver solution produces a red
spot, which on stirring diffuses a permanent faint,
4o
Practical Proofs of
reddish tinge throughout the liquid. With the
quantities prescribed this will be when about 25 cb.c.
of the silver solution have been added. We have
now added enough silver to combine with all the
chlorine contained in the solution, and one drop
over and above, which has formed the small
quantity of red silver chromate which imparts a
reddish tinge to the contents of the basin. The
necessity for avoiding the presence of free acids
throughout the operations arises from the fact that
silver chromate is decomposed and dissolved by
acids. Neglecting to take account of the drop in
excess (equal to about *05 cb.c), we may say that
the volume of silver nitrate solution added con-
tains just so much silver as is required to combine
with the chlorine contained in the 25 cb.c. of potas-
sium chloride solution. Successive determinations
are done with successive quantities of potassium
chloride solution, until practice in hitting the exact
point at which the reddish tinge appears enables
the pupil to make successive determinations not
differing by more than *i cb.c. The eye is assisted
in noting the exact point of change by comparison
with the contents of a dish containing potassium
chromate solution and a soluble chloride to which
some silver nitrate solution has been added, the
soluble chloride being present in excess. A sheet
of white paper placed beneath the dish is also of
assistance. The colour change can be observed
somewhat better by gaslight than by daylight.
Calculation and statement of results. — Multiply
Chemical Laws
4i
the weight of chlorine corresponding to 1 cb.c.
of the silver solution by the number of cb.c. used
for 25 cb.c. of the potassium chloride solution.
Multiply this number by 20 (i.e. 50OH-25), and we
obtain the weight of chlorine in the potassium
chloride which was weighed out. Subtract this
from the weight of the potassium chloride. The
difference gives the weight of the potassium.
Calculate by simple proportion the weight of
potassium which combines with weight 1 of chlo-
rine.
EXERCISE IX. — Determination of the ratio in
which potassium and bromine are combined in
potassium bromide.
We require the same apparatus and substances
as in the last exercises, except that potassium
bromide is used instead of potassium chloride.
Method of conducting the experiment. — Weigh
out accurately about *5 gram of potassium bromide,
and dissolve in 500 cb.c. of water. Proceed exactly
as in the last exercise.
Calculation and statement of results. — The com-
bining proportion of silver and bromine was deter-
mined in Exercise V. From the amount of silver
used in the titration of the potassium bromide
calculate the weight of bromine which it contains.
The weight of potassium is obtained by subtracting
the weight of bromine from that of the salt taken.
Calculate the weight of bromine which is combined
42
Practical Proofs of
in potassium bromide with just so much potassium
as has been found to unite with weight I of
chlorine.
We have now obtained all the necessary data
for a verification of the Law of Equivalent Pro-
portions.
Example of results obtained in verification of
the Lazv of Equivalent Proportions, — A pupil found
that
✓3*048 silver which combine with
I part by weight of chlorine^ 2*267 bromine
combines with m -1004 potassium which combine
with 2*269 bromine
The bromine, instead of being exactly equal in
each case, differs by 2 parts in 2,268 ; therefore the
experimental error is moy per cent. The standard
number for the percentage of silver in silver nitrate
was employed by the pupil who obtained the above
result.
The same results may also be calculated, using
instead of weight 1 of chlorine, the weight of chlo-
rine which standard determinations have shown
to combine with weight 1 of hydrogen. The
advantage of this method of calculation is that, by
comparison with a table of equivalent weights^ the
degree of accuracy attained in each determination
is readily calculated. The above example calcu-
lated on this basis comes out thus : —
yio7'8o silver (error + -13 p. c.) which com-
35 '37 chlorine^ bine with 80*20 bromine (error + 55 p. c. )
combine with ^38 -92 potassium (error — -28 p. c.) which com-
bine with 80*26 bromine ( + error 60 p. c.)
Chemical Laivs
43
The errors recorded above represent the divergence
of the numbers from the standard determinations
of equivalent zveiglits. It will generally be found
that the results obtained in this verification of the
Law of Equivalent Proportions has a smaller error
than some of the individual determinations.
Note on early experiments relating to the Law of
Equivalent Proportions , and on Stas* determinations
This law, which is sometimes called Richter's law,
was first enunciated as a relation between the
combining quantities of acids and bases. Richter's
results, published about 1793, were very rough.
Thus, his determinations of the ratio between
equivalent quantities of carbonic acid and lime
differs from the standard numbers as determined
by modern workers to the extent of 9 per cent.
He failed to convince the scientific world of his
day of the importance of his work. Cavendish
also published determinations of equivalent quanti-
ties of compounds. His numbers are much more
nearly accurate. When the law was applied to
the case of elements, the first results were again
very rough. Thus we find that Dalton (' New
System of Chemical Philosophy/ published 1808,
Part I., p. 219) gives numbers which show the ratio
of equivalent quantities of silver and sulphur to
be 7*69. The same numbers are given in Part II.
of Dalton's book published two years later. This
result is 14 per cent, greater than the ratio 675,
which is that of our present standard numbers.
The result calculated from the second experiment
44
Practical Proofs of
cited in Chapter II. (in which silver was heated
with excess of sulphur) is 6725, which differs from
the standard number by — '4 per cent. The
standard of accuracy for determination of equi-
valent quantities of the elements was greatly raised
by Berzelius.
As an example of modern work in which the
law of equivalent proportions is verified, we proceed
to quote numbers obtained by Stas. They relate
to the same elements as those dealt with in the
exercises of the present chapter. The weight of
silver is taken as unity instead of the weight of
chlorine, but the divergence from theoretical accu-
racy appears in the same way in both calculations,
viz. in the slightly different numbers for the two
determinations of bromine.
Stas found that : —
.100,000 of silver which combine with
32,844*5 of chlorine^ 74,080*5 of bromine
combine with ^36,259*1 of potassium which combine
with 74,073*1 of bromine
The experimental error is here about *oi per cent.,
or 1 part in 10,000.
For Stas' numbers quoted above, see ' Bull, de
l'Acad. Roy. Belg.,' i860, No. 8, pp. 328 and 329, for
the weights of chlorine and of potassium equivalent
to 100,000 of silver ; and for the weight of bromine
equivalent to the above weights of silver and of
potassium, see ' Mem. de l'Acad. Roy. Belg.,' 18651
vol. xxxv., pp. 171 and 172.
Chemical Laius
45
CHAPTER V
THE SECOND PART OF THE LAW OF DEFINITE
AND OF CONSTANT PROPORTIONS
The second part of this law states that the definite
fixed proportion between the weights of two
elements combining to form a particular substance
is a constant proportion between the weights of
those two elements present in chemical compounds
containing also other elements.
For the verification of the first part of this law,
see Chapter III.
Exercise X. — Determination of the constancy of
the ratio, or proportion, between the weights of
potassium and chlorine in potassium chloride and
in potassium chlorate.
Having determined the proportion in which
potassium and chlorine are combined in potassium
chloride (Exercise VIII.), it may now be shown
that these two elements are combined in the same
proportion in potassium chlorate.
Apparatus and materials required. — A hard
glass tube in which to heat the substance, closed
46
Practical Proofs of
at one end, and about 4 cm. long, retort stand and
clamp, glass-ivool, potassium clilorate, the dilute
silver nitrate solution, and other things required
for titration, as in Exercise VI 1 1.
Metlwd of conducting the experiment. — Place
about 1 gram finely powdered potassium chlorate,
the weight of which need not be accurately known,
in the dry glass tube. Plug the tube loosely about
half-way up, or nearer to the open end, with glass-
wool. Support the tube in the clamp of the retort
stand near the open end of the tube, sloping it at
an angle of about 150. Heat the salt with the
flame of a Bunsen burner, holding the burner in
the hand and moving the flame to and fro at first
so as not to crack the tube. If the salt after fusing
shows a tendency to solidify, the heat must be in-
creased ; the evolution of gas will then for a time
become more rapid. When the oxygen has nearly
ceased coming off, it may be advisable to turn the
tube round in the clamp, so as to ensure that the
flame shall get to all parts of the salt. When no
more bubbles of gas come off, allow the tube to
cool, and when quite cold weigh the tube with its
contents. Place the tube in a basin of distilled
water, which must be free from chloride, and ex-
tract the residue of salt in the tube by leaving in
the hot water. Take out the tube, wash it inside
and outside with a stream of water from the wash-
bottle, allowing the washings to flow into the basin.
Set the basin with its contents aside to cool. Wipe
the glass tube with a cloth, and put it in a desiccator
Chemical Laws
47
to dry thoroughly. When dry weigh the empty
tube. The difference between this weighing and
that of the tube before extracting the salt gives
the weight of the salt extracted, in which we have
to determine the quantity of chlorine. Make the
volume of the solution up to 250 cb.c. and titrate
successive quantities of 25 cb.c. with the dilute
solution of silver nitrate used in Exercise VIII.
On comparing the results with those of Exercise
VIII., it will be found that for the same weight of
the salt in either case the same quantity of silver
nitrate is required ; hence each contains the same
quantity of chlorine.
For the purpose of this experiment it is not
necessary to have a standardised solution of silver
nitrate ; it will suffice to compare the quantity of
a solution of unknown strength required for a given
weight of a specimen of potassium chloride with
the amount required for the same weight of the
residue obtained after heating potassium chlorate.
This method of performing the experiment is
convenient if it is desired to verify the second part
of the Law of Definite and of Constant Proportions
before entering upon the Law of Equivalent Pro-
portions. The following example shows the results
obtained by a pupil working in this way.
Statement of results. — A pupil found that a
certain weight of potassium chloride required
38*4 cb.c. of a solution of silver nitrate of unknown
strength. The same weight of residue left on
heating potassium chlorate required 38*2 cb.c.
48
Practical Proofs of
As the amount of silver nitrate used is proportional
to the amount of chlorine present, it follows that
the weight of chlorine in equal weights of the two
materials is the same to within 2 parts in 383, i.e.
to *5 per cent.
The verification of the law is, however, some-
what more rigorous if the strength of the silver
nitrate, and the composition by weight of silver
chloride are known, as will be the case if the pupil
has conducted the preceding experiments. As-
suming this knowledge, the proportions between
potassium and chlorine in the two materials can
be directly calculated. The above experimental
result, if calculated out in this way, gives the pro-
portion *9 per cent, higher in the chlorate than in
the chloride. The fact that the potassium is
estimated by difference makes the experimental
error come out higher than in the first calculation.
The example of potassium chlorate and chloride
serves to verify the law to the degree of accuracy
required in a first approximation, but it appears
that the chlorate loses a small quantity of chlorine
on heating, and the method described above would
not be suitable for determinations of the highest
accuracy.
Note on Stas determinations relating to the
second part of the Laze of Definite and of Constant
Proportions, — It does not appear, as far as the
present writer is aware, that the earlier chemists
conducted experiments expressly designed to test
whether the proportion between the weights of two
Chemical Laws
49
elements which combine together to form a ' binary'
compound is the same as the proportion between
those two elements in a * ternary ' compound, i.e.
in a compound in which the two elements are
present along with a third. Ordinary analysis
furnished some evidence in favour of this conclusion,
but for the most part it seems rather to have been
assumed than to have been proved. If the atomic
theory be correct, it follows that the proportion by
weight of two elements, A and B, present in a
binary compound AB, will be the same as the
proportion between the weights of those two
elements in a ternary compound ABC ; or at any
rate there will be a very simple relation between
the quantities. Dalton's graphic symbols for
ternary compounds, e.g. alcohol, show that he as-
sumes this to be so. Stas, however, pointed out
that it was important for the proper substantiation
of the atomic theory that the point should be care-
fully tested. Having verified the fact that com-
bining proportions are independent of conditions
of pressure, temperature, &c. (by his analysis of
ammonium chloride, vide Chapter III.), he proceeds
to consider what further experiments are necessary
to establish the invariability of the combining pro-
portions of the elements. He says (' Mem. de
l'Acad. Roy. Belg.,' vol. xxxv. 1865, p. 61): 'The
constant composition of stable chemical compounds
being admitted ... it remains to be shown that
in binary and ternary compounds, for example,
having two elements in common, the elements
E
5o
Practical Proofs of
common to both are present in the same proportion
by weight. Thus in two bodies, AB and ABC,
the ratio of the weight of A to that of B should be
exactly the same in AB as in ABC. The solution
of the problem is independent of the ordinary
operations of analysis ; it is sufficient to determine
whether the ternary bodies can be reduced to
binary without any fraction, however small, of one
element common to both becoming free.'
To test this, Stas reduced by the action of
sulphurous acid the chlorate, bromate, and iodate
of silver to the condition of chloride, bromide, and
iodide respectively. No trace of silver or of the
other constituent was set free. To give an idea of
the accuracy of this verification of the law of
constant proportions, it should be mentioned that
considerable quantities of the salt were used, e.g.
in the case of the iodate about 70 grams. The
accuracy is probably considerably greater than in
the case of Stas' result relating to the law of equi-
valent proportions quoted in Chapter IV.
Chemical Laws
5*
CHAPTER VI
THE LAW OF MULTIPLE PROPORTIONS
Statement of the Law. — If there be more than
one substance formed by the combination of tzvo
elements, then, taking the weight of one element
as fixed in each substance, the weight of the other
element in the second compound bears a simple
proportion to the tv eight of that element in the
first.
Two methods of verifying the law are given in
this chapter. The first (Exercises XL, XII., and
XIII.) is the more complete. In these exercises
the proportions between chlorine and copper in
each of the two chlorides of copper is determined.
The second method (Exercise XIV.) is less com-
plete, but may be adopted instead of the first
method if time presses. The second method con-
sists in determining the quantities of bromine in
the two bromides of mercury. The quantity of
mercury in each case is calculated by difference,
and the proportions between mercury and bromine
in the two salts are then worked out.
First met! 10 d of verifying the law (Exercises
XL, XII., and XIII.) by the analysis of cuprous
e 2
52
Practical Proofs of
chloride ana cupric chloride. — We find how much
silver chloride is formed in each case and how
much subsulphide of copper. If the cupric chlo-
ride gives twice as much silver chloride as does
the cuprous chloride for the same weight of sub-
sulphide of copper, then it is evident that for the
same quantity of copper the amount of chlorine
in the cupric salt is twice the quantity in the
cuprous salt. It is not necessary to know the
composition by weight of either the subsulphide of
copper or of the chloride of silver for the purpose
of verifying the law.
The following are the apparatus and materials
required for this method of verifying the law : —
Cuprous chloride, cupric oxide ; strong, pure hydro-
chloric and nitric acids, large beaker, syphon-tube,
solution of silver nitrate, flowers of sulphur, vaseline,
supply of hydrogen or coal gas, and a double set of
the following apparatus, if, as is recommended, the
two determinations be carried on side by side, viz.
a 250 cb.c. measuring flask, 50 cb.c. pipette, Rose
crucible (if this be not available a clay tobacco
pipe fitted into an ordinary porcelain crucible may,
according to Fresenius, be substituted), a porcelain
crucible and lid, an accurately cut funnel with angle
of 6o°, watch-glass, beaker, iron tripod, pipe-clay
triangle, sand tray, evaporating basin, holding more
than 250 cb.c, water-bath, desiccator containing lime
or caustic potash, hollow glass-rod, porous or
biscuit porcelain, packet of Swedish filter papers,
stoutest platinum wire, camel-hair brush, or, better,
Chemical Laws 53
a stiff black feather \ glazed paper, filter stand, drying
oven with thermometer^ wash bottle, mortar.
EXERCISE XI. — Preparation of the two chlorides
of copper in a state of purity.
Cuprous chloride. — (The pupil should be ac-
quainted with the mode of formation of this salt.)
Take the salt as obtained from the dealers (which
is not pure), dissolve it in a little strong, pure
hydrochloric acid, and pour the clear solution into
a large beaker of distilled water. The cuprous
chloride is thrown down as a white precipitate,
which quickly settles to the bottom of the beaker.
Syphon off the bluish solution as quickly as pos-
sible ; the loss of some of the precipitate does not
matter. Fill the beaker again with distilled water,
stir up, allow to settle, and syphon off again.
Repeat the operation a third time, to ensure that
the washings are free from cupric salt. The pure
white cuprous chloride which remains at the
bottom of the beaker would readily oxidise if
dried. No attempt, therefore, is made to dry and
weigh the substance, it being sufficient for our
purpose to determine the ratio of copper to chlorine
in an unknown weight of the salt. Immediately
after the last washing add strong pure nitric acid,
free from hydrochloric acid, little by little until the
salt has dissolved. Make up the volume of the
liquid to 250 cb.c, and label the flask in which the
solution is contained. The next step is to deter-
54
Practical Proofs of
mine the quantity of cuprous sulphide and of silver
chloride respectively which can be obtained from two
equal measures (of 50 cb.c.) drawn from the flask
with a pipette. Should anything occur to prevent
the successful completion of either determination,
another portion of 50 cb.c. could be withdrawn from
the flask and the determination repeated. Cupric
chloride is analysed in the same manner ; the salt
is dissolved in water, without being dried or
weighed, the solution made up to 250 cb.c. ; two
measures of 50 cb.c. each are taken, in one of which
the copper is determined as subsulphide, and in the
other the chlorine is determined as silver chloride.
Economy of time is effected by conducting side by
side the two chlorine determinations, and afterwards
the two copper determinations.
Cupric chloride. — To prepare pure cupric chlo-
ride, powder finely some pure black oxide of
copper, and roast it in a porcelain dish over a Rose
burner. Treat an excess of the oxide with strong,
pure hydrochloric acid with the aid of heat ; pour
off from the excess of oxide, and cool the hot
solution rapidly, placing the boiling tube or
narrow beaker in which it is contained under water
running from a tap, and keeping the contents of
the vessel rapidly stirred with a glass rod, when
the cupric chloride crystallises out. Pour off the
liquid as completely as possible, and allow the
crystals to drain. Spread the crystals upon a
piece of porous porcelain, and allow them to dry
in a desiccator over quicklime or pieces of caustic
Chemical Laws
55
potash. Dissolve the substance in water. If any
matter of a bluish-white colour remain undissolved,
filter through Swedish filter paper. Make up the
liquid to 250 cb.c. Separate portions of 50 cb.c,
each can then be used for analysis.
Exercises XII. and XIII.— The analysis of
the tzvo chlorides of copper.
For the copper determination (Exercise XII.)
remove 50 cb.c. of the solution by means of a
pipette and deliver it into a small evaporating
basin. Concentrate . the solution on a water-bath
to small bulk. Weigh a Rose crucible with lid,
finish the evaporation to dryness in the Rose
crucible, which if too small for the rings of the
water-bath may be conveniently supported by a
pipe-clay triangle. When quite dry add flowers of
sulphur, place the perforated lid upon the crucible,
connect up with a slow stream of hydrogen, and
after having displaced the air from the crucible by
means of the hydrogen, heat the crucible gently at
first and afterwards more strongly with the flame
of the Bunsen burner, and finally with the foot-
blowpipe, using not quite the full blast, and discon-
tinuing the heating five minutes after the flame of
sulphurous acid or the fumes of sulphur have
ceased to be visible round the lid of the crucible.
Allow to cool in the current of hydrogen. Examine
the contents of the crucible, which should contain
a dark-coloured, shining, crystalline mass of sub-
56
Practical Proofs of
sulphide of copper with no visible sulphur and no
red colour, which would indicate reduced copper.
Weigh the crucible and lid with the contents.
For the chlorine determination (Exercise XIII.)
take up 50 cb.c. of the solution of copper salt in a
pipette and deliver into a small beaker. In the
case of the cupric chloride add a few drops of
nitric acid. Place the beaker on the sand-tray,
cover the mouth of the beaker with a watch-glass,
and heat to boiling point. Add solution of silver
nitrate. Boil for a few minutes, till the precipitate
collects in flocks. Allow the precipitate to sub-
side. Add a drop more solution of silver nitrate.
If no fresh precipitate is produced, all the chlorine
has been thrown down as silver chloride.
It is important not to add a great excess of a
reagent for the precipitation of an insoluble com-
pound ; on the other hand, if too little of the re-
agent be added at first, time is lost by having to
add a second dose. Whenever the pupil can form
an estimate of the weight of a reagent required
for precipitation, he should use such a volume of
the solution of the reagent as contains a weight
slightly greater than is absolutely necessary. The
reagents in a laboratory are made up roughly to a
certain strength (so many grams to the Winchester-
bottle of water), and the pupil should be acquainted
with the strength of the solutions and be able to
calculate approximately what weight of each re-
agent is contained in 1 cb.c. of the solution. It is
a good plan to write on the label of each reagent
Chemical Lazvs
3/
bottle the number of grams per Winchester-bottle
used in making up the solution. Dilute acids
should be marked I : 3, 1 : 5, &c, according to the
proportion of acid to water which has been used.
Having made sure that all the chlorine is pre-
cipitated as silver chloride, fit carefully a small
Swedish filter paper to a well-shaped glass funnel.
In order to hasten the filtering it is well to attach
to the funnel a glass tube provided with one bend
in order to create a suction. Before commencing
the filtration it is well to ascertain, by pouring
water on the filter paper, that the paper is strong
enough to stand the suction. The use of filtering
pumps in quantitative experiments should not be
attempted until the pupil has had experience in the
management of filtrations. Very slightly grease
with vaseline a small portion of the underside of
the rim of the beaker, and pour the liquid down a
glass rod (made from hollow tubing) into the funnel
to about two-thirds of the height of the filter paper.
The liquid should be poured so as to fall on the
side of the funnel, not into the bottom of the cone.
Have a perfectly clean beaker to catch the filtrate,
since, although it is intended to throw away the
filtrate, yet it is important that if through mishap
some of the precipitate should come through the
filter paper, this should be recoverable by filtering
again. When the first charge of liquid has run
through, fill the funnel two-thirds full again, and
repeat the operation till the liquid above the pre-
cipitate in the beaker has all been poured off.
58
Practical Proofs of
Then pour hot distilled water on the precipitate
in the beaker and boil up for one or two minutes.
Allow the precipitate to subside and filter again.
After sufficiently washing the precipitate in this
way (by ' decantation '), bring the precipitate care-
fully on the filter paper with the aid of the glass
rod. The last portions must be removed by the
aid of a jet of water from the nozzle of a wash-
bottle. Having brought all the precipitate on to
the filter paper, wash further with a stream of hot
water from the wash-bottle until the washings
appear to be pure water, giving no reaction for
copper, for an acid, or for silver. Having washed
the precipitate, wet a piece of filter paper, and
place it over the mouth of the funnel which con-
tains the precipitate. Press the edges of the filter
paper to the sides of the funnel and tear so as to
leave a circular portion of the filter paper tightly
stretched over the mouth of the funnel. Place the
funnel in a drying-oven, or ' air-bath,' heated to a
temperature not much exceeding ioo° C, and leave
till dry. The temperature of the air-bath should
be ascertained from time to time by reading the
thermometer, which passes through a cork fitted in
the opening at the top. While the precipitate is
drying, weigh a porcelain crucible and lid. To
determine the weight of the ash of the filter paper,
take four to six filter papers of the same packet,
wrap them tightly in a small bundle and hold by
two turns of stout platinum wire. Set fire to the
bundle and burn so that the ash will fall into the
Chemical Laws
59
crucible. In order to ensure this see that the
burning paper is not exposed to draughts. When
the bundle of paper has ceased to glow tap the
wire, if necessary, so as to make the bundle fall
into the crucible. Strongly heat the crucible,
placing the lid loosely on, till there is no black
carbonaceous material left with the ash. Place the
crucible in a desiccator, allow to cool, and weigh.
Divide the total weight of the ash of four, or six,
filter papers, by the number of papers used. The
quotient will be the weight of ash of one filter paper
of the packet. This weight should be written on
the band wrhich holds the papers, and should also
be entered in the pupil's note-book. It is not ad-
visable to rely upon the weight of ash printed on
the wrapper. Having determined its weight, the
ash may be thrown out of the crucible, and after
testing again the weight of the crucible and lid,
the crucible may be used in the weighing of the
precipitate.
Take the funnel from the oven. If no steam
is escaping, if the paper does not stick to the glass,
and if the precipitate is loose and crumbling, the
substance is dry, and may safely be removed from
the filter paper. The removal of a dried precipitate
from the filter paper to the crucible is an important
piece of manipulation in quantitative analysis. A
beginner should rehearse the manipulation before con-
ducting the quantitative analysis. In the previous
experiments in which silver chloride has been dealt
with the whole operation was conducted, without
6o
Practical Proofs of
transference, in one vessel. It may probably be
assumed that transference is always accompanied
by loss. With the small quantities used in ordinary
chemical analyses it is, however, possible by proper
methods of manipulation to lose so little that the
loss is inappreciable with the balances employed.
If the loss is too small to affect the balance it is
the same, so far as numerical results are concerned,
as if there were no loss at all. The removal of a
precipitate from filter paper to crucible is a perfectly
legitimate operation provided we are dealing with
small quantities, and can only use a balance of
ordinary sensitiveness. In researches such as those
of Stas, in which large quantities and very delicate
balances are used, the processes are arranged so as
to avoid transference of material.
The crucible must be placed upon a piece of
dark-coloured glazed paper or upon a large clock-
glass, so that if any substance be spilt it may be
recovered. Most of the precipitate on the filter
paper can be made to fall into the crucible by
scraping gently with a loop of stout platinum wire.
The remainder must be gently brushed off with a
stiff black feather, taking care not to brush off any
of the surface of the filter paper. The paper,
having been freed as much as possible from the
precipitate, is rolled up into a small tight scroll and
burnt over the crucible, being held by one turn of
stoutest platinum wire. It is convenient to use a
piece of platinum wire sufficiently long to fix one
end on the stem of an inverted glass funnel, which
Chemical Laws
61
serves for a holder, while the paper is burning.
Care must be taken to burn the carbonaceous
matter as completely as possible, but the residue
should not be very strongly heated on the wire, as
the reduced silver would be apt to adhere to the
platinum instead of remaining in grains and falling
into the crucible with the little roll of ash. We
have now the silver chloride in the crucible. The
silver there is just so much of the silver in the
silver nitrate solution as was needed to seize upon
and throw down in the form of silver chloride all
the chlorine which was originally combined with
the copper in the 50 cb.c. of solution of the copper
salt. The rest of the silver remained in solution,
and was washed away in the filtering process. All
the chlorine in the 50 cb.c. of the solution was
thrown down as silver chloride ; but in burning the
filter paper the organic matter will have reduced
to metallic silver so much of the chloride as adhered
to the paper. It follows that the material in the
crucible weighs less than the silver chloride
precipitated, owing to the fact that a small quantity
of silver has been deprived of its chlorine. We
have verified by previous experiments the fact that
the proportion by weight in which silver and
chlorine combine does not depend upon the nature
of the reaction by which the combination is brought
about. Therefore by causing the small quantity of
reduced silver to combine again with chlorine we
shall obtain the true weight of the silver chloride
which was precipitated. To effect this, moisten
62
Practical Proofs of
the contents of the crucible with one drop of pure
strong hydrochloric acid, heat very gently at first
and afterwards more strongly, but use a small
flame of the Bunsen burner, and move the flame
about slowly so that no part becomes very highly
heated. The lid is not placed upon the crucible
during this operation, which can be watched until
the substance begins to melt round the edges of
the crucible, when the lamp should be withdrawn.
Place the crucible in a desiccator, replace the lid,
allow to cool, and weigh. From this weight sub-
tract the weight of crucible and lid plus the weight
of the ash of one filter paper. The difference is
the weight of the silver chloride formed from the
chlorine contained in 50 cb.c. of the solution of
copper salt.
Example of the calculation and mode of statement
of results. — The manner of stating the verification
of the Law of Multiple Proportions may be shown
by the following example from determinations
conducted when rehearsing the experiment to
ascertain if it were suitable to be tried for teaching
purposes. Results obtained by pupils are given
further on.
Equal portions of cupric chloride solution gave '3139 grams copper
subsulphide and I'H4I silver chloride
Equal portions of cuprous chloride solution gave -1872 grams
copper subsulphide and '3351 silver chloride
Therefore, so much cupric chloride as would give '1872 grains
copper subsulphide would give -6644 silver chloride
Now
6644
3351
= 1-983 (found) as against 2*ooo
Chemical Laws
63
calculated on the assumption of the Law of Multiple
Proportions. The experimental error therefore is
17 parts in 2000, = — '85 per cent. The error
appears with the minus sign. If we had taken the
weight of chloride as constant, and had calculated
the amounts of sulphide, the error would have been
the same in amount but opposite in sign. It is a
matter of convention therefore, but one or other
system of calculation must be adhered to, and the
sign ( + tive or — tive) stated, if the results of a
number of determinations are to be compared.
For teaching purposes it may be thought con-
venient to have some check upon the experiments
during the course of the work. For this purpose
standard numbers may be taken for the percentage
composition of the subsulphide of copper, and the
chloride of silver, and of the two chlorides of
copper, and the results obtained by the pupil may
be compared with these standard numbers as the
exercises proceed. The two following sets of ex-
amples of results obtained by two pupils show this
mode of statement.
The first pupil obtained the following results : —
Found
Copper . 48-00 p. c.
Chlorine . 52*00
Cupric Chloride
Calculated from
standard numbers
Experimental error
calculated for chlorine
+ -82 p. c. of the
47-18 p. C.
52-82 „
total quantity of
the salt used
Found
Cuprous Chloride
Calculated from
standard numbers
calculated for chlorine
+ *57 p. c. of the
Experimental error
Copper . 64*68 p. c.
Chlorine . 35-32
64-11 p. c.
35-89 „
total quantity of
the salt used
64
Practical Proofs of
Therefore, by simple proportion, in cuprous
chloride, the ratio of copper to chlorine was found
to be as 48 : 26*2, against 48 : 52 in cupric
chloride.
52
Now
26-2
1*985 (found) as against 2*000
required by the Law of Multiple Proportion, expe-
rimental error — 75 per cent.
The second pupil obtained the following re-
sults : —
Found
Copper . 48-04
Chlorine . 51*96
Cupric Chloride
Calculated rroTi
standard numbers
47-18 p. c.
52-82 „
Found
Copper . 64*46 p. c.
Chlorine . 35*54 >>
Cuprous Chloride
Calculated from
standard numbers
6d ' 1 1 p. C.
35^9 »
Experimental error
calculated for chlorine
+ -86 p. c. of the
total quantity of
the salt used
Experimental error
calculated for chlorine
+ '35 P- c- °f the
total quantity of
the salt used
Therefore, by simple proportion, in cuprous
chloride the ratio of copper to chlorine was found
to be 48*04 : 26*48, against 48*04 : 51*96 in cupric
chloride.
Now 5— §^ = 1*962 (found) as against 2*000
26*48
required by the Law of Multiple Proportions,
experimental error — 1*9 per cent.
Second method of verifying the Law of Multiple
Proportions {alternative to that of analysis of
chlorides of copper}.
Chemical Laws
65
EXERCISE XIV '. — {Alternative to Exercises XLy
XII, and XIII.), the determination of the per-
centage of bromine in the two bromides of mercury.
By this exercise the Law of Multiple Propor-
tions can be verified on the assumption that the
salts known as mercurous bromide and mercuric
bromide are composed only of the two elements
mercury and bromine. The pupil should be ac-
quainted with the methods of preparation of the
salts. The exercise is intended to furnish a shorter
method of verifying the law ; the salts are not
prepared by the pupil but are obtained direct from
the dealers, by whom they are supplied in a fairly
high state of purity. No attempt is made to
determine directly the quantity of mercury ; it is
arrived at by subtracting the weight of the bromine
from the weight of the salt. The method of
determining the bromine volumetrically has been
employed in Exercise IX., and the composition by
weight of silver bromide was determined in Exer-
cise V. Strictly speaking, the pupil should make
use of the numbers obtained by himself in Exer-
cise V. when calculating the results of the present
exercise. In the following account of an experi-
ment actually performed, the determinations were
carried out by a practised chemist, not by a pupil.
Standard numbers were assumed for the composi-
tion of silver bromide.
Account of an actual experiment, as conducted. —
A strong solution of caustic potash containing
F
66
Practical Proofs of
about 20 per cent, was used for the decomposition
of the salts. The sample was free from admixture
of chloride. Mercurous bromide, I gram, was weighed
out and covered with 20 cb.c. of the caustic potash
solution in a small porcelain dish, and warmed on
the water-bath for half an hour. The decomposi-
tion, however, was almost immediate. Water
was added to dilute further the caustic alkali, the
liquid was filtered, and the filtrate made up to a
volume of 250 cb.c. The mercury is all left on the
filter paper as oxide, &c, and all the bromine is in
the filtrate as potassium bromide. In the filtrate
the bromine was determined volumetrically by
exactly neutralising 20 cb.c. with pure nitric acid,
free from chloride, and titrating with a solution of
silver nitrate of known strength, using potassium
chromate as an indicator (see Exercises VIII. and
IX.). Two determinations gave the weight of
bromine, calculated for the whole 250 cb.c. as
1st determination . . . '283 grams bromine
2nd . . '284
Mean -2835
The weight of mercurous bromide was exactly
I gram, therefore the composition of the salt is : —
Calculated from
Found standard numbers
Bromine . . 28*35 p. c. 28-57 p. c.
Mercury . . 71*65 exp. error — *22 p. c.
In the analysis of the mercuric bromide, 1 gram of
the salt was weighed out and dissolved in about
100 cb.c. of boiling water. Twenty cb.c. of the
Chemical Laws
67
solution of caustic potash was added, the mercury
being thrown down from solution in the form of
the red oxide. The liquid was then filtered from
the precipitated oxide, Swedish filter paper being
used. If a small quantity of the precipitate should
have passed through the filter paper the titration
could still have been proceeded with, as the pre-
cipitate quickly settles to the bottom of the
measuring flask. The filtrate was neutralised as
in the last experiment, the volume made up to
250 cb.c., and 20 cb.c. were titrated with the same
solution of silver nitrate as was used for the
mercurous bromide.
The weight of bromine found in three determi-
nations was : —
•446 grams bromine
• *443
• *437
• *442
The weight of the mercuric bromide was exactly
1 gram, therefore the composition of the salt is : —
Calculated from
Found standard numbers
Bromine . . .44*2 p. c. 44-4 p. c.
Mercury . , . 55-8 p. c. exp. error - "20 p. c.
Mode of stating the above results as a verification
of the Law of Multiple Proportions.
In mercuric bromide 55*8 mercury are combined with 44*2 bromine
In mercurous 71-65 „ 28*35 >>
Therefore, in mercuric bromine 71*65 mercury are combined with
56*75 bromine
F 2
1st determination
2nd
3rd
Mean
68 Practical Proofs of
Now ===2'CK)2 (found) as against 2-ooo
calculated on the assumption of the Law of Multiple
Proportions. The experimental error is therefore
+ *i per cent.
Note upon early experiments relating to the Laiv
of Multiple Propoi'tions and upon Stas criticisms of
these experiments.
The results obtained by chemists in the earlier
part of the century were for the most part less
nearly exact than those given above.
Dalton, ' New System of Chemical Philosophy,'
p. 318, quotes in support of the law the following
values obtained by Sir Humphry Davy for the
composition by weight of two oxides of nitrogen : —
Nitrogen Oxygen
Nitric oxide . . 5-3 7
Nitrous oxide . . .11 "46 7
Now l_L4^=2*l6 (found) as against too re-
quired by the Law of Multiple Proportions, the
experimental error is therefore + 8 per cent.
Dalton's own results for the hydrocarbons and for
the oxides of carbon are even further from the
numbers required by the law.
The results obtained by Berzelius are more
nearly exact. Thus, e.g., he found the ratio
between the quantities of the base in the sub-
arsenite and arsenite of lead to be as
I : 1 -974 exp. error - 1 -3 p. c.
Chemical Laws
69
and he found the oxygen in chromic oxide and
chromic acid to be as
1 : 2*062 exp. error +3'i p. c.
Probably chemists were influenced in their ac-
ceptance of the Law of Multiple Proportions upon
results so rough by the circumstance that it
suggested a ready explanation (that of chemical
atoms) of the better established law of definite,
fixed proportions. Stas remarks in the introduc-
tion to his 1 Nouvelles Recherches sur les Lois des
Proportions Chimiques/ that chemists and physi-
cists have long been in the habit of crediting the
existence of a simple mathematical relation when-
ever phenomena present an appearance of regu-
larity. This prejudice, he says, leads them to ascribe
observed deviations ivholly to experimental error,
and he cites the case of the hypothesis, called
1 Prout's,' that the weights of all chemical atoms are
exact multiples of the weight of the hydrogen
atom. Stas verified the Laws of Definite, of Con-
stant, and of Equivalent Proportions, as well as the
Law of Conservation of Mass, but did not examine
the Law of Multiple Proportions considered as
Lot mathematique and not merely as Lot limitee.
Although the discovery of the Law of Multiple Pro-
portions may have suggested Dalton's atomic theory,
Stas considers (Joe. czt., p. 60 of the ' Mem. de Y Acad.
Roy. Belg., vol. xxxv., 1865) that chemists have
relied more upon the definite and constant propor-
tions of chemical combination as evidence of the
truth of Dalton's theory.
7o
Practical Proofs of
CHAPTER VII
THE LAW OF SIMPLE VOLUMETRIC PROPORTIONS
IN THE CHEMICAL REACTIONS OF GASES
Upon the molecules of gases. — The development
of the modern theory of chemical atoms and
chemical molecules from Dalton's atomic theory
commenced with the discovery of a relation be-
tween Dalton's 'chemical atoms' and the 'molecules'
of gases. The molecule of a gas is defined by
physicists as ' a small mass of matter, the parts of
which do not part company during the excursions
which the molecule makes when the body to which
it belongs becomes hot ' (Clerk Maxwell, ' Theory
of Heat,' 6th edition, p. 305). This physical
definition is independent of chemical considerations.
It appears from purely physical facts that the
number of molecules in a given volume is the same
for all gases at the same temperature and pressure
(Joe. city pp. 3°l-3l7)-
The researches of Gay-Lussac ('Mem. Soc.
d'Arcueil,' vol. ii;, 1809, reprinted by the Alembic
Club, ' Reprints,' No. 4) showed, within fairly wide
limits ol experimental error, that the volumes of
gases which react chemically together bear a
Chemical Laws
simple ratio, or proportion, to one another^
Chemists were at first inclined to infer from this
that the chemical ' ultimate particles ' or ' atoms '
of gases are identical with the physical ' molecule.'
Further examination of the matter showed that in
the case of most chemical elements in the gaseous
state, this could not be so. We shall return to
this point after formally stating Gay-Lussac's
' Laws of Volume ' in a condensed form ; merely
stating here that the conclusion arrived at has been
that the molecule of a compound gas is identical
with Dalton's ultimate particle or reacting unit,
but that in the case of elementary gases the
molecule may contain one or more chemical atoms.
Most of the common elementary gases (e.g. hydro-
gen, oxygen, nitrogen, and chlorine) have two
atoms in the molecule. The following is a con-
densed statement of the Laws of Simple Volu-
metric Proportion : The volume of an element in
the gaseous state bears a simple proportion to the
volume of the compound gas of which it is a con-
stituent. From this statement it would follow as a
corollary : —
Corollary I. — The volumes of the gaseous constitu-
ents of a compound gas bear a simple proportion
to one another.
It may be further deduced (assuming the laws
of definite and of constant proportions) that the
volumes of the combining or equivalent weights of
gaseous substances bear a simple proportion to one
another, or, as it may be stated : —
72
Practical Proofs of
Corollary II. — The relative densities of gases
stand in a simple ratio to their chemical equi-
valents.
This relation was experimentally observed
about the same time that the laws of volumetric
proportions were discovered. In the case of the
elementary gases the corollary may be expressed by
saying that the relative densities stand in a simple
ratio to the relative weights of the atoms.
Having now explained the connection between
Gay-Lussac's volumetric laws and the gravimetric
laws which formed the foundation of Dalton's
theory, we pass on to describe an experimental
verification of the law of simple volumetric pro-
portion.
EXERCISE XV. — The determination of the volume
of nitrogen obtained by the decomposition of a
knoivn volume of ammonia}
The reaction employed is the decomposition of
ammonia gas, contained in a graduated tube, by a
solution of sodium hypobromite, the hydrogen of
the ammonia being oxidised and the nitrogen set
free. The pupil should acquaint himself with the
preparation and properties of the hypobromites
(and hypochlorites), and with the evidence showing
1 This experiment is described by Professor Ramsay in his
Experimental Froofs of Chemical Theory \ from which it is taken
with his permission. The directions for performing the experiment
have been to some extent modified in accordance with the present
writer's experience.
Chemical Laws
73
that ammonia consists wholly of nitrogen and
hydrogen.
Apparatus and materials required. — Strong
solution of ammonia (' liq. amm. fort.'), bromine
(not bromine- water), solid caustic soda, dilute acetic
acid, retort stand with rings and clamp, sand tray,
two flasks, wide-mouthed bottle, or Woulff's bottle,
three corks, or rubber stoppers, glass tubing and
connecting rubber tubing, graduated gas-measuring
tube of 50 to 100 cb.c. capacity, not too wide to
close with the thumb, and the narrower the better
a basin in which to invert the tube (a small pudding-
basin or a clean glass mortar is more convenient
than the ordinary shallow evaporating basin), a
thermometer, a large and deep vessel, or a pneumatic
trough deep enough to immerse the graduated
tube.
Before proceeding to the actual experiment, it
is well that the pupil should rehearse the operations
of closing the tube, placing under water, shaking
the tube, &c, in order to make sure that he is
master of the manipulation. A dish containing
water to which a little acetic acid has been added
should be at hand when the experiment is being
conducted. After the hand has been immersed in
the caustic soda solution it should be immediately
and thoroughly washed in the basin.
Pireparation of sodium hypobromide. — Make up
a strong solution of caustic soda in a flask. In
order to judge of the volume which will be required,
pour into the pudding-basin (or glass mortar) as
74
Practical Proofs of
much water as will cover the thumb to the depth
of an inch when the hand is put into the water in
the position required for inverting the tube in the
experiment. The volume of the caustic soda
solution required will be from 50 to 100 cb.c.
more than this. The flask should be corked while
the solution is proceeding, and the flask should be
cooled under the tap before adding the bromine.
Add the bromine gradually to the cooled solution
in the draught chamber, agitating the flask till the
heavy red liquid has dissolved in the soda solution,
and then adding more bromine. The temperature
may be kept low, if necessary, by putting the flask
under the tap. The formation of the hypobromite
is accompanied by change to a greenish-yellow
colour. Ten cb.c. of bromine may be used to
300 cb.c. of caustic soda solution. If 50 cb.c. of the
solution enter the tube during the reaction, the
amount of bromine (in the form of hypobromite)
entering the tube would be largely in excess of the
amount used up in the decomposition of 100 cb.c.
of ammonia gas. A large excess of hypobromite
greatly hastens the reaction, whereas, if the solution
of hypobromite be a weak one, the last part of
the reaction goes very slowly and the experi-
ment cannot be completed within a reasonable
time.
To fill the tube with a nni 10 uia gas, warm gently
some strong ammonia solution in a small flask
fitted with a one-way cork. The gas should be
passed through a dry flask or a Woulff's bottle, in
Chemical Laws
75
which may be put some cotton wool to retain drops
of moisture, or lumps of quick-lime may be em-
ployed for the same purpose. Collect the gas by
upward displacement in the graduated tube. The
delivery tube should reach nearly to the top of the
graduated tube, which must be dry. After use in
the experiment the tube should be dried, before
using again, by warming and blowing air through
from the bellows. It is convenient, however, to
have more than one tube and to start two or three
experiments without waiting for the completion of
the first. The precautions taken as to dryness are
designed to exclude drops of moisture, which would
hold in solution relatively large quantities of
ammonia gas and would vitiate the results. As
the tube is open to the air the gas will not, pre-
sumably, be dry in the more precise sense of the
term, but will contain vapour of water. The stopper
of a bottle of hydrochloric acid brought near to,
and a little above, the mouth of the graduated tube
will show by forming white fumes when the tube
is practically filled with ammonia gas. The
evolution of gas should be allowed to continue for
some time afterwards. Observe the temperature
of a thermometer suspended near the tube. Finally
raise the graduated tube very slowly, so that the
entering gas may fill the space previously occupied
by the narrow delivery tube. The tube should be
held in a twist of paper, or a glove should be worn,
lest the heat of the hand should expand the gas.
Close the mouth of the tube firmly with the thumb
76
Practical Proofs of
and plunge the tube mouth downwards to the
bottom of the basin which has been filled to a
sufficient depth with the solution of caustic soda
and hypobromtte. Remove the thumb, keeping
the .tube upright. The liquid rushes into the tube
and effervescence takes place. Clamp the tube in
the upright position ; watch and make notes of the
progress of the reaction. The large excess of
caustic soda retards the absorption of ammonia by
the liquid. After the first violence of the reaction
is over, most of the chemical change appears to
take place near the surface of separation between
the liquid and the gas. If the liquid be de-
colorised to a small depth, this indicates that the
hypobromite has been deoxidised in this part of
the tube. In this case the action mainly takes
place at the upper surface of the coloured liquid,
as may be seen by noticing the level from which
the bubbles of (nitrogen) gas rise. The rise or fall
of the level of the liquid at any particular time
depends upon whether the absorption of ammonia
or the evolution of nitrogen is proceeding the more
rapidly. The liquid may, for instance, rise two-
thirds or more up the tube and finally sink to one-
half way up. When the action becomes slow the
tube may be shaken from side to side, taking care
to keep the bottom of the tube safely under the
liquid in the basin. If time presses, the tube may
even be closed with the thumb and the contained
liquid and gas be agitated together. If this be
done the liquid should be agitated in such a way
Chemical Laws
77
that the thumb remains covered by it. When no
more bubbles of gas come off, close the tube with
the thumb under the surface of the liquid in the
basin and transfer the tube to the large and deep
vessel of water, the temperature of which should
be brought to an equality with the temperature
registered by the thermometer which was sus-
pended near the graduated tube when the ammonia
was being collected. After the measuring tube has
remained in the water for five or ten minutes, raise
it until the level of the water is the same inside
the tube and outside, and read off the volume of
the gas to the bottom of the meniscus. The tube
should be held by a clip or by a twist of paper,
not by the unprotected hand, lest the heat of the
hand should cause the gas to expand.
Example of the calculation and mode of statement
of results.— A senior student obtained the follow ing
results from two experiments which were completed
in 2\ hours, using two tubes of different sizes. The
first tube had been dried and was being used again
for a third experiment, which was not quite com-
pleted at the end of the 2\ hours. The apparatus
had been got together and fitted upon the previous
day. First experiment^ done with a rather wide
measuring-tube graduated in \ cubic centimetres.
The divisions not being continued to the open end
of the tube, the distance from the last division to
the end of the tube was measured, and found to be
equal in length to 15 of the cubic centimetre
divisions. The volume of the tube was therefore
7§
Practical Proofs of
taken to be 115 cb.c. The results were as fol-
lows : —
Ratio 58 : 115 = 1 : 1*983 (found) as against
2*000 which is the nearest simple ratio. The
experimental error is therefore —'85 per cent.
The minus sign indicates that the volume of
nitrogen was found to be more than one half the
volume of the ammonia.
Second experiment^ done with a narrow eudio-
meter tube graduated in millimetres. The volume
of the ungraduated part at the upper end of the
tube was known to be equal to 3 of the m.m.
divisions. The number of m.m. divisions was 250,
and the length from the last m.m. division-mark to
the open end of the tube was found to be 57*5 m.m.
The volume of the tube was therefore reckoned
to be 3 + 250+57*5 = 310*5. For the purpose of
the experiment it is not necessary to know the
actual volume of one division. The results were as
follows : —
Volume of ammonia . . . .310*5
nitrogen . . . .154
Ratio 154 : 310-5 — 1 : 2-oi6.
The experimental error is therefore + '8 per
cent., the volume of nitrogen being found to be
slightly less than one half the volume of ammonia.
Note upon early experiments relating to the law
of simple volumetric ratios, or proportions. — Gay-
Lussac, calculating from H. Davys analyses, finds
Volume of ammonia .
nitrogen .
Chemical Laws
79
the volumetric proportions in the oxides of nitrogen
to be as follows. The numbers given in the last
column show how fa rthese results depart from
the numbers required to give the nearest simple
ratios.
Nitrogen
Oxygen
Error
Nitrous oxide
IOO
49 '5
I p. C.
Nitric oxide .
IOO
108-5
8-5 .,
Nitrogen peroxide .
IOO
2047
2-35 »
Dalton's nearest result for the volumetric composi-
tion of water vapour was : —
Oxygen
Hydrogen
Error
Water (vapour)
IOO
197
i -5 p. c.
These are examples of the results upon which the
law was accepted by chemists.
Of recent years the volumetric composition of
water has been carefully determined by Scott, who
finds the ratio to be 1*9965, which confirms the
law to within *i8 per cent.
Upon the relation of the {physical) molecule of
nitrogen gas to the {chemical) atom of the element
nitrogen, — Exercise XV. not only confirms the law
of simple volumetric proportions but affords evi-
dence as to the relation between the molecule of
nitrogen gas and the chemical atom of the element.
Equal volumes of nitrogen and ammonia contain,
8o
Practical Proofs of
according to the physical theory, an equal number
of molecules. It was found in Exercise XV. that
ammonia when decomposed yields half its volume
of nitrogen. There are, therefore, only half as
many nitrogen molecules as there were molecules
of ammonia. It follows that each molecule of
nitrogen gas has received a contribution of nitrogen
from tivo molecules of ammonia. If the molecule
of ammonia contain only one chemical atom of
nitrogen, then the molecule of nitrogen gas con-
tains two chemical atoms, and no more. It would
lead us too far to discuss the evidence, which
appears to show that the molecule of ammonia
contains only one chemical atom of nitrogen.
According to the view at first adopted by
chemists, that the chemical atom of an elementary
gas was identical with the molecule, it would follow
that no compound (such as ammonia) could yield
less than its own volume of any gaseous constituent
(such as nitrogen). Experiments such as that in
Exercise XV. show that the ultimate particles of
an elementary substance, even in the attenuated
form of a gas, may consist, like those of compound
bodies, of two or more chemical atoms united
together. The properties of an elementary sub-
stance constituted in this manner depend upon the
properties of a group of chemical atoms. The
atoms of nitrogen appear to be firmly united in
the molecule of nitrogen gas, and this two-atom
group shows little chemical activity ; it does not
readily take part in chemical reactions. Nitrogen
Chemical Laws
81
atoms in many chemical compounds, on the other
hand, readily take part in reactions. Under some
conditions, therefore, the atom of nitrogen is
chemically active, although, when it is joined up to
a second atom of its own kind in nitrogen gas, it
is chemically inert. The learner must be upon
his guard against the confusion which sometimes
arises from the promiscuous use of the phrase ' the
properties of nitrogen,' or * the properties of
oxygen,' to denote either the properties of nitrogen
gas (or oxygen gas), or the properties of the
chemical atom of the element.
Upon the relative weights of the chemical atoms ;
upon the choice of a unit to which the weights of the
atoms are referred ; and upon Front's hypothesis. —
The study of the densities of gases and of the
volumetric proportions in which gases react, showed
that chemical elements do not always unite atom
for atom, but that the atom of one element may be
equivalent to more than one atom of another
element. Now in the case of hydrogen, we do not
know of an instance in which the atom is equiva-
lent to more than one atom of another element.
Further, the equivalent weight of hydrogen is less
than that of any other known element. It ap-
peared, therefore, to be logical and convenient to
choose the equivalent weight of hydrogen as unity
(or 1 weight I ') in tables of equivalent weights,
and also to choose the weight of the atom of
hydrogen as unity in the table of ' atomic weights/
i.e. the table expressing the relative weights of the
G
82 Practical Proofs of
chemical atoms. Thus, if the atoms of chlorine,
oxygen, nitrogen, and carbon combine respectively
with one, two, three, and four atoms of hydrogen,
we have the following numbers : —
(as given
Equivalent weights
by standard determinations)
Multiplied by
Atomic
weights
Hydrogen
I
I
I
Chlorine
35 '37
I
35 "37
Oxygen
7-98
2
15-96
Nitrogen
4-67
3
14-01
Carbon
2 "992
4
1 1 '97
It would be beyond the scope of this book to
discuss in detail the considerations which assist in
the determination of the multiple of the equivalent
weight which gives the atomic weight. The most
important consideration is the weight of the element
in the molecules of its gaseous compounds. The
determination of the density of a compound gas
gives the weight of its molecule, and analysis gives
the per cent, of this weight which is due to the
element in question. The smallest weight of the
whole of an element present in the molecule of any
of its compounds, or, the least difference between
the weights of the element present in the molecules
of its compounds gives a probable value for the
atomic weight.
Hydrogen is the lightest gas known, and its
molecule contains two atoms. It appears, there-
fore, to be logical to reckon the density of hydrogen
as the unit in the table of the relative density of
Chemical Laws
83
gases. If a gas is stated to have a density of 20,
we mean that, bulk for bulk, it is twenty times as
heavy as hydrogen gas, and that its molecule is
twenty times as heavy as the molecule of hydrogen,
and forty times as heavy as the atom of hydrogen.
The molecular weight of such gas is said to be 40,
i.e. the weight of the ATOM of hydrogen is taken as
the unit in the table of MOLECULAR weights as well
as in that of atomic weights.
There are, however, practical objections to the
logical system of reckoning the atom of hydrogen
as the unit weight. Few of the equivalent (and
hence of the atomic) weights are directly determined
relatively to that of hydrogen, whereas a great
number are determined relatively to that of oxygen,
as, for instance, by converting a metal into its
oxide or reducing an oxide to metal. The result
of each such experiment has to be combined with
the determination of the combining proportion of
hydrogen and oxygen in order to calculate the
equivalent, or the atomic, weight of the element in
question in terms of that of hydrogen. Thus any
alteration in the received experimental numbers
for the composition by weight of water alters a very
large proportion of the numbers expressing atomic
weights. This is highly inconvenient, as the com-
bining proportion of oxygen and hydrogen is
difficult to determine with great precision, and as
successive experiments are constantly giving
slightly different numbers.
The atomic weight of oxygen is about 16
84
Practical Proofs of
times that of hydrogen. (Lothar Meyer and
Seubert's tables give 15*96, Clarke's tables give
15-963 as the most likely value). Probably the
best plan to adopt {vide Ostwald's ' Outlines of
General Chemistry/ pp. 14-15) is to take the
atomic weight of oxygen as the standard of
reference, giving to it the number, not of unity, but
16. New determinations of the composition by
weight of water will then only affect the precise
number for hydrogen, which at present stands on
the above system at 1*0032, leaving the other
atomic weights practically unaffected. When this
plan is adopted it is usual to state that the atomic
weights are given in terms of
O = 16
The learner must bear in mind that this is simply
a mode of statement adopted for convenience ; it
does not mean that approximate numbers arc em-
ployed, as when we say that the weight of the
oxygen atom is about sixteen times the weight ot
the hydrogen atom. Further, it does not mean
that any assumption is made that the weight of the
oxygen atom is really an exact multiple of the
weight of the hydrogen atom.
P routs hypothesis. — Not long after the publica-
tion of Dalton's atomic theory many chemists
inclined to the view that there existed a relation,
or law, among the atomic weights similar in form
to Dalton's law of multiple proportions. This
idea, which has appeared in several modifications,
Chemical Lazvs
85
is best known as Front's hypothesis. The simplest
and most important form of the hypothesis sup-
poses that the weight of all the atoms are whole
multiples of the weight of the atom of hydrogen.
This simple relation has not been proved.
The examination of a table of atomic weights
shows, however, that there are many more than
half of the elements whose atomic weights are
within ± I of whole multiples of the atomic weight
of hydrogen. There is a tendency to approximate
towards whole numbers, but this does not show
that there is any simple mathematical law such as
regulates the combining proportions of the ele-
ments.
There has been a prepossession in favour of
Prout's hypothesis due to desire for simplification.
It has been thought that if the atomic weights
could be shown to be integral multiples of that of
hydrogen, then all elements might be regarded as
condensed forms of a single stuff, which in its
lightest form is hydrogen.
If, however, it should be shown that there is no
such simple relation between the atomic weights,
there would be nothing in the discovery to negative
the existence of a single primary stuff or matter,
for we have no experience of the formation of our
chemical elements from any simpler materials, and
we have no knowledge whether our law of con-
servation of mass would hold in such a process.
86
Practical Proofs of
CHAPTER VIII. {Supplementary)
EXERCISES SUPPLEMENTARY TO THE COURSE
ILLUSTRATING THE SCOPE OE THE TERM
EQUIVALENCE IN CHEMISTRY
WHEN time and opportunity serve, it is well for
the learner to carry out a set of experiments which
will illustrate the scope of the term equivalence in
chemistry.
The set of experiments shown in the subjoined
diagram will serve the purpose (see table of con-
tents at the beginning of the book). The learner
may with advantage practise himself in setting
out such diagrams in illustration of the connection
between different reactions with which he be-
comes acquainted in the course of his studies.
The numbers printed on the subjoined diagram
are the ' round numbers ' nearest to the standard
numbers.
Chemical Laws
87
Supplementary Exercise I. — The determina-
tion of the volume of hydrogen evolved during
the solution of a knozvn weight of zinc in dilute
sulphuric acid. (See Exercise IV. (a), Chapter
in.)
The method of performing this experiment has
already been described. For the purpose of the
comparison with oxygen in Supplementary Exer-
cise VI., the volume of the gas obtained should be
reduced to what it would occupy at a temperature
of 0° C. and at a pressure of 760 m.m. of mercury,
according to the rules given in works upon physics.
Example of results obtained. — A pupil found
88
Practical Proofs of
that i litre of hydrogen at cr C. and 760 m.m. was
evolved during the solution of 32*8 grams of zinc.
Experimental error + 1 per cent.
Supplementary Exercise II. — The determina-
tion of the weight of copper deposited, and the
volume of hydrogen evolved, during the passage
of an electric current through a solution of copper
sulphate and through dilute sulphuric acid.
The following are the apparatus and materials
required: — An electric battery, three or four bichro-
mate cells may be used, preferably mounted so
that the carbons may be suspended above the
liquid when not in use, covered copper-ivire, binding
screzvs, &c, platinum dish, two retort stands with
rings and clamps, two glass funnels ^ one of them
having the neck cut short and stoppered with a
paraffined cork through which pass platinum elec-
trodes, gas-measuring tube as in the last exercise,
dilute sulphuric acid and pure copper sulphate,
alcohol, ether. It is also very desirable to have a
box of resistance coils, and a key for starting and
stopping the current.
Mode of conducting the experiment. — The gas-
measuring tube, filled with dilute sulphuric acid, is
inverted in the stoppered funnel, which is supported
on the ring of a retort stand, and the end of the
tube is brought over one of the platinum electrodes,
in which position it is clamped. The other platinum
electrode is connected with the carbon end of the
Chemical Laws 89
battery. Care must be taken that the measuring
tube is placed so that no bubbles of oxygen can
mix with the hydrogen which will be evolved from
the electrode covered by the measuring tube. The
weighed platinum dish containing a solution of
pure copper sulphate is supported upon the ring of
the second retort stand. In the solution of copper
sulphate there dips a copper electrode which is
connected with the electrode under the gas-
measuring tube. When the current is passing
oxygen will be evolved from the solution of copper
sulphate, and in order, to avoid spirting, it is well
to support an inverted glass funnel above the
platinum dish, the platinum wire attached to the
copper electrode passing down the neck of the
funnel. When all is ready, the platinum dish is
connected with the zinc end of the battery, the
current passes, copper deposits on the platinum
dish, and hydrogen gas collects in the measuring
tube. It is advisable to adjust the resistance of
the circuit so that the evolution of gas is slow ;
good results have been obtained with a current
giving 2 cb.c. of hydrogen per minute. The current
should be allowed to pass until not less than
60 cb.c. of hydrogen have been collected. The
gas is measured in the usual way, the temperature
and the height of the barometer being noted.
The platinum dish with its deposit of copper is
weighed. Precautions must be taken to prevent
the film of copper from oxidising, as readily
happens if it be heated. A good way of washing
H
go
Practical Proofs of
and drying the copper film is to pour off the
solution of copper sulphate, pour into the dish cold
distilled water and decant this off once or twice,
then wash out the water with alcohol, and then
wash out the alcohol with a little ether. The ether
is readily removed by blowing air from a bellows
into the dish. The dish can then be weighed.
Examples of results obtained. — Pupils obtained
results for the weight of copper corresponding to
I V2 litres of hydrogen, which differed from the stan-
dard numbers by — '6 per cent., — I per cent, + 5
per cent., and — '35 per cent.
Supplementary Exercise III.— The determina-
tion of the proportion by weight in which zinc
and oxygen combine.
A weighed quantity (nearly I gram) of pure
zinc may be dissolved in dilute sulphuric acid,
excess carbonate of ammonia added, and the whole
evaporated down to dryness. The substance may
then be ignited strongly in a loosely covered
porcelain crucible till the weight is constant, when
the carbonate has been converted into oxide.
Another method of converting zinc into the
oxide is to dissolve the metal in nitric acid, evapo-
rate to dryness, and heat strongly till the weight is
constant.
Chemical Laws
9i
Supplementary Exercise IV. — The determina-
tion of the proportion by weight in which copper
and oxygen combine.
This may be carried out by converting pure
electrolytic copper to oxide as follows. Dissolve
the copper in nitric acid, add solution of caustic
potash to the hot solution of the nitrate. For
details of the method of collecting and weighing
the oxide of copper, consult Thorpe's ' Quantitative
Analysis/ under the head of ' Copper Sulphate,' or
any similar work on analysis.
Example of results obtained. — A pupil found
that '5025 grams copper combined with '1272 grams
oxygen. Therefore 31*59 parts of copper combine
with 7*996 parts of oxygen. The standard number
is 7*98, therefore the experimental error is + '2 per
cent.
Supplementary Exercise V. — The determina-
tion of the proportion by weight between the zinc
dissolved and the copper deposited when pure
metallic zinc is placed in a solution of a copper
salt, the copper salt being in excess.
The experiment is done in a platinum dish.
The copper deposited is dried and weighed as in
Supplementary Exercise II.
Supplementary Exercise VI. — The determina-
tion of the proportion in which hydrogen and
oxygen combine.
The direct determination of the combining
weights of hydrogen and oxygen by Dumas's
92 Practical Proofs of Chemical Laws
method (reduction of copper oxide in hydrogen
and weighing the water formed) is not easy to
conduct. The volumetric proportion in which
hydrogen and oxygen combine may be more
readily determined if suitable apparatus be at
hand. This determination is constantly made in
the ordinary operations of gas analysis, and shows
that 5*6 litres of oxygen combine with IV2 litres
of hydrogen. If this result be obtained, it will
remain in order to complete the set of experiments
shown in the diagram, to show that 5*6 litres of
oxygen gas weigh 8 grams, the weight which was
found to combine with 31^ grams of copper and
with 32^ grams of zinc. For this purpose the
pupil may repeat the well-known exercise of heating
potassium chlorate, determining the volume of
water expelled from an aspirator, and noting the
loss of weight suffered by the potassium chlorate.
PRINTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
LONDON
October, 1895.
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