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Full text of "Object lessons and how to give them, second series for intermediate and grammar schools"

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THE OBJECT LESSON BOOK 



By GEORGE RICKS B.Sc. (Lond.) 



NATURAL HISTORY OBJECT LESSONS 

A Manual for Teachers and Pupil Teachers 



OBJECT LESSONS AND HOW TO GIVE THEM 
First Series for Primary Schools 



OBJECT LESSONS AND HOW TO GIVE THEM 

Second Series for Intermediate and Grammar 
Schools 



OBJECT LESSOM^ 

AND HOW TO GIVE THEM 

Secon& Seriee 

For Intermediate and Grammar Schools 



By GEORGE RICKS B.Sc. (Lond.) 

INSPECTOR OF SCHOOLS TO THE SCHOOL BOARD FOR LONDON 
AUTHOR OF "natural HISTORY OBJECT LESSONS " ETC. 



BOSTON, U.S.A. 
D. C. HEATH & CO., PUBLISHERS 

1S93 






«<DviCArk)K nE3^ 



cZ-C- 



CONTENTS. 



utBBOir P-^on 

INTRODUGTION TO *' OBJECT LESSONS, AND HOW TO GIVE THEM " . ix 
INTRODUCTION TO OBJECT LESSONS FOR JUNIOR AND SENIOR 

SCHOOLS . xiv 

FIRST STAGE. 

I. — SOLID AND LIQUID 3 

II. — " PROPERTIES " OF BODIES 5 

III. — WEIGHT 7 

IV. — POROUS. ABSORBENT 9 

V. POROUS BODIES. FILTERS 11 

VI. — NON-ABSORBENTS 13 

VII. — SOLUBLE 14 

VIII. — SOLVENTS — WATER, ALCOHOL, &C 15 

IX. — ADHESIVE . : 17 

X. — HARD AND SOFT 18 

XI. — BRITTLE, TOUGH, FLEXIBLE 20 

XII.— ELASTIC 21 

XIII. — PLASTIC 23 

XIV. — FUSIBLE 25 

XV. — ON SOME FURTHER PROPERTIES OF THE COMMON METALS . . 26 

XVI. — LEAD ... 27 

XVII — SULPHUR • 30 

SECOND STAGE. 

I. — WATER. — ITS PROPERTIES , • 35 

a.— WATER. — A SOLVENT . ••••.•••37 



5'J.i224 



vi CONTENTS. 

LKSSOW PAOB 

Tir. — WATER A«» VAPOUR. DEW 40 

IV. — WATER A8 FOG, MIST, CLOUD, RAIX, AND STEAM . • .41 

V. — WATER AS SNOW AND ICE . - 45 

VI. — MERCURY, OR QUICKSILVER 46 

VII. — AIR— A SUBSTANCE, INVISIBLE, OCCUPIES SPACE, HAS WEIGHT . 48 

VIII. — AIR PRESSES IN ALL DIRECTIONS 50 

IX. — AIR IS ELASTIC 

X. — GAS ..... -. , ... 

XI. — COAL-GAS • 

Cn. — TAR 

Xin. — CABBONIC ACID 

XIV.— PARAFFIN OIL 62 



XV. — CANDLES 



52 
54 
56 
58 
60 



64 



XVI. —SOAP-BURBLES AND WHAT THEY TEACH 65 

XVII.— BALLOONS - • • .68 



TniRD STAGE. 



I. — MOLECULES , , i 



3 

II. — STATES OF MATTER 75 

III. — ADHESION 77 

IV. — CAPILLARY ATTRACTION 79 

v.— PROPERTIES OF SOLIDS EXPLAINED 81 

VI. — FORCE OF GRAVITY — WEIGHT. 83 

VII. — THE SURFACE OF A LIQUID AT REST IS ALWAYS LEVEL . . 85 

VIII. — PRESSURE IN LIQUIDS. 1 87 

IX. — PRESSURE IN LIQUIDS. II. 

X. — BUOYANCY OF LIQUIDS 

XI. — WEIGHT OF THE ATMOSPHERE 

XII.— PRESSURE OF THE ATMOSPHERE 

XIII.— THE BAROMETER 

XIV. — THE SYRINGE 

XV. — THE COMMON PUMP 

XVI.— FORCE-PUMPS 

XVII. — THE SIPHON 

X.VI1I.— TU? AIR-PUMP 



90 

92 

94 

97 

99 

101 

103 

106 

108 

111 



CONTENTS. 



Vll 



LESSOjr 

I. 



III.- 

IV.- 
v.- 

VI.- 

VII.- 

VIII.- 

IX.- 



XII. 
XIII.- 
XIV.- 
XV.- 
XVI.- 
XVII.- 
XVIII.- 



FOTJRTH STAGE. 



-EFFECT or HEAT ON BODIES (1). EXPANSION AND CONTRACTION 



-HEAT THE CAUSE OF MOTION IN THE AIR 

-DEW AND HOAR-FROST 

-RAIN, SNOW, HAIL, SLEET . 

-SPECIFIC HEAT . 

-LATENT HEAT 

-COOLING BODIES AND FREEZING MIXTURES 

-SPECIFIC GRAVITY OF SOLIDS 



AND VAPORIZA- 



-EFFECT OF HEAT ON BODIES (2). — LIQUEFACTION 

TION 

-THE THERMOMETER . . • 

-FREEZING OF WATER . 

-BOILING OF WATER — CONVECTION 

-DISTILLATION .... 

-EFFECT OF PRESSURE ON THE BOILING POINT OF LIQUIDS 

-STEAM AND THE STEAM-ENGINE . 

-CONDUCTION OF HEAT. — CLOTHING 

-RADIATION OF HEAT. — RADIATORS 

-HOW HEAT AFFECTS THE ABSORPTION OF WATERY 

ATMOSPHERE .... 



VAPOUR BY THE 



PAGE 

117 



119 
120 
124 
125 
128 
130 
132 
136 
138 

410 
143 
145 
147 
149 
151 
153 
155 



SUPPLEMENTARY LESSONS ON PROPORTION 



II. 

III. 

IV. 



158 

161 
163 
165 



FIFTH STAGE. 



1. — FORCE . . . . . 

II. — CHEMICAL ATTRACTION 
ni. — ELEMENTS AND COMPOUNDS • 

IV. — THE AIR A MIXTURE OF GASES . 

V. — WATER A COMPOUND OF TWO GASES 



171 

173 
175 
178 
180 



vni CONTENTS. 

LESSON PAa« 

VI. — COMBUSTION • . « • . 182 

■VII. — THB CHEMISTRY OF A CANDLB 184 

VIII.— BLBCTBIC AND MAGNETIC F0BCE8 ...... 186 

IX. — GENTKB OP GRAVITY . . ... • • • .188 

X. — LEVERS AND THEIR USES 1 192 

XI. LEVERS AND THEIR USES. — II 195 

XII.— LEVERS AND THEIR USES. — III 197 

Xin. — THB PULLEY 

XIV. — THE WHEEL AND AXLE • 

XV. — TJIE INCLINED PLANE •••>.•• 

XVI. — THE WEDGE 207 

XVII. — THB SCREW .••••..•.. 208 

XVUI. — FRICTION • . . • e . ^ « • • c 210 



200 
203 
205 



INTRODUCTION TO 

OBJECT LESSONS 

AND HOW TO GIVE THEM, 



Our "knowledge of tlie material world is obtained through the 
senses. The organs of sense are the eye, the ear, the nose, 
the tongue and palate, and the nerves of touch located in the 
skin. The special nerves of these organs are acted on by 
things external to the body; the effect is conveyed to the 
brain ; and mental impressions or ideas are the result. Thus a 
red colour acting on the retina, the sound from a whistle acting 
on the auditory nerves, or the smell of an onion on the olfactory 
nerves produces a definite mental impression. The five sen- 
sory organs, then, are so many doors and windows by which 
knowledge enters the mind. 

There is, however, another source of knowledge of material 
bodies. In this case the mental impressions are derived from 
within the body, and are due to muscular exertion. It is b} 
muscular feeling that we estimate the amount of force required 
to overcome resistance. Thus we get ideas of elasticity and 
weight from the amount of active energy put forth by the 
muscles to overcome inertia in the one case and gravitation 
in the other. If a weight is placed in the hand we are con- 
scious of a certain amount of force expended to keep it from 
falling ; if the weight is increased we are conscious of an 
increased expenditure of muscular energy. 

The mental impressions, formed by and through the senses, 
including muscular feeling, are called sensations. 

By the organs of sense we are said to perceive^ or to make 
mental notes of external bodies, and these mental notes we 



,^. , , , . ,. ; OBJECT LESSONS. 

call perceptmis, , Perception is the first step in knowledge ; 
|atlen.tiv^,*j:reri:jpptfw leads to observation; observation is the 
forerunner of comparison ; while comparison is the basis of 
classification ; and these together constitute the foundation of 
all knowledge. 

The primary purpose of lessons on common objects and 
natural phenomena is to cultivate the senses, to train to 
habits of attention, intelligent observation, and accurate 
comparison, and so to lead up to the higher processes of the 
mind — reason and judgment. Of course the acquisition of 
information is an important aim ; but the object lesson is 
designed to assist and guide the child to discover properties 
of things, and thus acquire knowledge for himself, rather 
than to pour information into his mind like wheat into a 
eack. 

Mental impressions are formed at a very early period of 
childhood. A bright light or a shining object attracts 
attention before the child has acquired the power of 
taking hold with its hands; and a certain amount of dis- 
crimination, enabling it, for instance, to distinguish the face 
of its mother from that of a stranger, quickly follows. The 
power of recognising resemblances and differences rapidly 
increases, new ideas are as rapidly acquired ; and when the 
child enters school, he enters it with his perceptive faculties, 
to a certain extent, cultivated, and with his mind a treasury 
of simple ideas. 

The natural course for the teacher would seem to be to 
gather up into something like order, and to perfect, that 
which has been so far imperfectly accomplished; and then, 
starting from this as a basis, to evolve a systematic course of 
training, proceeding step by step in a natural order, each 
step being a logical sequence of the preceding. Further, 
the teacher who would best succeed must take childhood's 
method of imbibin^^ knowledge and adapt it to her own use. 
Restless activity, insatiable cariosity, and love of imitation 



HOW TO GIVE THEM. xi 

characterize childhood. What the child sees he wants tc 
know about, to handle, and examine, and, if possible, to take 
to pieces, or otherwise experiment upon ; what he sees done 
he wants to do, and if opportunity be not found for the 
indulgence of his natural activity, he will find the oppor- 
tunity for himself. An object lesson, then, besides fulfilling 
some definite purpose in training the perceptive faculties, 
should provide something for the children to do to satisfy 
their innate activity, something to examine and discover to 
arouse their curiosity, and something to cojyy to gratify their 
desire for imitation. Herein lies the secret of securing 
attention, of begetting a state of vigorous mental activity, 
and of associating pleasure with instruction. 

The selection of lessons, and their adaptation to the capacities 
of the scholars, or to their different stages of advancement, 
is another point of fundamental importance. A child of four 
years of age is a different being, intellectually, from a child 
of seven ; and a lesson suited to the capacity of the one must 
be totally unsuited to the mental condition of the other. 
The mental faculties of a child are strengthened and invigo- 
rated by proper exercise, but are weakened and depressed by 
being exercised on subjects beyond his powers of compre- 
hension. To graduate the lessons to the mental condition 
and previous training of the scholars necessitates a complete 
system. It is not sufiicient to select a lesson at random, no 
matter how skilfully it may be handled. Each lesson, whilst 
fulfilling its own special purpose, must form a link in the 
chain, a unit in the whole. 

Nor is the method of giving the individual lessons of less 
importance than their selection and adaptation. Occasional 
information given about things of every-day life does not 
serve the distinctive aim of object lessons. To be a passive 
recipient of information gives no pleasure to a child. To hold 
an object before it, and enumerate its general properties — 
what it is composed of, or where or how it is made — and 



XU OBJECT LESSONS. 

then to get the information returned hy questioning, is at 
best but a mere exercise of the memory ; it does nothing in 
the way of exercising and developing the more important 
mental powers. 

" To tell a child this, and to show the other, is not to teach 
it how to observe, but to make it a mere recipient of another's 
observations — a proceeding which weakens rather than 
strengthens its powers of self-instruction, which deprives it 
of the pleasure resulting from successful activity, which pre« 
sents this all-attractive knowledge under the aspect of formal 
tuition, and which thus generates that indifference and even 
disgust with which these object lessons are sometimes re- 
garded. On the other hand, to pursue the true course is 
simply to guide the intellect to its appropriate food, and to 
habituate the mind from the beginning to that practice of 
self-help which it must ultimately follow. Children should 
be led to make their own investigations and to draw their 
own inferences. They should be told as little as possible, 
and induced to discover as much as possible.*' * 

Having formed new ideas of things by the method of obser- 
vation and experiment under the guiding hand of the teacher, 
our next step is to endeavour to fix these ideas in the minds of 
the children by means of language. But in every case words 
miiBt follow ideas; in fact, terms should not be given till the 
necessity for them is felt. Thus, suppose " a liquid " to be 
the subject of the lesson. The children are led by experiment 
on several liquid bodies to note that they all have certain 
common properties — such as flowing in a stream, finding the 
lowest level, spreading out and filling up hollows, easily 
flowing in drops, having no definite shape, but taking the 
shape of the vessel into which they are poured — and the 
necessity is felt for one term which at once embodies all 
these properties. The extension of the children's vocabulary 
in this way is one of the minor advantages of object lessons; 
• Tlorbert Spencor. 



HOW TO GIVE THEM. Xlll 

and furtlier, to secure freedom and accuracy in speech, the 
children should be encouraged to answer all questions, as 
far as possible, in complete sentences, or at any rate in 
complete phrases. 

The teacher may commence object lessons by taking some 
familiar object — such, for instance, as the black-board — and 
lead the children to observe its colour, shape, substance, 
surface, and so on ; but then we have so many properties in 
combination that the scholars are not likely to get very clear 
notions of any. It is desirable, therefore, if not actually 
necessary, that lessons on objects should be preceded by a 
special training in colour, form, size, weight, hardness, and 
others of the more conspicuous properties of bodies. Lessons 
on objects may then be introduced gradually, and, to a large 
extent, they may be made to constitute simple practice in 
the application of previously acquired knowledge. 

In dealing with the properties or qualities of objects, those 
only should be dwelt upon which render the objects valuable 
for the several uses in which they are employed. Thus, all 
children are alive to the fact that we cannot see through 
sponge, cork, india-rubber, or leather; but to stop to describe 
these objects as opaque is a waste of time. On the other 
hand, although the children know equally well that we can 
see through glass, its property of transparency must be made 
a cardinal point in a lesson on glass, because it is this 
property which makes glass specially useful. 



INTRODUCTION" 

TO 

OBJECT LESSONS 

FOR JUNIOR AND SENIOR SCHOOLS. 

" The practice of every art implies a certain knowledge of 
natural causes and effects, and the improvement of our arts 
and industries depends upon our knowing the properties of 
natural objects which we can get hold of and put together." 

" No line can be drawn between common knowledge of 
things and scientific hiowkdge, nor between common reasoning 
and scientific reasoning. In strictness all accurate knowledge 
is science J and all exact reasoning is screntific reasoning." 

'* The method of observation and experiment by which 
such great results are obtained in science, is identically the 
same as that which is employed by every one, every day of 
his life, but refined and rendered more precise." 

** The way to science lies through common knowledge, 
we must extend that knowledge by careful observation and 
experiment, and learn how to state the results of our inves- 
tigations accurately, in general rules or laws of nature, and 
we must learn how to reason accurately from these rules, 
and thus arrive at rational explanations of natural pheno- 
mena, which may suffice for our guidance in life." * 

On these principles laid down by Professor Huxley the 
following simple lessons on natural objects, and natural 
phenomena have been constructed. 

* Professor Huxley, 



INTRODUCTION XV 

Their primary purpose is to develop the faculties of the 
mind, to quicken the intelligence, to train to habits of accu- 
rate observation, exact comparison, and sound reasoning, 
and to excite an interest in all those objects with which we 
daily come in contact, and those natural phenomena which 
constantly appear before our eyes. 

The lessons are suggestive rather than exhaustive ; some 
are little more than outlines ; a few are worked out more 
fully as models for young teachers. 

The experiments are numerous and interesting, yet simple 
and inexpensive. 

The lessons of the First Stage* deal with the more 
common properties of solids, and show how these properties 
make them specially useful in the arts and industries. They 
are lessons almost entirely of observation, experiment, and 
comparison. Those of the Second Stage deal similarly with 
the properties of liquids and gases illustrated by icater and 
air. The Third and higher stages demand a closer observa- 
tion; the reasoning faculties are gradually brought more 
fully into exercise; and the simplest facts and laws of nature 
are explained in the simplest possible way. 

* The standards for wliicli the stages are suited must depend in a great 
measure on how far the children have been trained in the infant school, but, 
generally, Staj?e 1. will be suitable for Standard II., Stage II. for St .ndard 
III., and so on. Lessons suitable for Standard I. will be found in ** Object 
Lessons, and How to give them. First Series, for primary schools." 



FIEST STAGE. 



FIRST STAGE. 

LESSON I. 

SOLID AND LIQUID. 

Articles for illustration : any specimens of well-known solid bodies, 
water, a piece of sponge, and one or two glass vessels. 



I. To show that the minute particles of which solids are built 
up are held more or less firmly together. 

Experiment 1. Take two lumps of loaf sugar ; and, rubbing 
them together, show that the lumps are made up of graim. 

Exp. 2. Crush or pound the grains to show that these 
may be divided into smaller bits as fine as flour or dust, and 
too small to be easily seen as separate particles. 

Exp. 3. Rub lumps of chalk together, to show how ex- 
tremely fine are the tiny particles of which chalk is made. 

Exp. 4. Hand a small cubical lump of loaf sugar, or a 
piece of brick, to a scholar, and ask him to separate it into 
tiny bits. He fails. Why ? The sugar, or brick, is too 
hard. The grains, or particles, are held together too firmly. 

II. To show that water and oil are also composed of minute 
particles ; hut that these are held together less firmly than are 
the grains of sugar and salt 

Exp. 5. Take some water in a tumbler, and sprinkle a 
little on the floor, or blow a little through a syringe, to show 
the tiny dro^s. 



4 OBJECT LESSONS. 

' .' Msip, 6, Into a test-tube three-parts full of water pour a 
single drop of olive oil. SLake well; the single drop is 
divided into thousands of tiny drops, giving the water a milky 
appearance; but the drops are too small to be seen separately. 

" Now which is the more easy to divide, the oil and the 
water into tiny drops, or the sugar and salt into grains ? " 
The water and oil. " And why ? " Because the drops of oil 
and water are not held so firmly together as are the grains in the 
sugar and in the salt, 

" Here are pieces of stone and wood. You can handle them, 
pass them round, throw them up, and they are not altered. 
Now, can we take a piece of this water out of the glass and 
pass it round the class? No ; it would break into drops at 
once, and fall on the floor." 

" I want you to remember that all those things in which the 
tiny particles are held firmly together we call solids ; and 
that all those things which are made up of tiny drops not held 
firmly together we call liquids." 

III. To show that solids have shapes of their own ; but that 
liquids, having no shapes of their own, take the shapes of the 
vessels in which they are placed. 

The teacher may proceed somewhat as follows : — 

" Here is a small block of wood.* What is its shape P " 
A cube. 

** I put it in this tumbler ; is its shape altered ? " No. 

*' What shape has it still P " A cube. 

** I take out the cube, and fill the tumbler with water. 
What is the shape of the water in the tumbler P " The same 
08 the tumbler. 

* Kinder-garten cube, for instaiiM. 



FIRST STAGE. O 

** Now I pour some of the water into this wine-glass. 
What shape has it now ? " The shape of the glass. 

" Now I fill this small bottle. What shape has the water 
now ? " The shape of the bottle. 

" The second fact I want you to remember about solids and 
liquids is this : That solids have shapes of their oivn ; but that 
liquids have no shapes of their own^ and therefore they take the 
shapes of the vessels in which they are placed.** 

Metals and stones, desks and forms, houses and trees, books 
and pencils, all have shapes of their own, and we can only 
alter their shapes by cutting, or hammering, or pulling, or 
squeezing. Water, and other liquids, have no shapes of 
their own; the moment we remove the vessel which holds 
them they break up into numberless drops. 

IV. To show that the particles of some solids are held more 
firmly together than those of other solids. 

Call upon one child to try and break off a piece of iron 
with the fingers, then with a hammer. A second may take 
a flint, or a piece of brick. A third a piece of loaf sugar. 
A fourth may break a piece of chalk with the fingers, and a 
fifth a piece of table- salt. 

" What do we learn from these experiments ? " That the 
tiny grains or bits are held together more firtnly in some solids 
than in others, 

LESSON II. 

"PROPERTIES" OF BODIES. 

Articles for illustration : orange, onion, sugar, salt, glass, sponge, 
water. 



I. The distinguishing marks of bodies are called their "proper- 
ties." 

Teacher, showing an orange, asks ; — " What is this ? 



6 OBJECT LESSONS. 

How do you know it is an orange ? " By its shape. By its 
colour. By its size. ** What shape is it ? What colour has 
it?" 

** Here is an onion. How can we tell oranges from onions 
without looking at them ? " By their smell. 

" Here is a book and here a sheet of glass. What can we 
say about the glass which we cannot say of the book ? " 
We can see through the glass ; but we cannot see through thi 
hook. 

** I daresay you remember what we say of glass because we 
can see through it ? " Glass is transjwrent. 

We can tell an orange by its colour and shape and scent ; 
we know an onion by its strong smell ; we can tell glass 
because we can see through it ; we know sugar by its sweet 
taste, and salt by its salt taste. 

There is something about almost everything in the world 
which helps us to tell or distinguish it from everything else. 
The taste of sugar, the smell of an onion, the shape of an Qg^y 
the colour of an orange, the transparency of glass, the softness 
of sponge, the lightness of cork, are all marks which help 
us to point out or distinguish these things one from another, 
and from other objects in the world. And what I want you 
to learn in this lesson is that to these special marks we give 
the name of Properties. 

II. Bodies possess many properties. 

Exp. 7. " Here is a piece of dry sponge. Squeeze it." 
It feels soft. " Put it on the water." It is light. It sucks 
up water. " Look at it." It is full of little holes. 

"The sponge has many properties. Name some of them.*' 

JSjt]). 8. *' Take this piece of glass. Squeeze it." It is 
hard. "Hammer it." It breaks easily. "Puss the tips of 
your fingers across it." It feels smooth, " Place a book 
behind it." We can see the book through the glass. 

"Glass has many properties. Name some of them." 



FIRST STAGE. 7 

III. Bodies may have some properties alike, or in common. 

Exp. 9. " I take this lump of sugar and this lump of 
salt, and set them in a little coloured water on this plate. 
What do you see?" The sugar and the salt suck up the 
water. 

** I put the sugar in this glass of water and the salt in 
another. What happens ? " Both dissolve in the water. 

'* Now you can name for me two properties which belong 
to sugar, and also to salt." Both suck up water; both 
DISSOLVE in water. 

"Taste the water in the gLisses. Now you tell me in 
what respect sugar and salt are unlike.** They are unlike in 
taste. 

" Name one property which the following pairs of bodies 
have in common.'' 

1. Soda and ice. Both are transparent. 

2. Cork and sponge. Both are light. 

3. Lead and gold. Both are heavy. 

4. Wool and sponge. Both are soft to the touch. 

5. Ripe orange and sugar. Both are siveet to the taste. 

6. Lemon and rhubarb. Bath are sour to the taste. 



LESSON III. 

WEIGHT. 
Articles for illustration : pieces of metals, woods, cork, and sponge. 



I. Weight is pressure downwards. 

Exp. 10. Direct some of the scholars to hold in their hands 
any heavy substances within convenient reach. What can 
they say about them ? They are heavy. 



8 OBJECT LESSONS. 

Next take lighter bodies, such as light wood, bark, cork, 
&c. What can be said of these bodies ? They are light 

Exp. 11. Place these bodies in a vessel of water ; some 
float, some fall to the bottom. Distinguish again the light 
and heavy bodies. When placed in water in what direction 
do bodies press ? Downwards. 

Exp. 12. Place a heavy weight on some soft yielding 
substance such as putty, clay, snow, or sponge. In what 
direction does the weight press ? Downwards. 

Exp. 13. Direct a boy to place his right arm and hand 
in a horizontal position. Place weights — books for instance — 
on the hand until it is visible to the class that the hand is 
pressed downwards. 

We call this pressure downwards Weight. Weight is a 
property common to all bodies. 

II. Actual weight depends partly on size. 

" Wood floats on the water, iron sinks to the bottom. 
Which, is the heavier body ? " Iron, 

" Here is a small piece of iron, and here a large block of 
wood. Which is the heavier ? " The wood. 

*' What do we mean, then, when we say that iron is 
heavier than wood ?" We mean that when we take pieces of 
iron and wood of the same size, the iron is heavier than the 
tcood. 

When we compare the weights of bodies it is always 
understood that we are speaking of pieces of the same size. 

III. The meaning of heavy and light. 

** Here is a small block of wood. Has it any weight ? 
Does it press down ? Now take this small block of lead of 
the same size. Which prcases down the harder? Which 



riRST STAGE 9 

has the greater weight ? (compare the weight of the lead and 
the wood." The lead is heavy, the wood is light. 

" What do we mean when we say that wood is light ? " 
We mean it has little weight. 

'* And what do we mean when we say the lead is heavy ? '* 
We mean that lead has much weight. 

All bodies, then, have weight ; and when we say that bodies 
such as wool, feathers, cork, and sponge are light, we mean 
that they possess but little weight, compared with such bodies 
as stones and metals. 

IV. The uses we make of bodies as dependent on their weight. 

The teacher will illustrate how heavy substances are used 

for weights, &c. ; and light substances, as cork, for instance, 

for making life-buoys, cork jackets, &c. 

*^* As these lessons have a natural sequence, the introduction of any one 
lesson should, as a rule, be a recapitulation of the salient points in the preced- 
ing lesson. 



LESSON IV. 

POROUS. ABSORBENT. 



Articles for illustration : sponge, bread, piece of cane, sugar, salt, chalk, 
water, a couple oi plates, and a couple of tumblers. 



I, Meaning of porous. 

Porous bodies absorb water. Select substances in which 
the pores can be readily seen — such as sponge, crumbs of 
bread, and a piece of cane — to show the meaning of porous. 
Let the children handle and examine the specimens, and 
discover the holes for themselves. 

Exp. 14. Pour a little coloured water into a plate, and 



10 OBJECT LESSONS. 

iu it place the sponge and the bread. The children will see 
the water gradually rising. 

" Where does the water go? " It filk up the tiny holes. 

** I place a piece of flint and a piece of lead in the water. 
Do these substances suck up the water ? " No. 

"Why not ? " They have no little holes.* 

Exp, 15. Place the piece of cane, together with a slate 
pencil, in a bottle half filled with spirits of turpentine. In 
a few minutes the turpentine will have ascended through the 
pores to the top of the cane, and on the application of a 
lighted taper will burn with a smoky flame. The turpentine 
does not ascend through the slate pencil. Why not ? 

Exj). 16. Next let the children examine pieces of chalk, 
loaf sugar, table-salt, &c. Can they see any pores ? No, but 
they are there. Show this by placing the chalk, &c., in the 
coloured water. The water is absorbed, we can see it rising ; 

hence the pores must be there. 
They are too small to he seen. 

Eocp. 17. Take a piece of loose 
twine,, and, after immersing it in 
water place one end in a glass of 
water, and the other end in an 
Fig 1.' empty glass placed at a lower ele- 

vation (Fig. 1). 
After a time it will be found that all the water has been 
transferred to the lower glass. How has this been brought 
about ? The twine is porous ; the water ascended through the 
pores to the top of the glasnyjust as it went up the cane; and then 
trickled down through the pores into the lower glass. 

* Note. — All snbstancos are more or less nbsnrlicnt. It will be sufficient, 
however, at this stage that tht- children 8^lOulddi^tinliui8h between substances 
inanifcstly absorbent, and those which absorb so very little as to be practi- 
cally non-absorlwnt.^ 




FIRST STAGE. 11 

II The uses to which we put some porous bodies 

1. From this last experiment the children can be led to 
see why we make candles with wicks in the centre ; also the 
use of wicks in oil-lamps. 

2. Most articles of clothing are porous. Show the use 
of the '' house-flannel." 

3. Blotting-paper is absorbent. Write upon it. The ink 
spreads about — runs into the pores. How is blotting-paper 
useful ? 

** Writing-paper" is not porous, the pores have been filled 
up with size. 

The teacher will doubtless have other substances at hand 
still further to illuitrate the fact that porous bodies absorb 
water. 



LESSON V. 

POROUS BODIES. FILTERS. 



Articles for iUusiration : a small fiower-pot, spuiiyu, cnarcoal, sand, 
Hour, blotting-paper, and a small funnel. 



I. A sponge filter. 

Exp. 18. Take a small common flower-pot — clean of 
course. Put a piece of sponge at the bottom. Pour in a 
little dirty water. When the pores in the sponge become 
filled with the water, the latter passes slowly through, but 
is not cleansed. 

*' Why ? " The pores are too large to 'preveni tiie tiny 
particles of mud from passing through. 

Put sawdust or sand with the water. These substances do 
not pass through. 

*• Why ? " The pores are too small to allow the particles 
of saivdust or sand to pass through. 



12 OBJECT LESSONS. 

II. The charcoal filter. 

Exp, 19. Show that charcoal is porous by standing a 
piece in water on a plate, as shown in the previous lesson. 

Exp. 20. Place layers of powdered charcoal and sand on 
the sponge in the flower- pot. Pour in water ; this, as it 
slowly trickles out at the bottom, will be found at first to be 
coloured with very fine particles of charcoal, but presently 
the drops will be clear and colourless. 

Prepare a mixture of flour and water — half a tea-spoonful 
of flour well stirred into a tumbler of water. The mixture 
when poured into the filter will have a milky appearance, 
but the water will trickle out clear and bright. 

III. Blotting-paper filter. 

Exp. 31. Cut circles of blotting-paper say 3J or 4 inches 
in diameter. Take two thicknesses and fold twice, as in the 

cut (Fig. 2). Then 
open out to form a cone. 
The filter will have 
the paper two thick- 
nesses on one side, and 
six on the other side. 
Place the cone in a small funnel and pour in the flour- 
mixture. 

Clear water passes through, but the flour is left behind in 
the filter. The pores in the blotting-paper are too small to 
allow the flour-dust to pass through. 

IV. The earth-filter. 

Have you ever seen a spring ? The water comes out of 
the ground clear and bright. Where did the spring get its 
water ? From the clouds. The rain fell on the soil and 
became muddy and dirty ; but it trickled slowly through the 
soil and the sand, and gravel, and rocks, and, as it comes out, 




Fig. 2. 



FIRST STAGE. 13 

we see it quite clean again. How is this ? The earth through 
which it has passed has acted as a huge filter, 

V. Uses of filters. 

To drink dirty water makes people ill. We can cleanse 
the water by filtering it. Spring water is best to drink, 
because it is clean, it has been filtered. When water is 
supplied to us through pipes, and we have to keep it in 
cisterns, it is best to filter the water before we drink it, 
because the pipes may not be clean, and dust and dirt may be 
present in the cistern. 



LESSON VI. 
NON-ABSORBENTS. 



Articles for illustration : ^any non-absorbent substances, as metals, 
glass, leather, clay, putty, &c. 



I. Some non-absorbent bodies. 

Show by experiment that many substances, such as glass, 
india-rubber, leather, metals, horn, ivory, &c., do not absorb 
water. 

II. Non-absorbent substances do not allow water to pass 
through. 

Refer to glass, earthenware, and china vessels, which we 
use for holding water and other liquids. 

III. Uses to which some other non-absorbent substances are 
put. 

India-ruhher for making waterproof clothing. 

Leather for boots and shoes. Refer to the necessity for 



14 OBJECT LESSONS. 

keeping tte feet dry. Bottles and drinking vessels were 
formerly made of leather. 

Paint and tar are put on wood to prevent absorption of 
water, and consequent decay of the wood. 

Dry wood absorbs water and swells. Window frames not 
painted would not fit closely. They would be too large in 
damp weather, or too small in dry weather. 

Putty is used in glazing to prevent water passing through 
between the wood and the glass. 



LESSON VII. 

SOLUBLE. 

Arttclfs for illnstration : eohible and insoluble substances ; sugar, 
alum, salt, soda — caniplior, chalk, marble, wood. 



I. The meaning of soluble. 

Exp. 22. Put a teaspoonful of salt into a medium-sized 
test-tube three-parts filled with water. Stir, or shake ; 
in a short time the salt has disappeared, and the water 
is just as clear as it was at first. Where is the salt? Clearly 
it is in the water. We can taste it, but not see it. It is 
invisible. 

Repeat Exp. 6, p. 4. The oil is split up into such 
tiny drops that we cannot see them separately. 

In the same way the salt splits into such tiny particles 
that, although we can taste them, we can neither see nor feel 
them. 

Exp. 23. We can recover the salt from the water. Boil a 
little brine in the evaporating dish until the water has all 
been converted to steam. The salt is left behind.* 

• Another and a pretty experimf nt to show that water may contain solids 
in solution, althoiif^li wo cannot soo thorn. To tho solution of salt add a ivw 
drops of nitrate of silvrr. A dense white curdy-lookiiig solid is seen lloatiug 
about. Add a litth- ammonia solution ami (he .sulid is again dissolved. 



FIRST STAGE. 15 

II. Some substances are soluble in water, and others insoluble. 

Exp. 24. Show the solubility of sugar, alum, soda, salt, 
&c. Use glass vessels for clearer illustration. 

Then the insolubility of other bodies, such as stone, chalk, 
coal, and wood, may be demonstrated. 

Ask the children to name some of the things they know 
which dissolve in water, and others which do not so dissolve. 
Arrange the names in two columns on the blackboard. 

III. Manufacture of salt and sugar. 

The teacher may illustrate the use made of the solubility 
of substances by showing how salt is prepared from the water 
of brine springs and from sea water by evaporation ; and 
also how sugar is prepared from the juice of the sugar-cane 
in a somewhat similar manner.* 



LESSON VIII. 

SOLVENTS— WATER, ALCOHOL, &c. 

Articles for illustration : small quantity of alum, saltpetre, lime, 
camphor, spirits of wine, benzine, and naphtha. « 



I. Water can dissolve only a certain quantity of a solid. 

Exp. 25. Show this by putting more salt or sugar in a 
test-tube of water than the water can dissolve. 

[Hasten the solution by boiling in the flame of the spirit- 
lamp.] 

II. Some substances dissolve best in hot water, others in cold 
water. 

Exp. 26. Make a hot saturated solution of alum, and set 

* The manufacture of salt and sugar may form subjects for separate 



16 OBJECT LESSONS. 

it aside to cool. A large proportion of the alum assumes the 
solid form.* Why ? 

Exp, 27. Pour upon a clean piece of window-glass a hot 
saturated solution of saltpetre. Allow the liquid to drain 
off, hold it up to the sunlight, and beautiful crystals will be 
seen to spread over the glass. Why ? 

Tell the children that salt dissolves equally well in hot or 
cold water ; but that lime dissolves best in cold water. 

III. Water dissolves more of some substances than of others. 

Exp. 28. Compare the solubility of salt and lime for 
instance, by trying to dissolve equal quantities in equal 
volumes of water. 

Lime appears to be almost insoluble. 

Exp. 29. Show that some lime is dissolved, by filtering 
the solution through blotting-paper (see p. 12) and then 
sending the breath into the clear solution through a glass 
tube. The water becomes milky, showing that there was 
something in the water which is not found inordinary water. 
This must be lime, for nothing else was put in, 

IV. Other solvents. 

1. Alcohol, or spirits of wine. 

Camphor dissolves in spirits of wine ; but only to a very 
slight extent in water. 

Exp. 30. Pour water into a solution of camphor, and the 
camphor becomes a feathery looking solid in the mixture. 

2. Benzine. 

Fat dissolves in benzine. Hence we use benzine to remove 
grease-spots from clothing. Show this by experiment. 

• Take some small article made of ^re, such as a basket, covrr the wire 
with worsted, place the basket in n hot saturated solution of alum, and leave 
to cool slowly without distuihunce. In a iow days a pretty cryetol buaket 
will be seeu 



FIRST STAGE. 17 

3. Naphtha. 

India-rubber dissolves in naphtha. The solution can be 
spread over articles of clothing to make them wateipiouf. 



LESSON IX. 

ADHESIVE. 

Articles for illustration : any of the common well-known adhesive 
substances — Plaster of Paris, or cement, putty, and glue, also slaked hme, 
and saud. 



I. Meaning of adhesive. 

Exp. 31. Take any of the well-known sticky substances 
such as gum, sealing-wax, white of e^^y paste, treacle, &c., 
and show by actual experiment how they stick things together. 
Such substances are said to be sticky, or adhesive. 

Exp. 32. A piece of gold-leaf adheres to the finger and is 
not easily removed. 

Exp. 33. Plunge the finger into water ; some of the water 
adheres to the finger. It is easily wiped ofi', because but 
little adhesive. Glue is not easily removed from the finger. 
Why ? Glue is more adhesive than water, 

Exp. 34. Plunge the finger into mercury. None of the 
mercury adheres. Why? Mercury is not adhesive to the 
finger. 

Show how any of the above substances are useful because 
of their adhesive properties. 

II. Some adhesive substances used in building. 

1. Plaster of Paris. 

Exp. 35. Take two pieces of marble or slato, or a couple of 



18 OBJECT LESSONS. 

bricks. Place in water. Mix a little plaster of Paris with 
water till it has about the consistency of thick cream. 
Remove the marble, slate, or bricks from the water, spread 
the plaster of Paris thickly over one piece, and press the other 
firmly over it. In a few minutes the plaster of Paris will 
become hard, and the solid substances are bound firmly 
together. 

2. Cement. 

Exp. 36. Cement such as is employed in making concrete 
foundations for walls, &c., may be used in the same way as 
plaster of Paris. For this illustration the cement should be 
mixed with about twice its volume of fine sharp sand. 

3. Mortar. 

Exp. 37. Show how mortar is made. [Ordinary mortar 
consists of slaked lime and sharp sand well mixed with 
water.] Illustrate the use of mortar. If possible show old 
mortar. 

4. The uses of putty* and glue may also be illustrated. 



, LESSON X. 
HARD AND SOFT. 



Articles for illnstntion : as many as possible of the following : — glass, 
flint, steel, coins of various sorts, iron nail, tin, pewter, lead, chalk, rock- 
salt, wood, cork. 

I. How one solid is shown to be harder than another. 

The children may be called upon first roughly to distinguish 
between the hard and soft substances on the table hy feeling 
them. The articles should be arranged in two divisions, 

• Putty if composed of whiting and linseed oil, mixed and well worked 
together. 



FIRST STAGE. 19 

and their names written in two columns on the black- 
board. 

Secondly, they should again separate the articles into three 
divisions by scratching. 

Exp. 38. Try with the finger-nail. 

Chalk, rock-salt, lead, wood, &c., can be scratched. 

Set these on one side as forming the 1st division. 

Exp. 39. Try the remainder with the point of an iron 
nail. It will scratch gold, silver, copper, pewter, &c. 

These will form the 2nd division. 

Exp. 40. Those which the iron nail will not scratch, viz. 
flint, glass, steel, &c. 

These will form the 3r«? division, including the hardest 
substances. 

Lastly, show the class that we can tell the harder of two 
bodies by rubbing them together. The harder will cut, or 
scratch the softer. 

Exp. 41. Try iron with copper, brass with copper, iron 
with glass, and so on. 

Tell the children that of all known bodies the diamond is 
the hardest. It will cut or scratch every other known 
substance. Instance the glazier cutting glass for windows. 

Describe the diamond as looking like beautiful clear 
glass. Why so tiny a bit in the glazier's tool ? 

If a glass-cutter can be borrowed for the occasion, and its 
use shown, so much the better. 

II. How some metals are made harder. 

1. If steel be made red hot, and then cooled quickly by 
plunging into cold water, it becomes much harder. 

2. Pure gold is almost as soft as lead, and if used for 
coins would soon wear away ; a little copper mixed with it 
makes it much harder, and it does not wear away so quickly. 
Silver is hardened in the same way. Copper is also hardened 



20 OBJECT LESSONS. 

by mixing with it a little tin and zinc — ^two other metals. 
The mixture is called bronze, and pennies, halfpennies, and 
farthings are made of ic. 



LESSON XI. 
BRITTLE, TOUGH, FLEXIBLE. 



Articles for illustration : pin, needle, various wires, old " kid " glove, 
chalk, and glass. 

I. Meaniag of brittle, tough, and flexible. 

Exp. 42. Take a pin ; ask a child to break it. It bends, 
but does not break. Try a needle ; it bends and then breaks. 
In the same way try a piece of lead wire, or copper wire; and 
a piece of chalk, or slate pencil. The lead wire and the 
copper wire bend, but do not break. The chalk and the 
slate pencil break easily. 

Exp. 43. Test by striking each article with a hammer. 
The same result : the pin and the lead and copper wires bend, 
but do not break ; the needle, chalk, and slate pencil break 
into pieces. 

Tell the class that when we can break articles into sharp 
pieces with the fingers, or by throwing them on the floor, 
or by striking with a hammer, we say they are brittle. 
When they bendy but do not break, we say they are flexible. 

Exj). 44. Next compare lead wire with copper wire by 
bending backwards and forwards. The lead wire breaks 
easily, the copper does not. We say the copper wire is 
tough. 

Exp. 45. Next try to tear a piece of thin leather — an 
old "kid" glove for instance — and then a piece of brown 



FIRST STAGE. 



21 



paper. The paper is easily torn, t"he leather is not easily 
torn. Both the paper and the leather are flexible ; but 
the leather is tough also, it is not easily torn. 

II. Bodies which are orittle, &c. 

By similar experiments the children may be led to see 
that such substances as 



1. flint 
china 
glass 
cast iron 

2. chalk 
salt 
bread 

3. copper 
wrought iron 
brass 

hard wood 

4. cork 
sponge 
india-rubber 



are hard and brittle. 



are soft and brittle. 



are hard and tough. 



are soft and tourrh. 



LESSON XII. 

ELASTIC. 

Articles for ilkistration : balls of wool, india-rul)ber, clay or patty, an 
orange, a piece of sponge, strip of glass, cork, and piece ot " elastic." 



I, Meaning of elastic. 

Exp, 46. Take balls of wool, india-rubber, and clay or 
putty, an orange, and a piece of sponge. Let individual 



22 OBJECT LESSONS. 

scholars be called in front of the class to try the effect of 
squeezing each. 

The orange, the wool and india-rubber balls, and the 
sponge take their own shapes again when the pressure is 
removed. They are said to be elastic. Clay, and putty, and 
butter are not elastic. Why ? 

Exp. 4!7, "Pull india-rubber, woollen cloth, or flannel. 
What is the result?" These substances stretch or become 
longer. " Let go with one hand. What follows ?" They go 
hack again to the length they had before being stretched. Try a 
band of india-rubber by actual measurement. 
. Why do we say that india-rubber, woollen cloth, flannel 
and such like articles are elastic ? 

Exp. 48. Take a flat ruler, a cane, or a piece of whale- 
bone. Bend them and . then let go with one hand. What 
is the result ? They spring back again. These bodies also 
are elastic. Why do we say so ? 

Call the attention of the scholars to the three kinds of 
elasticity here illustrated, and give other examples of each 
kind. 

II. Some bodies are more eiastic than others. 

Exp. 49. Test an ordinary wooden penholder, a quill 
pen, a strip of glass, and a slate pencil. The quill pen can 
be bent almost double before it breaks, the glass bends a 
little then snaps, and so of the wood ; the slate pencil bends 
scarcely at all before it breaks. 

In the same way compare other substances, such as — cork 
with sponge, leather with flannel, a book cover with a sheet 
of paper, and so on. 

III. Uses we make of elastic substances. 

1. Cork for stopping bottles. Show how the cork is com- 
pressed on passing through the narrower part of the neck of 



FIRST STAGE. 



23 



the bottle, and how it opens out and fills the larger part of 
the neck. 

2. Itidia-ri^bher for bands, ^' eiastw," &c. [Cold has a 
curious effect on india-rubber : it makes the rubber non- 
elastic. Advantage is taken of this in the manufacture of 
" elastic." India-rubber threads are stretched, wound on 
rollers, and kept in the cold for a few days. They are then 
woven with the woollen, cotton, or silk threads into bands. 
The bands are passed over a hot roller and the rubber 
becomes elastic again.] 

3. Sponge for wasliing purposes. "We are able to squeeze 
out the dirty water. The sponge expands again and is ready 
to take up more water. 



LESSON XIII. 

PLASTIC. 

Articles for iUustration : well-kneaded clay, and one or two moulds. 



I. Meaning of plastic. 

Exp. 50. Take a lump of clay [previously well 
kneaded], and having sprinkled over it a little fine sand 
press it into a mug, or cup, 
or '* mould" of any form. 
Press well in in order that 
the clay may take the exact 
form of the inside of the 
vessel in which placed. 
Break the mould, or if of 
shape to allow it, turn out 
iihe "cast." [Plaster of 
Paris may be used in the place of clay.] 

We have made the clay into a certain shape, the shape ol 




MUG, 



Fig. 3. 



CLAr. 



24 OBJECT LESSONS. 

the vessel into whicli it was pressed. "We call the vessel a 
mould, and because the clay can be formed or moulded, we 
say it is plastic. Plastic yneans capable of being moulded, or 
formed into shape. 

Clay can be moulded into shape by the hand. Show this. 

II. Things made in moulds from clay. 

Exp. 51. Bricks. Show how bricks are made. A 
common slate pencil box with the bottom removed will serve 
as a mould. 

Drain-pipes, tiles, &c., made in moulds from clay. 

Explain that these things are baked to make them hard. 
Contrast the bricks before and after baking, with regard to 
properties. Before baking, soft, plastic, and non-porous; after 
baking, hard, brittle, oxidi porous. 

III. Things made from clay by the hand— earthenware and 
china. 

Make a rough tea-cup to show the process. Stick on a 
handle. 

Describe the manufacture of earthenware. A fine kind of 
clay, and burnt flints ground to powder, are well mixed. 
A tough paste is thus made, and from this the articles are 
formed. The articles are now baked in an oven. Next they 
are dipped in a mixture and baked again. The second 
baking produces the glaze, and renders the articles non- 
porous. Colours are next put on, and the articles again 
baked. 

Ornaments, vases, figures, &c., are made of potters' clay 
mixed with fine sand, and then baked. These are called 
terra cotta, which means baked earth. 

When warmed, gutta-percha is plastic; ornaments, solea 
for lxK)ta, &c., are made from it. 



HUST STAGE. 26 

I-ESSON XIV. 
FUSIBLE. 
Articles for illustration : lead, tin, cast articles, salt, and Buirar 



I. Msaningf of fusion. 

Exp. 52. Melt lead in an iron spoon. Pour it out. 

•'* It flows in drops. In what state is it ? " In a liquid state, 

** In what state was it before melting ? " In the nolid 
state. 

" Then what change have we brought about by heating ? *' 
We have changed the lead from the solid to the liquid state. 

Things which can be changed from the solid to the 
liquid state by heating are said to he fusible. 

Ice is fusible. It does not change to water on a very cold 
day, except we bring it into a warm room, or hold it in the 
warm hand. It does not require much heat to melt or fuse it. 

XL Common substances which are fusible.* 

1. The metals. Some require very great heat. Melt a 
little tin ; the iron spoon does not melt. Why ? Show 
articles made of cast or fused iron — nails, hinges, grates, &c. 
Show any articles cast from bronzefcor bell-metal. 

2. Salt is fusible at a great heat. Makes a glaze for 
some kinds of drain-pipes. 

3. Sugar. Melt a little in the evaporating dish, and 
compare the fused sugar with the original. 

4. A mixture of flints and soda is fusible ; and when fused 
it makes a beautiful transparent glass. 

* Of course, in strictness the term In used comparatively. Some substances 
are more fusible than others ; and it is oidy to the former, those which are 
evidently fusible, that the term i-i here applied. With sufficierib heat all 
solids are fusible. But those which are combustible " take tire " before 
mnrh^ng the point of fusion, unless totally excluded from the ftcctss of 
oxygen. 



26 OBJECT LESSONS. 



LESSON XV. 

ON SOME FURTHER PROPERTIES OF THE COMMON 
METALS. 

Articles for illustration: any specimens of malleable and ductile 
meluis, various wires, a thin rod of glass. 



I. Malleable. 

Ei'p. 53. Take a piece of lead, place it on a block, and 
hammer it. What is the result ? It is flattened or spread 
out. 

A bit of copper wire may be treated in the same way. 

Refer to the blacksmith heating iron to a white heat, and 
then hammering it into various shapes. 

Most of the metals can be hammered out without break- 
ing ; but gold and silver can be hammered out into thin 
leaves finer than the finest tissue paper. Show gold and 
silver "leaf" and tin "foil," and plates of any other metals. 

Substances which spread out without breaking when 
hammered might be said to be hammerahle. And that is 
just what " malleable " means. It is formed from the Latin 
word {malleus) for a hammer. But there is a kind of ham- 
mer made of wood, used by carpenters, called a mallet (little 
hammer), and gold and silver are beaten out with wooden 
hammers or mallets ; hence we say malleabley and not hani- 
merable, but the two words have the same meaning. 

The teacher should now show the many uses to which 
these metals are put because of this property of malleability. 

II. Ductile. 

JSxp. 54. Take a thin glass rod. Hold it in a lamp or 
gas-flame until it is soltened, then gently draw it out into a 
thin thread. Tell the children then the word ductile is used 
for ^an be drawn out. Solid glass is not ductile. Glass 



FIRST STAGE. 27 

softened by heat is very ductile. Most of tlie metals are 
ductile, some mucli more so than others. Gold and silver 
wire can be made as fine as the finest thread. Show steel, 
copper, lead, and zinc wires. 

III. Tenacious. 

Exp. 55. Direct some of 6he children to test the strength of 
some of the specimen wires by trying to break them. Glass 
wire snaps very easily^ It can scarcely hold together. Lead 
and zinc wire break more easily than copper, and so on. 

We say that lead holds together more firmly, or is more 
tenacious^ than glass ; and copper is more tenacious than lead. 
The tiny particles in some metals hold together more firmly 
than in others. Why cannot we make so fine a wire or so 
thin a leaf of lead as we can of gold or copper ? The tiny 
particles of lead do not hold together so firmly as those of gold 
or silver. Lead is not so tenacious. 

Direct the attention of the children to some of the more 
common uses of wire.* 



Note. — Lessons on any of the more common solids may 
now be introduced. In a great measure they should serve 
as the medium for a recapitulation of the ideas developed in 
the previous lessons. The following lessons on lead and 
sulphur are given as examples. 



LESSON XVI. 
LEAD. 

Articles for illustration : lead in as many forms as may conveniently 
be procured, as pipe, sheet, foil, and wire ; also galena. 



I. Its properties. 

(a) "I want Harry to come to the table, and take this 
• The coinmon uses of metals may form the subject for other lessons. 



28 OBJECT LESSONS, 

piece of pipe in his hand. Look at it. I think you can tell 
me of what it is made ? " It is made of load, 

" What can you say about its weight ? " It is vert/ heavy. 

(b) " Take this nail, and try to scratch it. Now take this 
knife and try to cut it. What else does this teach you about 
lead ? " It is a soft metal. 

(c) " John shall take this piece and hammer it on the 
block of wood. What happens?" It spreadii otit, or flattens. 

** What do we say of lead because we can hammer it 
out ? " It is malleable. 

** Here is a piece of lead ' foil.' What does that teach 
us ? '* That lead is malleable. 

(d) " Here is a piece of lead wire. What can we learn 
from this ? " That lead is ductile. 

(e) " Bend the wire. It bends easily. What do we learn 
from this ? *' That lead is very pliable. 

" It does not go back again to its former shape. What 
does this teach us ? " That lead is not elastic. 

(/) " Bend it backwards and forwards two or three times. 
What happens ? " It breaks. 

" What does this teach us ? " That cead wire is not very 
strong. It is not tenacious. 

{(j) " I melt this piece of lead in the iron spoon. What 
does this teach ? *' That lead is easily melted. 

" What difference do you see between the freshly melted 
lead and the piece of lead before it was melted ? *' The fresh 
piece is much brighter. It shines more. 

(h) " Lead pipes and sheet-lead plates on roofs of houses 
last for very many years. What does this teach us P " 
That it does not wear away quickly. 

" Yes, and when a thing does not wear away quickly we 
say it is durable.** 

" We have learnt a good many things about lead. Tell 
me what they are once again.** Lead is heavy ^ soft, malleable^ 
ductile^ pliable, fusible^ and durable. 



FIBST STAGE, 29 

II. Itsnses. 

We have now to find out how all these properties of 
lead make the metal useful for various purposes. 

*' Here is our piece of lead pipe. Now why is lead 
specially useful for making pipes ? " 

[The teacher should here trace the course of one of the 
gas-pipes in the room, and show how it has to be bent and 
turned. Cast-iron pipes would do for straight tubes, but 
they could not be bent. Wrought iron would be much 
dearer, and would not bend so easily as lead. Silver or 
copper would do for gas-pipes, but they are too dear. Silver 
would be better than lead for water-pipes, but would cost too 
much. Copper would not do, because it would rust, and the 
rust of copper is % poison.] 

Lead is very useful for making pipes, because — 

1. It is easily bent. 

2. It is soft enough to be cut with a knife or saw. 

3. It does not rust. 

4. It does not allow gas or water to escape. 

5. It is cheap and durable. 

For similar reasons it is useful as sheet-lead for covering 
the roofs of houses or floors, for lining wooden cisterns foi- 
holding water, and so on. 

III. Alloys of lead. 

[Mixtures of metals are called alloys.] 

Lead and zinc melted or fused together make a verv 
ductile alloy. 

Advantage is taken of this to make wire, which, besides 
being cheaper, is softer and more easily bent and twisted 
about than copper or iron wire. Useful in the garden for 
tying up trees and shrubs. Why better than twine ? 

When a little of another and very hard metal called arsenic 



30 OBJECT LESSONS. 

18 fused with lead, the alloy is harder than pure lead. This 
alloy is used for making shot, [Test the hardness of common 
shot by hammering.] 

When another metal, very much like arsenic, called 
anlimonyy is fused with lead, it produces an alloy — type- 
metal — used for making type for printing. [If possible, 
show specimens.] 

Solder is a mixture of lead and tin. [If possible, show its 
use.] 

Pewter is an alloy of tin and lead. 

IV. Whence obtained. 

Tell the children that we get lead from mines — not pure 
lead, but lead mixed up with other substances. 

[Show any ores of lead. The more common one, viz. 
galena^ from which lead is smelted, is very plentiful.] 

The lead is melted out, and run into moulds. 



LESSON XVIX. 
SULPHUR. 



Articles for illustration : roll sulphur, " flowers " of sulphur, an olive* 
oil flusk, and the spirit-lamp. 



I. Its properties. 

The children will discover the more obvious properties of 
sulphur under the guidance of the teacher. It is of a pale 
yelloio colour, hard, brittle, itijlammabley inaoluble in water, and 
heavier than water. 

JExp. 56. To show that sulphur is fusible. Put powdered 
roll sulphur, or flowers of sulphur, in an ordinary olive-oil 
flask ; heat gently over the spirit-lamp. The sulphur easily 



FIRST STAGE. 31 

changes into the liquid state, when it has the colour of 
amber. As the temperature rises it becomes darker in 
colour, and takes about the consistence of treacle. Pour into 
cold water, and the once hard and brittle yellow sulphur is 
now soft and tenacious, and much like india-rubber. 

Let the children compare the properties of this changed 
substance with those of roll sulphur. 

Exp. 57. Although svilphur is insoluble in water, it is 
soluble in alcohol, or spirits of wine, and some other liquids. 
The teacher will dissolve a little in alcohol, or in bisulphide 
of carbon. 

It readily takes fire, and from this property follow its 
chief uses. 

II. Its uses. 

1. Lucifer matches. 

The children will be interested in learning how their 
grandfathers and grandmothers managed to get a light 
before " matches " were invented. Describe the *' flint and 
steel" and the tinder-box. Show how sparks were ob- 
tained by striking a piece of steel. Use the back of a knife 
on the sharp edge of a broken flint. Next, show how the 
light was obtained by using strips of paper, or splinters of 
wood, the ends of which had been dipped in melted sulphur. 

Next followed the improved sulphur matches, and in 
many places they are still used. The splinters of wood 
were dipped in melted sulphur as before ; but in addition 
just the ends were dipped in a mixture which, when it became 
dry, ignited on being rubbed on a roug^h surface. This did 
away with the flint and steel, and the tinder-box. 

Now the best matches are made without sulphur. "Why ? 
We are glad to be rid of the sulphur because of its suf- 
focating smell. 

2. Gunpowder. 

This is an intimate mixture of about 15 parts, by weight, 



32 OBJECrt LESSONS. 

of Ditre^ 2 parts sulphur, and 3 of charcoal. These are well 
ground, then well mixed, and made into a paste with water. 
The paste is pressed into hard cakes, these are broken into 
grains, and then dried. 

Exp. 58. The teacher may make a little of the paste, 
not too soft. It burns with a hissing noise, and throws off 
showers of sparks. 

3. Sulphur is also used in bleaching straw and wool, &c. 
Erp. 59. Show this property of bleaching by holding a 

flower in the fumes of burning sulphur for a few seconds^ 
most of the colour disappears. 

4. Occasionally used as a medicine. The children may 
have heard of " brimstone and treacle." 




SECOI^D STAGE, 



SECOND STAGE. 

LESSON I. 

WATER.— ITS PROPERTIES. 

Articles for illustration : a " pop-gun " or substitute, a piece of gk 
tubing, a piece of lead or wooden pipe, and a bottle and cork. 



I. Water is a liquid. [See First Stage, Lesson I., page 3. J 

We may say water is a liquid because — 

1. It may he made to flow in drops. Show this by letting 
water drop from a sponge, or from a bottle. 

2. It cannot be grasped by the hand. Why ? Its particle 
do not hold together firmly enough. 

3. It cannot be made to form a heap. Try it on a slate or 
plate. It spreads out, and seeks to find the lowest place. 

4. It has no shape of its oivn. It takes the shape of the 
vessel in which it is placed. 

II. Water is clear, colourless, transparent, tasteless, and 
odourless. 

All these simple properties may be readily elicited from the 
children by directing them to use their senses of sight, taste, 
and smell. 

III. Water cannot be squeezed into a smaller space. 

£Jxp. 60. To show this the teacher will require a tube and 
piston of some kind. A child's "pop-gun" will answer 



■I I i'l 



36 OBJECT LESSONS. 

very well ; or take a straight quill, and a slice of raw potato, 
with a little stick for a piston. Plug one end of the quill by 
pushing through the potato. Nearly fill the quill with water, 
and then plug the other end by pressing the potato on the 
quill till the latter cuts through. The plug 
may be forced out or broken, but the column of 
water cannot be made shorter. A piece of strong 
glass tubing,* with pellets of "tow"t instead 
of potato, and a piston rod made of hard wood 
shaped as in the cut (Fig. 4) will form a better 
instrument, and will be useful in future lessons. 
IJxp. 61. Fill a bottle with water and try to 
force the cork in. Some of the water is squeezed 
out by the side of the cork as the cork is pressed in. 

IV. Water presses sideways as well as downwards. 

Children will appreciate tbe fact of the pressure down- 
wards by trying to lift a bucket full of water. 

&p. 62. To show the pressure sideways 
take a piece of tube — any kind will answer ; 
it may be cardboard, wood, or lead, provided 
we cm readily pierce the walls. Plug one 
end of the tube firmly ; make tiny holes in - 

positions as shown in Fig. 5, and plug with ,Jt 

wooden spikes. Fill with water, and remove /f^ 

the spikes. 

Direct the children to note carefully what 
follows. 

Three tiny streams spout out. Are they 

alike ? How do they differ ? The top stream jpig, 5 

does not run out with so much force. The 

bottom stream seems to be in a greater hurry ; it is pushed 

harder and so rushes out farther. 

* Country boys make their pop-guns by forcing out the pith from a straight 
stick of elder- wood about an inch in diameter, 
t Unravel a piece of old twine. 



s 



•J 



SECOND STAGE. 37 

Note also that as the water in the tube lessens in volume 
the force of the streams lessens. 

** What do we learn from this experiment ? " Two things. 
(1) That water presses sideways as well as downwarch. (2) 
That the deeper the water the greater the pressure. 

]>^OTE. — Pressure in all directions is dealt with in a future 
lesson. 



LESSON II. 

WATER— A SOLVENT. 



Articles for illustration : substances soluble in water, acetate of lead, 
liquid ammonia, tumbler, j ug, and water. 



I. Action of water on salt, sug'ar, alum, soda, &c. 

Exp. 63. Show the solvent power of water by dissolving a 
little salt, sugar, alum, and soda in separate glasses or test- 
tubes. 

How has the water changed the solid ? 

Can we see the salt or alum in the solution ? The water 
has broken up the salt, &c., into such tiny particles that we 
cannot see them. It has made the solids invisible. 

How do we know that the salt or sugar is everywhere in 
the water ? Taste the smallest drop. It is salt or sweet as 
we dissolve the one solid or the other. 

We should remember then that even the brightest and 
clearest liquid may contain some solid in solution although 
we cannot see it. Sometimes, indeed, we cannot discover 
the solid either by sight, taste, or smell. 

Exp. 64. " Here is a liquid which contains a solid." 
[Dissolve lead in white vinegar or acetic acid.] You can- 
not see it ; but it is there as I shall show you. 

Here is another liquid [liquid ammonia], but this contains 



38 OBJECT LESSONS. 

no solid. I pour a little of this into the first, and what do 
you see ? A white solid shows itself, which slowly sinks to 
the bottom ; the solid is painters' " white lead." 

Exp. 65. ** Here is another clear liquid [lime- water], of 
which you may drink. It is quite harmless ; but it contains 
a solid in solution, which you cannot discover by sight, taste, 
smell. A boy shall blow through it by means of this glass 
tube. It gets milky-looking. By-and-by a white dust 
will fall to the bottom. This is chalk." 

We can recover solids by evaporation. Show this by 
means of the evaporating dish. 

Bodies which can be broken up or dissolved by liquids are 
said to be soluble, and the liquid which dissolves the solid is 
called the solvent. Water is therefore a solvent for salt, 
soda, &c. 

IT. Action of soluble bodies on liquids which dissolve them. 

Exp. Q>Q. Fill a tumbler full of water to the brim. Care- 
fully pour it into an empty jug and add a couple of ounces 
of salt. A tumbler of water weighs about half a pound, so 
that the weight of the salt solution will be about ten ounces. 

Now pour the solution into the tumbler. It is again 
exactly full and no more. Half a teaspoon ful of water would 
have caused an overflow, but two ounces of salt have made no 
change in the size [volume] of the water. 

What can we learn from this experiment ? That when we 
dissolve salt in watery the salt does not increase the bulk of the 
water. 

This is true also of other solids which dissolve in water. 
They do not increase the volume of the water. We may 
conclude from this that water is porous^ although the pores 
are too small to be seen ; and that the tiny particles of the 
solid fill up the pores. 

If we add another ounce of salt to our solution we shall find 



SECOND STAGE. 39 

that the whole is not dissolved. A boj can eat only a certain 
quantity of bread at a time, and water can dissolve only a 
certain quantity of salt at a time, and no more. When the 
pores are full no more salt can be taken up. 

Time permitting, the teacher may here show the different 
solvent power of water on different solids, and how heat 
affects this solvent power. 

III. Uses of water dependent on its solvent power. 

Pure water is obtained by boiling, and then changing the 
steam back to water [distillation]. 

Rain water and snow water are nearly pure ; but river and 
spring water always hold solids in solution. 

Where do these waters get their solids from ? 

When rain fulls what becomes of it ? 

1. Part runs away in streams to the river and thence to 
the sea. 

2. Part *' dries up *' — evaporates into the air. 

3. Part soaks into the ground. 

That which flows into rivers dissolves certain substances 
as it moves along, rubbing against the soil and sand and 
stones. 

That which sinks into the ground dissolves a good deal 
more of rocks, &c., than river water. This water rushes out 
in springs. The best drinking water comes from springs. 

Spring water contains solids. Instance the '* fur " on the 
kettle. 

Let the children taste distilled water. They will find it 
not agreeable ; in fact, it will remind them of rain water, 
and they will learn that the best drinking water holds sub- 
stances in solution. This occasion may be taken to show what 
are the impurities that make water bad for drinking, and what 
is the difference between mineral water and dirty water. 

Refer to the fact that plants are dependent for much of 
their food on the solvent power of water. 



40 OBJECT LESSONS. 

LESSON ITT. 

WATER AS VAPOUR. DEW. 

Articles for illustration : freshly cut leaves, flower in water, lump of 
ice. 



I. Vapour, evaporation. 

Suppose we hang out a wet cloth to dry. The cloth dries ; 
but where does the water go to ? You sprinkle a little watei 
on the floor; it soon dries up. The roads may be watered; 
but they are soon dry again. What do we mean by dries up ? 
Where does the water go ? 

You say it is gone away ? I will tell you how it went away, 
and where it is gone. 

You will remember how the water split up the salt into 
such tiny particles that we could not see them, and how even 
the tiniest drop of water got its share of the salt. Well, 
very much in the same way the air splits up the water into 
very tine particles, too small to be seen, and then the water 
mixes with the air, as the salt mixed with the water. 

" I want you to remember that the water which is in the 
air, but which we cannot see, is called vapour ; and that the 
change of water to vapour is called evaporation** 

II. Other sources ot vapour. 

We see that the ground, and houses, and trees, and plants 
are all wet after a shower ; and we see them dry very soon 
after, and we know that much of the water has become vapour ; 
but there is water also going into the air in the form of vappur 
from leaves and from the bodies of animals. 

Exp, 67. Place a few freshly cut leaves under a dry 
tumbler. The inside soon becomes covered with moisture. 
Why? 

In the same way, if the naked arm be inserted in a jar, the 



SECOND STAGE. 41 

jar after a time will become covered with moisture, showing 
that, like the leaves, the skin gives off water. 

Water also is always coming from the lungs. Breathe on 
cold slate or glass. What is the result ? 

The children will now be prepared to answer such questions 
as the following : — 

*' Why do flowers soon droop and wither after they are 
cut ? " 

" Why do we put flowers in water when we want to keep 
them fresh and bright ? " 

" Why does the water in the vessel decrease in quantity ? '* 

" Why do we desire more to drink on a hot than on a cold 
day?" 

" What do we mean when we say that * ink dries ? ' " 

III. How to collect vapour from the atmosphere. 

3xp. 68. The teacher may show how to get vapour from 
the air, by bringing a glass of iced water into a warm room. 
The moisture soon covers the outside of the glass.* 

In a similar way vapour fromi the air is settled at night on 
the cold grass and leaves. This is called dew.f 

We often see moisture on the windows of a warm room, 
or on those of a closed carriage. Where does it come from ? 
What causes it to settle on the window ? 



LESSON IV. 

WATER AS FOG, MIST, CLOUD, RAIN, AND STEAM. 
Articles for ilhistration : apparatus for boihng water. 

I. What IS fog ? 

" What did we learn about vapour in our last lesson ? " 
It is in the air but we cannot see it. 

* The moisture is better seen perhaps on the outside of a silver vessel. 
t A full explanation of the formation of dew is given in the Fourth Stage, 
Lesson XIII., page 145. 



42 OBJECT LESSONS. 

" What do we say of things which cannot be seen ? " 
They are invisible, 

" Can you tell me why vapour is invisible ? " Yes, the 
little particles when floating about in the air are too small to he 
seen. 

" In this lesson I am going to show you how this vapour 
changes back to water, and in what/orms we see it in the air. 

" But first of all I must tell you that when vapour changes 
back again to water we say the vapour condenses. To 
condense means to press into a smaller space, and so to 
thicken. Every one has heard of 'condensed milk.* The 
substance of the milk — the nourishing part — is pressed into 
a smaller space, and therefore thickened. Then it is said to 
be condensed. Just so when vapour falls back into a liquid 
state its particles are pressed together into smaller space. 
It is * condensed.* 

" You know that vapour condenses, because you have seen 
the water on the cold tumbler and on the window panes ; 
but what causes the change ? 

" Breathe on this hot slate." No moisture. 

" Breathe on the cold slate.** We see the water. 

" Breathe on this cold glass.** We see the water. 

" Where did the water come from that is on the slate ? ** 
From the lungs. 

** Have you ever seen what looks something like smoke 
coming out of the mouth on a cold day ? ** Yes. 

** That is the moisture of the breath condensed by the cold 
air.** 

" It is the cold, then, which condenses vapour.'* 

And I must tell you now that cold air makes the tiny 
particles of vapour join together — in companies, as we may 
say — to make tiny drops of water large enough for us to be 
able to see a mass of them together, and not large enough to 
be seen singly. The cold air changes the vapour into what 
we may call water-dust. This water-dust is/og. 



SECOND STAGE. 43 

On a foggy morning you may see a little white snh^tance 
settled on the loose fibres of wool on your coat. Under a 
magnifying- glass we see that this is formed of rows of water- 
beads, so tiny that it would take fifty of them to make a 
drop the size of a pin's head. Then on spiders' webs you 
ma}' see water-beads a little larger, 

II. What are clouds? 

The teacher may lead the children to answer this question 
for themselves by some such simple narrative as the fol- 
lowing : — 

" The morning was misty, but there was every prospect of 
a fine day, so we — that is, my brother and I — made up 
our minds to climb to the top of one of the high mountains 
we had seen a few miles off as we entered the village on the 
previous evening. Immediately after breakfast we trudged 
ofi", and soon arrived at the foot of the hill. So far we had 
walked in a thick mist ; but, after climbing for about an 
hour, we walked, almost suddenly, out of the mist into the 
bright sunshine, and a glorious view burst upon us. Above 
were cloud-capped mountain-peaks ; around, on every side, 
the lesser hills were bathed in a flood of sunlight ; below, a 
great white sea of fog hid every house and tree. The moun- 
tain was steep, and we were glad now and then to take a 
rest, and to watch the fog as it slowly melted away. In 
another hour it had entirely disappeared, and we saw below 
us lesser hills and valleys, lakes, and streams, with villages 
dotted here and there, stretching for miles away. 

" When we had reached within half a mile or so of the top 
of the peak we were climbing we entered another fog, very 
much like the one we had left in the valley in the morning, 
only this was colder and wetter. It was not pleasant, for we 
could see but a yard or two before us. However, we 
struggled on to the very top and rested awhile, hoping the 
fog would disperse. But we waited in vain, and were obliged 



44 OBJECT LESSONS. 

to descoTirl without enjoying the splendid view we had 
promised oui'selves from the top. We soon got hack into 
the bright sunshine of the valley, but on looking behind us, 
there, on the top of the peak, resting like a nightcap, was the 
cloud through which we had passed.*' 

III. Mist and rain. 

" Where does the rain come from ? " It comes from the 
clouds. 

" How does the rain fall ? " It falls in drops. 

" Are the drops always of the same size ? '' No ; they are 
sometimes small, at other titnes large. 

" Do clouds always send down water ? " No. 

** How often can we see clouds ? " Almost ahcays. 

" But it only rains now and then. How is this ? I will 
tell you. When the clouds do not rain, the particles of water 
are all in small companies ; but when the clouds get colder, 
then the particles gather into larger companies, so as to form 
drops. These drops fall, and we say it rains. When the 
drops are small we say it is a misty rain, or a * Scotch mist.* ** 

IV. Steam. 

Exp. 69. Show, by boiling water in a kettle or ** Florence 
flask " * that the steam, as it issues from the spout or neck, 
is invisible. 

It is commonly said that we see the steam as it issues from 
the steam-engine or the kettle, but this is not strictly true. 
As the steam spreads out in the air it gets a little cooled, 
and a steam-fog is formed. The steam-fog is what we see. 
Bteam-fog is just like common fog, only it is hot. 

Steam-fogs soon change to vapour, and then are, of course, 
invisible. Common fogs, too, evaporate ; but more slowly 
than the steam-fog. 

* Flasks mudti of thin glass in whioh olive-oil is imported. Being thin, 
they are less liable to crack . 



SECOND STAGE. 45 

[Tlie fuller explanation of steam and its uses, and of 

the formation of rain, is given in the Fourth Stage, 
Lessons VIII. and XIV., pages 132 and 147.J 



LESSON V. 

WATER AS SNOV\/ AND ICE. 
Articles for illustration : water and a lump of ice. Snow if possible. 



I. Snow, ice. and water compared. 

Under the guidance of the teacher the children may first 
compare snow, ice, and water as to their chief properties. 
For example — 

1. Water cannot be grasped by the hand ; snow may be 
pressed into hard balls. 

2. Water and ice are alike clear and transparent. 

3. Ice is lighter than water, and therefore rests near the 
top of the water. 

II. Meaning of "frozen." 

" When ice is taken into a warm room, what change 
takes place in it ? " It melts. 

" And what is it when melted ? " Water. 

" What do we call the change of ice to water ? " Melting. 

" If water is put in a very, very cold place, what change 
takes place ? " The water changes to ice. 

"And you know, I think, what we say when water is 
changed to ice ? " We say it is frozen. 

"Yes, and when we say that any substance which we 
usually see as a liquid is changed by cold to a solid we 
say it is frozen. Thus milk or quicksilver may be frozen. 
But when melted lead becomes solid again we do not say it 
is frozen. It is solidified. But frozen and solidified mean 
the same thing — a change from the liquid to the solid'' state 



46 OBJECT LESSONS. 

III. "What are snow, hail, and ice ? 

" Ice, you know, is solid water, but what are hail and 
snow ? " 

" Rain falls in drops — large and small. Sometimes rain- 
drops have to pass through very cold air in coming down. 
What happens ? " The drops freeze. The water becomes solid^ 
and these little solid balls of ice we call hail. 

** But suppose the vapour as it condenses into fog and 
mist to become frozen and to fall, what then ? " We have 
a snow-storm. 

" There is much more to learn about snow, and hail, and 
ice ; but all I want you to remember now is that ice is solid 
water, that hail is solid rain, and that snow is solid fog and 
mist." 

IV. Uses of snow and ice. 

The teacher may lastly call the attention of the children 
to the uses of snow and ice. 

Although snow is so cold it is like a blanket in this, that 
it does not let warmth pass through it easily. Sheep are often 
buried in the snow, and are found to be much warmer than 
they would have been in the frosty air. Snow keeps the 
earth warm, and partly protects the plants from the frost. 

Ice covers the water. It shuts off the cold air and so 
keeps the water beneath warmer for the benefit of the 
animals and plants which live in it. 



LESSON VI 
MERCURY, OR QUICKSILVER 



A.RTICLES for illustration : mercury and tin-toil, and, if possible, scalea 
for weighing, vermilion, and cinnabar — an ore of mercury. 



I. Its properties. 
The more evident properties — such as its great weighty its 



SECOND STAGE. 47 

state as a liquid, its easy ditmihility into small drops, and its 
beautiful silvery lustre — may be elicited from the children. 

Exp. 70. If the teacher has scales at hand he will com- 
pare the weight of mercury with the weight of water. 
A small cup, or small bottle of water weighs, say, half an 
ounce ; the same volume of mercury will be found to weigh 
nearly fourteen half- ounces ; that is, mercury is nearly four- 
teen times as heavy as water. 

This metal is a liquid at ordinary temperatures, but in 
the very cold regions of the world it freezes in winter and 
becomes a solid metal like a bar of tin or lead. In this 
state it is malleable like most other metals. This the 
teacher can only tell the children, but he may show them 
that, like other liquds, it can be made to *'boil'* and 
change to invisible vapour. 

Exp. 71. If the experiment be conducted in a test-tube, 
the vapour will condense again in tiny silvery drops on the 
cool glass near the open end of the tube. 

II. Whence obtained. 

Always obtained from mines. Somotimes found as pure 
liquid mercury in little hollows in rocks ; more often as an 
ore. This ore consists of sulphur and mercury. The ore 
is roasted, the sulphur burns away, and the mercury becomes 
vapour. This vapour condenses in cool earthenware pipes 
as liquid mercury. 

II r. Us-s. 

I. For "silvering" looking-glasses. 

A piece of tin-foil, of the same size as the glnss fo be 
" silvered," is spread on a perfectly flat and smooth stone. 
Mercury is poured on the tin-foil and made to cover it. 
The glass plate is then caused to slide gently over, not quite 



48 OBJECT LESSONS. 

touching the tin -foil. The glass thus sweeps off a large 
proportion of the mercury and all the air, leaving but a thin 
film of the liquid metal. Next the glass is heavily weighted. 
In a short time the mercury and the tin-foil form a solid 
amalgam J which adheres to the plate. 

Exp. 72. The teacher can illustrate the formation of 
amalgams by working up a little tin-foil, such as is used for 
wrapping round tobacco, with mercury until the mixture 
has the consistency of putty. 

2. For the extraction of silver and gold from their crushed 
ores. 

The mercury forms an amalgam with these metals. To 
obtain the precious metals the mercury is driven off by 
heat. It is, however, collected to be used again for a 
similar purpose. 

3. Mercury is also used for the preparation of a very 
blight red-coloured powder called vermilion. Yermilion is 
used as a paint, and for colouring aealiny-u-ax. Other uses 
will appear in future lessons. 



LESSON VII. 

AIR-A SUBSTANCE. INVISIBLE, OCCUPIES SPACE, HAS 
WEIGHT. 

Articles for illustration : a tumbler and basin of water, test-tubes, 
and a little ammonia and hydrochloric acid. 



I. Air— a substance, invisible, occTipies space. 

Hitherto we have dealt with things which we can see ; now 
we propose to find out something about sonic bodies which 
we cannot see. 



SECOND STAGE. 49 

**T have here a turabler. Is it full or empty ?" 

" Empty you say. I think not : I shall show you that it 
is full of something.'' 

Exp. 73. See ! I turn it bottom upwards, and press it 
down into this basin of w^ater. " Does the water fill the 
glass ? A boy shall come to the table and press it further 
down. Can he make the water quite fill the glass." No. 

*' Then there must be something in the glass. I lean 
it on one side, and out something comes in a great bubble. 
What was it." Air. 

" Then what was there in the glass ? " Air. 

**"What do you say about the air because you cannot see 
it ? " We say it is invisible. 

It is difficult for little children to appreciate the fact that 
bodies do exist although invisible, and the teacher should 
therefore multiply instances. He may refer to salt and sugar 
in solution, and to vapour and steam. 

JSxp. 74. A pretty experiment may be shown as a further 
illustration. Hold an inverted test-tube for a few moments 
over the mouth of the bottle containing ammonid- water, and 
another over a bottle containing hydrochloric acid. The 
tubes become filled with the gases that rise from the bottles, 
but nothing can be seen. Place the mouth o*f the first over 
the mouth of the second, and then invert. A white cloud 
appears in the tubes, which gradually falls as a white flaky 
solid to the bottom of the lower test-tube. Or, fill a bladder 
with air. Prick it. The children may feel and hear the air 
rushing out, although they cannot see it. 

II. Air has weight. 

That air has weight is readily shown by weighing a flask 
from which the air has been taken, and then weighing it 
again when the air has been allowed to enter ; but as few 
teachers will be able to command the necessary apparatus, it 

E 



50 



OBJECT LESSONS. 



must suffice here to show by a diagram on the blackboard 
how the weight of air is ascertained (Fig. 6). 




Fig. 6. 

A box measuring a foot in each direction will hold about 
an ounce of air. 



LESSON VIII. 

AIR PRESSES IN ALL DIRECTIONS. 

Articles for illustration ; tumbler, water in large basin, piece of thin 
card, a boy's sucker. 

I. Pressure downwards. 

Exp. 75. Fill a tumbler with water, invert it and, raise 
nearly out of the water (Fig. 7). The water does not fall out 
of the tumbler. Why not ? There must be some pressure 
on the free surface of the water. And this can only be the 
air, for nothing else rests on the water. 



Exp. 76. Repeat the same experiment, using a wide 
tube, securely corked, or covered with a piece of bladder at 
one end. Remove the cork or prick the bladder ; the water 



SECOND STAGE. 



51 



falls. Why ? If tlie tube is not too wide, the thumb placed 
over one end will answer equally well. 

The air presses downwards on the free surface of the 





water with sufficient force to hold the water up in the 
tumbler or tube. 

II. Pressure upwards. 

Exp. 77. Fill a tumbler, wineglass, or wide test-tube 
to the brim with water. Press over its open end a stiff piece 
of paper or a card. Hold the card in its place and invert 
the glass ; the water will not run out (Fig. 8). Why ? It 
is because the air presses upwards on the paper, and keeps 
the water in. 

III. Pressure sideways and in all directions. 

Eccp. 78. Cut a circle of about four inches in diameter 
from a piece of moderately thick leather. Soak till it is soft 
and flexible. Tie a knot at the end of a piece of string, and 
pass the other end through a small hole cut in the centre of 
the leather. Dip the leather in water, and then press it down 
on to a piece of slate or smooth stone. We can lift the slate 
or stone by means of the sucker, and the sucker adheres equally 
well, no matter in what position the slate or stone is placed. 

]^ow, why do we press down the sucker, and how is it that 



52 



OBJECT LESSONS. 



the leather holds en so firmly ? Leather is not adhesive, 
neither will water stick the leather to a stone. 

We press the leather to squeeze out all the 
air. We then raise the leather a little by 
pulling the string, and the pressure of the air 
on the leather on one side, and the slate or 
stone on the other, hold the two firmly to- 
gether. It is just as though I held the leather 
with my left hand and the stone with the right, 
and pressed the two together. 

You will learn more in a future lesson about 
the pressure of air. What I want you to re- 
member now is that air presses in all directions, 
downwards, upwards, and sideways. 

The teacher may refer to the manner in which 
limpets fasten themselves on the rocks when 
the tide recedes ; and to the suckers on the feet of flies 
enabling them to walk on the ceiling body downwards. 




Fig. 9. 



LESSON IX. 
AIR IS ELASTIC. 



Articles for ilhiptrntion : sponge, pop-gim, or cylinder and air-tight 
piston, bladder, or any air-tight bag. 



The teacher should introduce the subject of this lesson by 
referring to the elasticity of solids. Sponge may be taken as 
an example, because the elasticity of air is somewhat similar 
to the elasticity of sponge. 

Like sponge, air can be pressed into a smaller space. It is 
eonq)ressibk. And, like sponge, when the pressure is removed 
it opens out again. That is, air is clastic. 

£:vp. 79. The first property is easily sho'.vu by means 
of the pop- gun. 



SECOND STAGE. 53 

How does tlie pop-gun work ? When the cork is placed 
in the end of the gun, the barrel is full of air. If the cork 
were not in, the air would all be pushed out by the rod. 
But the cork keeps the air in. As the rod is pressed in the 
air is pressed closer together, and occupies a smaller space. 
When the rod is pressed half way, the air occupies half the 
space it occupied before. Now when air is squeezed in this 
way, it tries to open a way out for itself, and as the rod is 
pressed in still farther, the air forces out the cork all at once, 
and so makes the popping sound. It is not the rod alone 
which forces out the cork, for it is not long enough to touch 
the cork. It is the air between the rod and the cork trying 
to expand to its former size which makes the latter fly out. 
In other words, it is due to the elasticity of the air. 

The teacher may explain the action of the pop- gun made 
out of a goose quill, as described in Lesson I., p. 36. 

The different action of water and air in the ,^ 
pop-gun, should also be shown. 

Exp. 80. A more perfect apparatus for showing 
the elasticity of the air is an air-tight brass tube 
with a closely fitting piston, as shown in Fig. 10. 

When the piston is pressed down considerable 
resistance is felt, and if it is pressed down quickly 
and then released, it springs back again. 

The teacher may further illustrate the elasticity 
of air by means of any suitable articles within 
reach, such as a bladder, or an air-tight india- J^ 10. 
rubber cushion filled with air. The bladder or 
bag may be pressed at will ; bufc the air within always 
forces it into its original shape when the pressure is with- 
drawn. 



54 OBJECT LESSONS. 

LESSON X. 

GAS. 

Articles for inustratlon : sulphuric acid, a little Bulphide of iron, 
bottles, anil water. 

I. What is a gas? 

The teacher should call on the children to reproduce the 
ideas already acquired about solids and liquids. 

A solid is a substance which retains its form and size, 
unless acted on with more or less force. 

A liquid is a substance which keeps its own size, but takes 
up the shape of the vessel in which placed, and spreads 
itself out so as always to have a level surface. 

We have now to consider some other substances which are 
neither solids nor liquids. We call them gases. Most of 
you know one gas — that which we burn to give us light. 
Air is one of these gases. What have we already learnt 
about air ? It has weight, but is very light. It takes up 
room for itself, viz. occupies space. It is compressible and 
elastic. 

All gases have weight, [but as we shall learn by-and-by, 
some are heavier than others,] and they all occupy space. 
Like air, too, they are all very compressible and very 
elastic. 

There is one more fact to learn about gases, including air. 
They are always trying to spread out more and more, so that 
very little of a gas will fill a large space. We can half-fill 
a bottle with a liquid, but we cannot half-fill a bottle with a 
gas. The gas will spread out and fill the bottle. 

A rough idea of the constant tendency of gases to expand 
may be shown by allowing a little coal gas to escape. It 
soon fills the room, as may be detected by its unpleasant 
smeU. 



SECOND STAGE. 



55 



Exp. 81. A better method is to fill a bottle* with some 
strong-smelling gas, such as sulphuretted hydrogen,t and 
allow it to escape into the room. It will soon be discovered 
in every part of the room. 

We can now answer the question, " What is a gas ? " 
Gas is a substance which (unless confined) retains neither 
form nor size, and tvhich has no surface. 

II. How liquids and gases are alike. 

Show how Water may be made to flow in a stream. 

Uxp. 82. Then make a little carbonic acid gas by pour- 
ing very dilute sulphuric, or hydrochloric acid, on a few 
pieces of chalk, and show how this can be made to flow into 
another tumbler. We cannot see the gas flow out of one 
vessel into the other, but we can see its effect when poured 

* As in future lessons we shall often have to fill bottles with gases, it will 
be well to show the method here. (1) When the gas is not soluble in water, 
take any vessel, such as a wooden bucket, and fix a f^helf across it two or 
three inches from the top. Cut a hole in the shelf. Nearly fill the vessel 
with water. This forms a " pneumatic trough.'* Fill the bottle with water 




Fig. 11. 



Fig. 12. 



in the trough, and place it mouth downward on the shelf over the hole. The 
gas from the generating bottle passes along a tube, and bubbles up into the 
bottle through the hole in the shelf. (See Figs. 11 and 12.) (2) When the 
gas is soluble in water it must be collected by displacement of air. (See 
Fig. 67.) If the gas is lighter than air the jar must be inverted and the gas 
poured upwards. 

t Sulphuretted hydrogen is formed in abundance when diluted sulphuric 
acid is poured on sulphide of iron— both inexpensive substances. For pre- 
paration (see Fig. 67). Heat is not required. 



56 OBJECT LESSONS. 

on a lighted taper. The flame is extinguished. Both liquids 
and gases may be made to flow ; hence they are called fluids. 
The word fluid means flowing. 

The teacher may conclude this lesson by showing how 
extremely useful in nature is the constant expansion of gases. 
Stagnant pools, heaps of refuse, decaying vegetable and 
animal matter, give of gases which to breathe in quantity 
would cause illness, and perhaps death. But they soon 
expand, and are lost to the senses in the vast atmosphere 
around and above us. 



LESSON XI. 

COAL-GAS. 

Articles for ilhistration : long clay pipe, small coal, clay, soda-watei 
bottle, wire gauze. 

L Its properties. 

Attach a piece of india-rubber tubing to a gas-burner and 
collect the gas in a bottle, as described in the foot-note, 
page 55. 

Like air, coal-gas is invisible; but, unlike air, it has an 
unpleasant smell, and burns with a bright flanw. 

11. Its manufacture. 

Exp. 83. Take a clay tobacco-pipe with large bowl and 
long stem. Fill the bowl nearly to the brim with crushed 
coal, and stop firmly with well-kneaded clay. Put the 
charged bowl into a clear fire, and direct the children to 
take note of the result. First steam pours out of the stem. 
This is from the moisture in the coal. When this has 
ceased, a stream of coal-gas follows, which may be ignited. 
It burns like a candle. 



SECOND STAGE. 57 

At the present stage it will be sufficient to let the children 
understand that coal-gas is manufactured on a large scale in 
a manner similar to that we have employed in making a 
tiny quantity. Great iron vessels, called retorts^ are used 
instead of the bowl of the pipe, and long iron tubes instead 
of the stem. 

III. Coal-gas in mines. Fire-damp. 

Refer to what sometimes happens when coal-gas escapes 
into a room. It mixes with the air, some one brings in a 
light, and there is a sudden explosion ; the windows are 
blown out, or the walls thrown down, and perhaps people 
are injured. 

Exp. 84. Show slight explosion of a mixture of coal-gas 
and air in a soda-water bottle. Fill the bottle with water, 
admit air to fill about two-thirds of bottle, and then fill up 
with coal-gas [see note, page 55]. Apply a lighted taper 
to the mouth of the bottle. 

" What happens in mines ? " 

" Firstly, the owners of a coal-mine get all the fresh air 
they can into the mine, or the men could not breathe, and 
would die. 

" Secondly, gas often escapes in large quantities from the 
coal in the mine without any heat. 

" Thirdly, it is dark, and the men must have lights to see 
to work." 

" Here then we have everything wanted for a dreadful 
explosion. Fortunately when a light is placed in a lamp 
made of wire gauze it will not set fire to the mixture. But 
then sometimes the men are careless, and explosions happen, 
and many of the poor miners are killed." 

Exp. Sb. If a piece of wire gauze be held in a jet of 
gas, an inch or two above the " burner," the gas may be 
burned above the gauze without igniting the gas below. 



58 



OBJECT LESSONS. 



IV. TTsefal lessons to be learnt. 

1. Never sleep in a room into which coal-gas is escaping. 
It may poison you. 

2. Whenever there is an escape of gas open the doors 
and windows. Gas expands, and soon mixes with the air 
outside. 

3. Never take a light to see where the leakage is ; the 
mixture of gas and air might explode and kill you. 



LESSON XII. 

TAR. 

Articles fcft- illustration : tar, spirits of wine or ether, naphtha, and if 
convenient, carbolic acid, and any aniline colours. 



I. Whence obtained. 

Tar is one of the products formed during the manufacture 
of gas from coal. It comes over from 
the retort with the gas, and is col- 
lected in water through which the gas 
is made to pass. 

It may be made from wood in the 
same way. That is, the wood may be 
enclosed in an iron retort and heated 
just as we heat coal. In the one case 
charcoal is left behind, in the other 
coke. 

The teacher should show these sub- 
stances. 

Planks used in ship-building are 
covered with wood-tar made from 
logs of pine-wood. 
Wood -tar is obtained as follows: — a hollow or kiln is 




Fig. 13. 



SECOND STAGE. 59 

made of sugar-loaf shape (see Fig. 13) in the side of a hill, 
having a small opening at the bottom which leads to a tank. 
The kiln is filled with logs of pine- wood and covered with 
turf, a hole being left in the top, where the fire is kindled. 
The wood smoulders, and becomes charred from the top 
downwards, while the tar oozes out at the bottom. 

II. Properties. 

Many of the properties of tar* may be elicited from the 
children by leading questions. 

Wood-tar is of a blaclmh-hroion colour. 

Coal-tar is hlach, in each case a viscous fluid — viz. a fluid 
which is thick and sluggish — having the consistency of liquid 
glue, or treacle. Unpleasant smell, and bitter hnr}iing taste. 
Burns freely, giving off volumes of heavy smoke. A little 
heavier than water, and therefore sinks to the bottom when 
poured into water. Does not mix with water ; and soap and 
water will not remove it from the fingers. 

Dissolves partly in alcohol, and partly in ether and qyirits of 
turpentine, and mixes freely with oils and fats. 

To cleanse the hands of tar, rub with turpentine, or with a 
little oil or fat, and then wash with soap and water. 

The most important property of tar is its power of pre- 
venting decay. Salt and sugar are used to preserve meat ; 
but tar is a more powerful preservative than either. Tar is not 
used, however, for this purpose because of its unpleasant taste. 

III. Its uses. 

1. For preserving wood. Timber is sometimes steeped 
in tar. Sometimes the tar is spread over woodwork like 
paint. 

2. Napjhtha, useful for dissolving india-rubber ; carbolic 

acid, used for disinfecting purposes and in the making of 

* Tar is a mixture of many bodies. As these vary with the source whence 
the tar was derived, and the amount of heat used in distillation, its properties 
rary somewhat. 



60 OBJECT LESSONS. 

carbolic soap ; aniline colours^ used for dyeing calico, and 
cloth, and other important substances are got from tar. 

How far the teacher will pursue this subject must depend 
on the capacity and intelligence of the scholars. 



LESSON XIII. 
CARBONIC ACID. 



Articles for illustration : bottles, tumblers, chalk or marble, and 
hydrochloric acid. 

I. Its properties. 

To show its properties collect one or two bottles of the gas. 
(See Fig. 12.) 

Exp. 86. [Carbonic acid gas is readily obtained by pouring 
dilute hydrochloric acid on lumps of marble or chalk.] 

The gas is invisible and without smell. It is also a heavy gas ; 
it can be poured from one vessel to another. (See experiment 
Lesson X.) 

Its most striking property can be shown by plunging a 
lighted taper into a jar or bottle of the gas. The flame is at 
once extinguished. Or a stream from a jet may be made to 
play on a burning match. The flame is extinguished. Not 
only does carbonic acid gas not burn itself, but it prevents 
other bodies from burning. 

[A portable fire extinguisher has been constructed which 
makes carbonic acid gas, and pours it out through a tube.] 

We breathe out carbonic acid gas from the lungs. 

Exp. 87. Send a stream of carbonic acid gas into 
lime-water. The children will see that the gas makes the 
water look milky ; they may be told that the water held 
lime in solution, and that the gas united with it and 
formed chalk, which is insoluble. Hence the white colour. 



SECOND STAGE. 



61 



Now call on one of the scholars to hreathe out through a 
solution of lime-water and note a result similar to the above. 
What conclusion can the children draw from the experiment? 
We breathe out carbonic acid gas. 

It took a much longer time to make the water milky by 
breathing through it than by sending the gas from the 
bottle through it. Why ? Because tlie quantity we breathe 
out is verii small indeed. 



Exp. 88. A saucer of lime-water left to stand for a 
few hours will become milky-looking on the surface. Why? 
There is always a small amount of carbonic acid gas present in 
the air. 



II. Carbonic acid produced in burning. 

We have seen in previous lessons that we breathe out 
vapour of water from the lungs, and that water is produced 
by flame. In this lesson we have learnt that we breathe out 
carbonic acid gas, and now you 
have to learn that flame also pro- 
duces carbonic acid gas. 

Exp. 89. Take a bottle with 
wide mouth — a large ** pickle- 
bottle " will answer very well — 
invert it over a lighted candle 
(Fig. 14). When the candle is 
extinguished place the bottle over 
a wine glass of lime water (Fig. 

15). In a few minutes we see the milky looking surface, 
showing that carbonic acid gas was present in the bottle. 
This gas was produced by the burning of the candle. 

III. Carbonic acid gas is poisonous. 

The teacher will refer (1) to the fact that chalk or limestone 
when heated in kilns gives oflP carbonic acid gas [draw the 




62 OBJECT LESSONS. 

outline of a lime-kiln on the blackboard], and that people 
who have gone into kilns, recently emptied, for the sake of 
the warmth, have gone to sleep, and never wakened again ; 
(2) to the choke-damp (carbonic acid gas) formed by the 
explosion of fire-damp (coal-gas), which probably kills more 
miners than the explosion itself. 

The amount of carbonic acid gas in the air is not suffi- 
cient in quantity to do us any harm ; but if we shut ourselves 
up in a close room, and breathe the same air over and over 
again, the amount of the gas is increased, and we become 
heavy and sleepy, and perhaps get a headache. 

IV. A useful lesson. 

It is not healthy to live in a close room. Allow the 
foul air to escape, and the pure air to come in, by opening 
windows or doors. If opened ever so little the air is kept 
purer and more healthy than when closed. 



LESSON XIV. 

PARAFFIN OIL 

Articles for ilUistration, benzoline, paraltin oil, paraffin canrlles, and, 
if possible, benzoline and paraftin lamps. 



I. Propertiesi 

The properties more easily discerned should be educed in 
the usual way. Paraffin oil is a colourless liquid, liyhter than 
water, having an unpleasant smell. 

The oil is not explosive as is often supposed, and as a liquid 
it does not burn. 

Exp. 90. Pour a little into a cup, and plunge into it a 
lighted taper. There is no explosion, and the oil does not 



SECOND STAGE. 63 

burn ; on the contrary, the taper will be extinguished, just 
as if it had been plunged into water. 

Whence, then, do we obtain the beautiful light given by 
paraffin lamps? It is from the invisible vapour or gas into 
which the paraffin liquid changes when heated. 

A mixture of the vapour from paraffin oil and air is 
explosive, just like a mixture of coal-gas and air ; and it is 
the paraffin gas which burns just as coal-gas burns. 

II. Whence obtained. 

The children will remember how coal-gas was obtained 
from coal. A crude, viz. impure, oil is obtained from coal 
just in the same way, only the coal is not heated so much. 

The poor kinds of coal are used for the purpose of extract- 
ing this oil. In America the oil is obtained from oil-wells. 
It is called petroleum, that is, rock-oil, and millions of 
gallons are brought to this country every year. 

The crude oil is separated into benzoline, paraffin oil, and 
solid white paraffin. 

III. Uses. 

Benzoline is used for sponge lamps. A lamp should be 
shown, and the teachers should point out the danger in 
carelessly using benzoline. It gives off vapour in hot 
weather, and may thus form an explosive mixture with the 
air. It burns readily also on the application of a flame. 
Hence it should never be handled by candle-light. Benzoline 
is useful in removing grease- spots from clothing. It dis- 
solves the grease. This should be illustrated. 

Paraffin oil is used for burning in lamps. A lamp should 
be shown. 

Paraffin, when purified, is a pure white solid. It is used 
for making candles. 



64 OBJECT LESSONS. 



LESSON XV. 

CANDLES. 

Articles for illustration : as many kinds of candles as can be obtained 
the stem of a tobacco-pipe. 



I. Kinds of candles. 

1. The rushlight. This is now seldom used, but it was 
the light by which our grandfathers and grandmothers had 
to read and sew. It was made of \hQ pith of rushes dipped 
in fat. 

2. The common dip. Dips are made like the rushlights ; 
only instead of the rush-pith the wicks are made of loosely 
twisted cotton threads. The wicks are dipped in the melted 
tallow two, three, four or more times, but allowed to cool 
between each dipping. The cooling allows the tallow to set, 
and in the next dipping more tallow adheres. 

3. Mould candles. What is a mould ? Look at the 
candle. What is the shape of the mould ? Where must the 
wick be placed. Why called mould-candles ? The wick is 
plaited. Why ? It saves snuffing. It causes the wick as 
it burns to curve slightly outwards, and the wick is com- 
pletely consumed. [See Fifth Stage, Lesson VIL, page 184.] 

Paraffin, wax, oil obtained from the head of a whale, and 
many other fats and oils are used in the manufacture of 
candles. 

II. How a candle burns. 

The teacher should have a candle burning in front of the 
class, and call children to note first the solid fat, then the 
cup at the toj) and what is in it. What liquefies the hard 
fat? Next they should note the liquid fat going up the 
wick. Place a twist of cotton in water to show how the 
water asceitds. 



SECOND STAGE. 65 

At the top of the wick the liquid is changed to a gas 
by the heat. Show this by inserting a small tube into the 
middle of the flame ; the gas pours out at the other end and 
may be ignited. Compare with the paraffin oil going up the 
wick, and with turpentine up a piece of cane. 

The teacher should draw the attention of the children to 
the fact that whether we use candles, or oil, or coal-gas to 
light our houses, we always burn^rts of one kind or another. 



LESSON XVI. 

SOAP-BUBBLES AND WHAT THEY TEACH. 

Articles lor illustration : tobacco-pipe, soap and hot water, a Uttle 
oil, water, and quicksilver. 



I. How made. 

"To-day our lesson is to be on soap-bubbles and what 
they teach us. I must first of all show you how to make 
soap-bubbles, and then I dare say you will try and make them 
for yourselves." 

Exp. 91. "You all know what this is?" A tobacco- 
pipe. " And this I dare say you can tell by its colour and 
smell ? " Soap. 

" Yes, and here I have water. I will heat a little in this 
test-tube over the spirit lamp." 

" Now I shall dissolve some of the soap in the warm water. 
Next I warm the pipe and put a drop or two of the soap 
mixture in the bottom of the bowl." 

"Now I blow gently. There it is. What do you see?" 
A ball. 

"Yes, and we call this ball a soap-bubble. Look, I shake 
it off". There it goes. In what direction is it going ? " 
Upwards. 

P 



66 OBJECT LESSONS, 

" Ah, where is it now ? " It has burst. 

** I will make another. There it goes, up again.*' 

** Watch it. In what direction is it going now ? " It is 
coming down. 

" We will find out something more about these pretty 
balls. I put a little water in the pipe and blow. I make 
bubbles, but they break at once in the bowl of the pipe. 
Why ? I will tell you. The particles of water alone cannot 
stick together firmly enough to make the ball. The soap 
sticks them together, so that the thin covering of the soap- 
bubble is just a very thin sheet of soap and water. ^^ 

** But what is there inside the ball ? Think for a moment. 
How did I make the soap bubble ? " By blowing through the 
stern of the pipe. 

" And what did I blow through the stem ? " Air. 

" Now breathe gently on your hands ? How does the air 
which comes from the lungs feel?" It feels warm. 

" Then what kind of air did I breathe into the bubble ? " 
Warm air. 

" Now you can tell of what the bubble is made ? What 
is the covering ? " A sheet of soap and water. 

" And what is there inside ? '* Warm air, 

II. What they teach. 

" And now we have to ask the soap-bubble why it first 
went up, and then came down again." 

"And first why does the bubble ascend?" 

" Here is a test-tube, what is there in it ? " Nothing. 

** Oh yes, there is something in it although you see nothing. 
What fills it?" Air. 

" Now I pour in a little oil. Where does the oil go ? " 
To the bottom oj the tube. 

*' And what has become of the air which was at the bottom 
where the oil now is ? I will tell you, the oil has pushed it 
up. 



SECOND STAGE. 67 

"Now I pour in a little water. Where do you see the 
water ? " At the bottom. 

" And where is the oil ? " Just above the water. 

" And how has the oil been raised higher up in the tube ? ** 
The icater has pushed it up. 

" Lastly, I pour in a little quicksilver. Where does that 
go ? " To the botto?n. 

" And what has the quicksilver done to the oil and the 
water ? " It has pushed them higher up in the tube. 

" Now I will shake the test-tube. What do you see ? " 
The quicksilver is at the bottom, and the oil and icater mixed up. 

" Wait a minute or two. Now what do you see ? " Tlie 
oil is rising to the top of the water. 

" From these experiments you see that the heavier bodies 
always jor^ss up the lighter ones and take their places. And 
this is what I want you particularly to remember. Neither 
light bodies nor heavy bodies ascend of themselves. If they go 
up they are always jt?ws^e<^ up.^* 

" On a cold day stand under an open window. What do 
you feel ? " The cold air coming down. 

" Yes, it comes down and takes the place of the warmer 
air of the room. But where does the warm air go ? It is 
pushed up to the top of the room, and squeezed out wherever 
there are openings.** 

" When the air of the room is warm open the door about 
an inch. Hold a lighted candle near the top ; the flame 
is blown outwards. Hold it near the bottom ; the flame 
is blown inwards. The cold air is coming in at the bottom 
and the warm air is going out at the top, and the cold 
air forces up the warmer air just as water forces up oil, or 
quicksilver forces up water. Hence we know that the warm 
air is lighter than the colder air." 

" We will now return to our soap-bubble. What kind of 
air had it inside ? " Warm air. 



68 OBJECT LESSONS. 

" And which is the lighter, warm or cold air?" Warm 
air. 

'* Then why did the bubble ascend ? " The cold air pushed 
up the warmer, and therefore lighter air in the huhhle. 

" Now we have to ask, Why did the soap bubble come 
down again ? When you put hot water out in the cold does 
it remain hot ? " iVb, it soon gets cold. 

" And the warm air soon gets cool and heavier, and the 
soap-and- water- covering of the bubble helps to make it a 
little heavier still, and down it comes. 

" I think you must have learnt in this lesson why bodies 
lighter than water ascend in water, and why bodies lighter 
than air ascend in air. You will learn in the next lesson 
about other bodies which are lighter than air, and to what 
use we put them because they are light enough to ascend 
in the air. That is, they are light enough to allow the air 
to push them up." 



LESSON XVII. 

BALLOONS. 

Articles for illustration : a small collodion balloon.* 



I. A model balloon. 

Exp. 92. To inflate a balloon and despatch it to the 
ceiling is an interesting experiment; but, unfortunately, it is 
somewhat difficult of execution. The difference between the 
weight of coal-gas and common air is not sufficient to carry 
a balloon of less diameter than 18 inches, but a small collodion 
balloon will ascend if filled with hydrogen gas.t 

• These can be piirchased at a shilling. Great care must he taken in 
dnfolding, as they tire extremely (lelicate. 
t To prepare hydrogen gas place a little of granulated zinc at the hottom of 



SECOND STAGE. 



69 



JI. How balloons ascend. 

The teacher can make clear to the children the exceeding 
buoyancy of hydrogen gas. in an ocean of air by comparing 
it with the buoyancy of cork in water. Cork is four times 
lighter than water, but hydrogen is fourteen and a half times 
lighter than air. Now cork from its lightness compared 
with water can be made to hold up or carry up weights 





Fig 16. 



Fig. 17. 



in the water [show this by experiment], and just in the same 
way balloons are able to carnj up tveights in the air. 

By comparing the weights of equal volumes of air and 
hydrogen, it is easy to. see that hydrogen has a great lifting 
power. We have to remember that so long as the balloon, 

a bottle (Fig. 16), cover it with water. Insert cork with tubes as in figure'. 
Pour in through the long tube a little sulphuric acid. Hydrogen comes off 
in quantity. Allow sufficient time for the bottle to be filled with hydrogen 
to the exclusion of air, and then tie the collodion balloon over the tapering 
end of the shorter glass tube with a thread of silk. When inflated tie the 
mouth quickly but firmly. If the gas is required free from moisture and acid 
it must be passed through water, and a tube oontaining either lumps of 
unslalced limq, or of calcic chloride. 



70 OBJECT LESSONS. 

the hydrogen it holds, and the weight it carries, weigh less 
than the volume of air displaced the balloon will ascend, 
because the lighter body is always pressed up and its place 
occupied by the heavier body — in this case the air. 
Let the capacity of the balloon be 100 cubic feet. 

100 cubic feet of air weighs, say . . 7| lbs. 
100 „ „ hydrogen „ . . i lb. 

If the weight of the balloon itself is 4 lbs., the carrying 
power will still amount to 3 lbs., or thereabouts. Of course 
the larger the balloon the greater the carrying power. 
Coal-gas is now always used because of its cheapness. Its 
density varies from one-third to two-thirds that of air; 
hence its carrying power is much less than that of hydrogen, 
and the balloon has to be made so much larger. 

The teacher may enlarge on the management and uses of 
balloons : how, when they arrive in rarer air, they are made 
to ascend still higher, and how the aerial voyager manages 
to get down again. 

*,* A balloon 18 in. in diameter, made of f^old-beater's skin, costs about 
three shillings ; but this can be used many times. 




THIKD STAGE. 



THIRD STAGE. 

Hitherto our lessons have dwelt almost entirely on 
facts evident to the senses, or made evident by simple 
experiment. We have now to offer simple explana- 
tions of these facts, leaving the more difficult points 
for consideration in the higher stages. We start with 
the hypothesis that all matter is made up of molecules. 



LESSON I. 
MOLECULES. 



Articles for illustration : mercury, small piece of chamois leather, test- 
tube, drop of olive oil, &c., according to experiments .selected. 



The aim of the teacher in this lesson will be to show how 
minute particles of matter may be divided and subdivided until 
we can just distinguish them by the naked eye, then only by 
the aid of a common lens, then with the help of a microscope ; 
and, secondly, to show that we can further subdivide, so that 
with the aid of the most powerful microscope we fail to detect 
them, and thus lead up to the still smaller particle — the 
molecule. 

(1.) Exp. 93. Squeeze a drop of mercury through 
chamois leather, and let che drops fall on a piece of black 
cloth. Thousands of the tiniest drops are made from one 
drop. They are easily seen shining like silver on the black 
ground. Take one drop, the size of a pin's head. Spread 



74 OBJECT LESSONS 

it over the clotli with the blade of a knife, as you would 
spread butter on bread. Thousands of tiny drops from this 
one drop may be seen with a common lens. 

Exp. 94. Heat a few drops of mercury in a test-tube. 
In a few minutes the mercury boils and begins to change to 
invisible vapour ; but as it comes in contact with the cold 
glass in the upper part of the tube the vapour condenses into 
hundreds of thousands of tiny globes ; some join together 
and become large enough to be seen by the naked eye, while 
others come into view only with the aid of the magnifying- 
glass. Here, then, we have mercury divided up, first of all 
into particles (in the shape of vapour), too small to be seen 
at all, and then condensing into what seems to the eye the 
tiniest of silver balls. 

(2.) Exp. 95. Take a large test-tube, nearly fill it with 
water, and add one drop of olive oil. Shake violently, and we 
have split the single drop into thousands upon thousands, 
many of which can be seen under a lens as silvery globes 
gradually rising towards the surface of the water. 

(3.) Water changes to vapour, the particles of which are 
absolutely invisible under the microscope. It condenses into 
drops (in fog) very small, but not too small to be seen under 
the magnifying-glass. 

(4.) Eitp. 96. When water is added to a solution of 
gum-mastic in " spirits of wine," the gum-mastic becomes 
visible as very fine whitish particles. If we add one drop of 
the solution to half a pint of water, stirring well when we 
add the drop, the water assumes a milky tinge. This milky 
tinge is given by the particles of gum-mastic, but they are 
too small to be seen even under the most powerful microscope. 

Now the best microscopes will show solid bodies so small 
that the hole made in a sheet of paper with the point of a 
needle will hold many thousands. The particles of gum- 



THIRD STAGE. 75 

mastic must be smaller still, for they cannot be seen 
at all. 

The teacher may refer also to solutions of solids in 
liquids ; the solids in solution are invisible. 

From the above or similar experiments the children will 
be led to see into what extremely minute particles we can 
subdivide* matter, and they must be told that for reasons of 
which they will learn more in future lessons, we suppose 
that all bodies are made up of minute ball- shaped particles — 
particles so minute that it would take millions of them to 
make the tiniest drop of water we can see. These small 
particles are called molecules, a word which means little 
masses. 



LESSON II. 

STATES OF MATTER. 

Articles for iUustration : lead, mercury, water. 



If we agree to suppose that all bodies are built up of 
molecules, we can easily explain many facts not otherwise 
capable of simple explanation. 

I. Cohesion. 

Take a lump of lead. It is not easy to break. Why ? 
The molecules are held firmly together. Are the mole- 
cules tied together in any way ? No, but there is some 
power or force which holds them together just as if they 
were tied. If not, what would happen ? The lead would 
break into fine dust. What is the force that holds the 
molecules together ? We know not. We only know it is 
there, and we name it the force of cohesion, because the word 
cohesion means holding together. 

Let us break the lump of lead, and then press the broken 
ends together. Do they unite and hold together again ? 



76 OBJECT LESSONS. 

No, How is ibis ? If you could see the broken ends undei 
the microscope you would learn the reason at once. The 
broken surfaces are rough, and the molecules are not brought 
near enough to each other to hold together. 

Exp. 97. Boys sometimes amuse themselves by cutting 
a piece oif from two bullets ; then, scraping the cut faces as 
smooth as possible, they press them together. The pieces 
hold together so well that it takes a pretty hard pull to get 
them apart. How is this ? We have brought some of the 
molecules near enough together to hold on to or attract each 
other. 

Sometimes when sheets of glass have been pressed together 
it has been found impossible to separate them without break- 
ing. 

Exp. 98. In the same way if we divide a piece of 
india-rubber, making a smooth cut with a sharp knife, we 
can press the cut faces together and make the pieces adhere. 

II. Solid, liquid, gas. 

"We can now better understand the difference between 
6olids, liquids, and gases. 

As we saw in the lump of lead, the molecules in solids are 
held firmly together. It is this, in fact, which makes them 
solids. 

Here is a drop of mercury. I just put my finger on it 
and it is broken into many smaller drops. The molecules 
of mercury are not held so firmly together as the molecules 
in lead. It is the same with water : you can break it into 
pieces with the slightest touch. But is there no attraction 
or drawing together in the molecules of liquids ? 

I squeeze a drop of mercury between two pieces of glass. 
It is flattened. I remove the pressure, and the drop becomes 
a globe again. 

I pour a little oil on water; you see the oil floating in drops 



THIRD STAGE. 77 

I throw water on the floor. The molecules corabine to 
form ball-shaped drops. 

If the molecules of mercury, oil, and water did not attract 
each other they would be spread out. 

The molecules of liquids do not attract each other strongly ^ 
only just enough to keep together in drops. 

In some solids the attraction between the molecules, viz. 
the force of cohesion — is stronger than it is in others : chalk 
is more easily broken than flint, and flint than steel. In the 
same way the force of cohesion is greater in mercury than in 
water. 

A drop of water is spoilt when pressed with the finger ; 
a drop of mercury is not spoilt by a touch, it is only broken 
into smaller drops. 

Why is it that a fine needle laid on the surface of water 
will float ? Because its weight is not sufiicient to overcome 
the cohesion of the molecules forming the uppermost film of 
water. 

As to gas. You will remember that I allowed a bottle of 
an unpleasant gas to escape and it soon filled the room. How 
was this ? The molecules of a gas have no liking for each 
other ; on the contrary, they try to get as far apart as possible. 
Hence you cannot have a bottle half empty and half filled 
with gas. The molecules of the gas spread apart and fill 
the bottle. 



LESSON III. 
ADHESION. 

Articles for illustration : bit of ^gold-leaf, mercury, water, and any 
adhesive substances. 



I. What is the force of adhesion ? 

In our last lesson we considered the attractive force between 
molecules of the same kind of matter. In this lesson we have 
to consider another kind of attractive force. 



78 OBJECT LESSONS. 

Place your finger on a piece of gold-leaf ; it sticks so firmly 
that you can neither shake nor pull it off. We say the gold- 
leaf adheres to the finger. 

Put your finger in water. It comes out wet. Some of the 
particles of water adhere to the finger. 

Plunge your finger into mercury. The finger is not 
wetted ; the mercury does not adhere. 

With paste, gum, and sealing-wax you can make pieces of 
paper adhere. With glue you can join pieces of wood as 
well as paper. With mortar and cement you can cause stones 
and bricks to adhere. 

This kind of force which causes water to adhere to the 
finger, gum to stick to paper, glue to wood, and mortar to 
bricks is calk d the force of adhedon. We may call it a sticking 
together force. 

II. Why is it more difficult to stick solids together than it is to 
stick liquids, or semi-liquids, to solids ? 

That it is so is seen if we remember how water adheres to 
the finger, and gum to paper. 

Break a stone, a piece of cast iron, or a piece of wood. Feel 
the broken surfaces. Under the microscope polished surfaces 
are seen to be rough and uneven, just as we see the broken 
surface of cast iron with the naked eye. 

Solids pressed together can only touch here and there. 
Liquids flow into all the little hollows and fill them up, and 
so adhere firmly. 

III. Why does water wet the finger, while mercury does not ? 

We have seen in the last lesson that the force of cohesion 
in liquids is not very great. When we put the finger in 
water there are the two forces of cohesion and adhesion at 
work. There is the attraction of the molecules of water and 
there is the attraction between the solid and the liquid. The 
latter is greater than the former. Some particles of water 



THIRD STAGE. 79 

leave tlie ofher particles to stick to the finger. In the case 
of mercury the contrary is the case. There is more attrac- 
tion betweep ihe molecules of mercury than there is adhesive 
power between the finger and the liquid. 

Note. — ihe strength of the adhesion between water and 
glass may be tested as follows : — 

Exp. 99. Suspend a plate of glass from one arm of a 
scale beam and exactly balance it. Place a dish of water 
under the plate of glass, so that the surface of the water and 
the under surfaee of the glass just come in contact. Several 
grains may now be added to the weights on the other side 
without destroying the balance. 

By a similar experiment mercury may be shown to have 
some slight attraction for the glass. 



LESSON IV. 
CAPILLARY ATTRACTION. 



Articles for iUustration : capillary tubes, water, mercury, alcohol, 
and any common porous bodies. 



The force of adhesion manifested between solids and 
liquids explains many of the interesting facts about porous 
and absorbent bodies which we have noted in former 
lessons. Before, however, proceeding to these explanations 
the teacher should repeat some of the former experiments, 
such as — 

1. The absorption of coloured water by salt, chalk, and 
sugar. 

2. The ascent of water or oil in a cotton wick. 

3. The ascent of turpentine or paraffin oil in a piece of 
cane. 

The children may now be directed to examine the surface 



80 



OBJECT LESSONS. 




of water ia a very narrow test-tube. The surface is level 
except near the circumference, where the water is curved 
upward, looking very much like the interior 
surface of a watch-glass (Fig. 18). 

Exj). 100. Next place a glass tube with a fine 
bore in a glass of water : the liquid rises in the 
tube above the surface of the water in the 
tumbler. And the finer the bore the higher 
the water rises in it. 

The attraction of adhesion between the glass 
and the water is best shown in hair-like tubes, that is, with 
tubes as fine as a hair ; hence the attraction is called capil- 
lary attraction. The word capillary means hair-like. 

The teacher should now caU attention to the ascent of 
water in sponge, sugar, &c. The pores of sponge, wood, 
sugar, salt, chalk, blotting-paper, linen, cotton, &c., all form 
minute, hair-like tubes, in which the water rises by capillary 
attraction. 

Further illustrations of capilkry attraction may be found 
in the water rising from a saucer through the hole in the 
bottom of the flower-pot, and thence through 
the stems and leaves of the plant ; and in the 
luxuriant vegetation of the river margin. 



^"-= 



Exp. 101. The teacher may lastly test the 
adhesive force between glass and alcohol, and 
between glass and mercury, in minute tubes. 
The alcohol does not rise so high as the water, j^jg. iq 

and the mercury does not rise at all ; on the 
contrary, it is depressed below the level in the glass, and its 
surface is of the same shape as the outside of a watch-glass 
(Fig. 19). Why is this? Simply because the force of 
adhesion between the glass and the mercury is less than the 
force of cohesion between the molecules of the liquid metal. 



THIRD STAGE. 81 

LESSON V. 

PROPERTIES OF SOLIDS EXPLAINED. 

Articles for illustration : yipecimens used in preceding lessons for 
illustrating the more common properties of bodies. 



By various experiments on common solids, such as break- 
ing, bending, pressing, scratchiug, hammering, and so on, 
the teacher will lead the children to see that the force and 
kind of cohesion ii the molecules is very various, and gives rise 
to the special properties of solids. 

I. Hardness. 

Iron can be broken only with difficulty, marble requires a 
sharp stroke with a hammer, chalk can be broken with the 
fingers ; evidently, then, the cohesive force must be very 
ditferent in these bodies. 

The teacher will call on the children to tell how we compare 
the hardness of solids [see Lesson X., page 18] and if the class 
is sufficiently advanced he may introduce the " scale of hard- 
ness " as given below.* In this table each substance marked 
with a lower number will scratch one marked with a higher 
number. Thus topaz (3) will scratch quartz (4), and quartz 
(4), in its turn, will scratch felspar (5). If we find a substance 
which will scratch felspar, but is able in its turn to be 

 Scale of Hakdness of Bodies : — 

1. Diamond 

2. Corundum 



o rp / cannot be scratched with a steel iSde. 

4. Quartz (flint) 

5. Felspar 

-' T7,f ; can be scratched with a steel file. 

7. ±luor-^par ( 

8. Calc-spar / 

,^* rp I [ can be scratched with the finger-nail, 





82 OBJECT LESSONS. 

scratched "by quartz, we say the hardness of the body is 
between 4 and 5. 

II. Flexible and brittle. 

Question as to these properties. Bend a strip of cork. It 
is clear that the molecules on the upper side must be pulled 
a little farther apart than usual, while those on the under 
side must be pressed closer together. Such bodies as can be 
bent without breaking are said to be flexible. In some bodies 
the molecules will not bear this change of position — they 
part asunder. Such bodies are said to be brittle, 

III. Elastic. 

The teacher should first question on the different ways of 
testing the elasticity of solids (see Lesson XII., page 21). 
In an elastic body the molecules will bear not only to be 
pressed or pulled from their usual position ; but, on the 
force being removed, will return to it again. 

There is a limit, however, to the elasticity of bodies, and 
if the force applied exceeds a certain limit the form of the 
solid may be permanently altered. Thus the elasticity of 
springs of carriages may be permanently injured by over- 
loading. 

IV. Malleable, ductile, and tenacious. 

Hold a thin tube of soft glass in the flame of the spirit- 
lamp- It does not melt, but it softens so that it can be flat- 
toned, or drawn out into fine threads. How is this ? We 
displace the molecules, making them take updiflerent positions 
without destroying their cohesion. So it is with certain metals : 
we may hammer them out into thin leaves, or draw them out 
into fine wires without destroying the force of cohesion in 
their molecules. Such metals are malleable or dactile. That 
a metal may be ductile, it is evident that its molecules must 
possess a strong cohesive force, or breakage would follow. 



THIRD STAGE. 83 

Metals which possess this strong cohesive force are said to be 
tenacious. 

A metal cannot be ductile without being tenacious. It 
may, however, be tenacious without being ductile. Platinum 
is the most ductile metal, gold is the most malleable. 



LESSON YI. 

FORCE OF GRAVITY— WEIGHT. 
Articles for illustration : a few heavy and light substances. 



I. Meaning of **up " and " down." 

" I throw a ball in the air over my head. In what direc- 
tion do you say it goes ? " Upwards. 

*' And in what direction does it return ? " Doivmcards. 

" A boy shall lift this piece of iron. If he looses his hold, 
where will the iron go ? '^ To the floor. 

** And supposing the floor were not there ? " To the 
ground. 

" What do you mean by the ground ? " The surface of 
the earth. 

" You all know the general shape of the earth ; what is 
it ? " Round like a ball, or an orange. 

" And I dare say you know that New Zealand is on the 
other side of the globe, just about opposite to us. 

'*Now if a teacher were giving the same lesson in a 
school in New Zealand that I am giving you at this moment, 
and he asked a boy to let fall a piece of iron from the table, 
in what direction would the iron fall ? " Dowmvards to the 
ground. 

" But has downwards the same direction in New Zealand 
that it has in England ? Let me help you to find out. 

" Here I diaw a circle on the blackboard to represent the 



84 



OBJECT LESSONS. 



O A 



earth (Fig. 20). Suppose a man in a balloon, in the position 
which I mark a, throws out a stone, where will it go ? " To 
the ground. 

** Now suppose a man to be up in a 
balloon on the opposite side of the earth, 
and he should let fall a stone, where 
will the stone go ? *' To the earth. 

** But will the stone travel in the 
same direction as the first one ? " NOy 
in the opposite direction. 

" If there were no solid earth in the 
way, where would the stones meet ? " 
[Look at the lines. ] At the point marked 
c. " And this point is the centre of the 
earth. Just in the same way, wherever 
a body is let fall it falls towards the 
centre of the earth, 
" What does dotvn mean, then ? " Toivards the centre of the 
earth. 

" And what does up mean ? " Away from, the centre of the 
earth. 




II. Gravity or weight. 

" Now we have to inquire, what is it that causes bodies, 
when let fall from a height, to go towards the centre of ihe 
earth ? The earth attracts or draics ever (/thing towards itself. 
How this is done we do not know. That it is done is proved 
every time we lift a weight, or throw a stone. 

" I ask a boy to hold this piece of iron in one hand, and 
this piece of cork of about the same size in the other. 
What has he to note about these substances ? '* Iron is 
heavier than the cork. 

"What makes the iron heavier than the cork? I will 
tell you. It is because the earth draws the iron more strongly 
than it draws the cork. It is, in fact, the force with which 



THIRD STAGE. 85 

the earth draws everything towards itself that makes weight. 
And we name the force the force of gravity, because the 
word gravity means heaviness^* 

III. Explanations. 

A knowledge of this force of gravity helps us to explain 
many facts in nature. 

1. Why is lead heavier than wood ? In other words, why 
does the earth attract a piece of lead more than it attracts a 
piece of wood of the same size ? 

The molecules in the lead are closer together than the 
molecules in the wood. There is more substance in the lead 
than in the wood. 

2. Why does stone sink in water, and why does wood float ? 
Because the earth attracts the stone with more force than it 
does the water, and the water with more force than the wood. 

3. Molecules of gas are always trying to get farther and 
farther apart. Why then do they not get apart altogether 
and fly away into space ? Because the earth attracts them, 
and when the force of gravity is equal to the repelleni 
force between the molecules the gas cannot get any farther 
away from the surface of the earth. 

Note. — The time of another lesson may be profitably spent 
in reviewing these various attractive forces. Other kinds of 
attraction are referred to in future lessons, and the attraction 
of gravitation is more fully explained. 



LESSON VII. 

THE SURFACE OF A LIQUID AT REST IS ALWAYS LEVEL, 

Articles for illustration: — various glass vessels, and, if possible, a 
"spirit-level." 

I. Molecules of water. 
We saw in the last lesson that the properties of solids depend 



86 OBJECT LESSONS. 

on variations in the force of cohesion between the molecules 
In liquids the force of cohesion is but slight, almost nothing, 
and it is to this fact that the peculiar properties of liquids are 
due. In fact, the difference between solids and liquids may 
be thus expressed, that in solids the force of cohesion is 
greater than that of gravity, while in liquids the force of 
gravity is very much greater than that of cohesion. 

We have, then, to imagine that water is made up of globes, 
80 exceedingly fine that it takes millions to form a single 
raindrop. Further, these molecules must be so smooth and 
round that they move about and amongst each other with the 
greatest possible ease. 

It may be asked why we think they are round and smooth. 
It is because if they were rough, or had corners or points, they 
could not move about among each other so easily as they do. 
We cannot roll blocks or nails about as we can shot ; and the 
smoother the shot the easier it is to move them about. 

If the molecules of water were large enough for us to see 
them, the surface of still water would look like a level layer 
of fine, clear, and colourless shot ; and whenever there was 
the least motion in the water we should see the tiny mole- 
cules rolling about amongst each other with perfect ease. 

XL Water always tries to find its leveL 

We seldom see still water. As it moves so easily, it is 
nearly always in motion. The wind raises it into waves and 
ripples. It runs in streams and rivers, and wherever water 
is in motion it is simply trying to return to a level. 

Uxp. 102. The teacher may illustrate this with a basin 
of water. The surface is level as the basin rests on the 
level table. Raise the basin at one end, the water runs a 
little to the lower side, and becomes level again. 

Exp. 103. Water is always at the same level in the tea- 
pot, or the garden watering-pot (Fig. 21, 1.) 

If the watering-pot be turned up as in Fig. 21, 2. the 



THIRD STAGE. 



87 



level is still kept, but when turned up a little more, as in 
Fig. 21, 3, the water in the spout in trying to get on a 
level with that in the pot runs out. 

The teacher may use various glass vessels, and by placing 




them in different positions show with sufficient accuracy for 
our present purpose that when the liquid is at rest the surface 
is level. 

He may also show an interesting application of this property 
of liquids in the common spirit-level. 



LESSON VIII. 

PRESSURE IN LIQUIDS. I. 

Articles for illustration : a few glass marbles, a bag of small shot, a 
lamp chimney, and a boy's sucker. 



I. Pressure downwards and sideways. 

" What is weight ? *' Pressure doicnwards. Pressure 
towards the centre of the earth. 

" I place a lump of lead on the table ; in what direction 
does it press ? " Downwards. 

" Is there any pressure sideways or upwards ? '' None. 

" Why ? " Because the attraction of cohesion among the 
'particles is stronger than their gravity, or attraction towards thi 
earth. 



88 OBJECT LESSONS. 

Exp. 104. "Here is the tube which we used in a 
former lesbon (see page 36). I set it upright and place this 
iron rod in it. Does the rod press on the sides ? " No. 

'' I fill the tube with water. Now if I make a hole in the 
side of the tube, what will happen ? " The water will run 
out 

" What does that show about the water ? " That water 
presses on the sides of the vessels which hold it. 

" Liquids then differ from solids in this, that they exert a 
pressure sideways as well as downwards. Solids press only 
downwards. I will endeavour to explain how this comes 
about. 

" Tell me what we learnt about the molecules of water in 
the last lesson." They are very minute. They are round and 
smooth. They move about amongst each other with perfect ease, 

Exp, 105. A little experiment will now help us to see how 
water presses sideways. 

Take three marbles, place them side by side in the shape of 
a triangle on the smooth table, or on a piece of glass. Now 
put a marble on the top ; what happens ? Those below are 
pressed out sideways. 

And this is just what happens with the fine " water-balls " 
only the water-balls are so much smoother than the marbles 
that they move much easier. . 

Exp. 106. The teacher may further illustrate by 
making a hole in a bag of small shot. The shot close near 
the hole will be pressed out by those above, and these in 
their turn will be pressed out by those above them, and so 
on. Just so it is with the delicate water-shot. They are 
pressing down, and so pressing sideways those below. And 
if an opening is made in the side of the vessel that holds the 
water, the molecules near the opening will roll out like the 
shot, only a great deal easier because so much smoother. 



rHIRD STAGE. 



89 



II. Pressure upwards and in every direction. 
(a) Thrust your hand into a vessel of water. You press 
some of it down, but not all of it. Some of it is pressed 
upwards, for you see it is higher in the vessel. 

(&) JExp. 107. Place a flat piece of cork on the surface of 
water and try to press it down. Or, take an air-ball and push 
it down into the water. You find it difficult to push the 
cork or the ball beneath the water. Why ? Because of an 
upward pressure. 

(c) Pour water into a coffee-pot. Some of it is pressed up 
the spout till the liquid in the pot and in the spout have the 
same level. 

(d) Exp. 108. Cover one end of a glass cylinder — a 
lamp chimney will answer the purpose — ^ith the leather of 
a boy's " sucker," and let the string pass upwards through 
the chimney. Hold the leather firmly against the base of 
the chimney, and lower carefully into a jar of water. Drop 
the string, and the leather is held in place 

by the upward pressure of the water (Fig. 
22). 

It should of course be noted that this up- 
ward pressure in liquids operates only up to 
the natural level of the surface and not 
beyond. At the surface there is no upward 
pressure. 

This upward pressure in water explains 
why solids weigh less in water than out of 
it. 

Water, then, presses downwards and sideways and upwards. 
It presses in all directions. 




Fig. 22. 



90 OBJECT LESSONS. 

LESSON IX. 

PRESSURE IN LIQUIDS. (I. 

Articles for illustration : chimney -glass and boy's sucker, hollow 
india-rubber ball, and bent glass tubes as in Fig. 23, glass vessel for 
holding water. 

I. Pressure increases as the depth increases. 

It is quite certain that the pressure downwards in a 
column of water must increase with the depth, for a tall 
column must be heavier than a shorter one. 

Exp. 109. The teacher may next show that the pres- 
sure nideways increases with the depth by the tube as 
described on page 36. 

The chimney-glass and boy's sucker (see the preceding 
Lesson) will serve to show that pressure upwards increases 
svith the depth. 

Press the chimney-glass a short distance into the water. 
Pour water into the chimney-glass and note how tall a 
column of water is required to overcome the upward pres- 
sure and set free the sucker. 

Remove the glass from the water, and again holding on 
the sucker, press down further into the wa4;er. Pour water 
into the chimney until the sucker is released, and compare 
this column of water with the previous one. It will be 
longer, and therefore heavier. Why must it be longer and 
heavier? Because it has to overcome a greater upward 
pressure. And it will be found that the deeper we press in 
the chimney- glass the greater will be the upward pressure 
on the leather sucker. 

Sailors sometimes amuse themselves by partially filling a 
bottle with water and then after corking it tightly, letting it 
down by means of a long string into deep water. The great 
pressure of the water forces in the cork, and the bottle 



THIRD STAGE. 



91 



comes up full of water. No matter in what direction the 
neck of the bottle may point when let down, the result is 
the same. This shows that the water presses with equal 
facility in all directions, and that, deep down, the pressure 
is much greater than near the surface. 

ir. At the same depth the pressure is equal in all directions. 
Exp. 110. Take two glass tubes bent as in Fig 23. 
Put mercury in the lower part of each tube, so as just to fill 
the short arms. Lower both tubes into a glass vessel of 




Fig. 23. 

water. The water will force the mercury up the longer arm 
of each tube, and if the mouths of the short arms are at the 
same level, the mercury in the long arms will stand at the 
same height, showing that the downward pressure and the 
side pressure are equal. 

III. Liquids transmit pressure. 

The teacher will first explain the meaning of the word 
transmit. 

Exp, 111. Then take a hollow india-rubber ball having 
a hole in it, and fill with water. Prick the ball with a 
needle in several places. Close the large hole with the 
thumb, and squeeze the ball. The water will issue forth in 
tiny jets in every direction, showing that the water trans- 
mits in every direction the pressure put upon it. 



92 OBJECT LESSONS. 

LESSON X. 

BUOYANCY OF LIQUIDS. 

A.RTICLBS for illustration : pieces of cork, sponge, wood, and metals, 
salt, an egg, mercury, and spirits of wine. 



I. Buoyancy of water. 

The teacher niay take a hollow india-rubber ball, a piece 
of cork, or a bladder filled with air, and direct some of the 
scholars to press the object down beneath the surface of 
water in a bucket or tub. 

Some effort is found necessary to press it into the water. 
The children can feel that there is an upv/ard pressure. 
This upward pressure is called the buoyancy ^ ov floating power 
of water. 

Bodies which swim about partly below the surface of the 
water are said to float. 

Some bodies, such as cork and dry sponge, sink very little 
into the water. The floating power of the water prevents 
their sinking far down. These are very light bodies. Other 
substances, such as wood and india-rubber, sink farther into 
the water. These are heavier bodies, and the floating power 
of the water — the pressure upwards — is unable to do more 
than just keep a small part above the surface. We say that 
wood and india-rubber are light bodies. 

Other bodies again, such as stone and iron, overcome the 
floating power entirely and sink to the bottom. Such we 
may call heavy bodies, but they vary very much in weight. 

The teacher may here refer to the special uses of light 
bodies, such as cork for life-buoys and lifeboats, wood for 
ship-building, and so on. 

II. Bnoyancy of other liquids. 

Exp. 112. Take a small cup of mercury and place a lump 



THIRD STAGE. 93 

of iron on it. The iron floats. Wliy ? Because mercury 
is heavier than iron, and has so much greater floating 
power than water. The teacher may direct the children to 
put cork, wood, copper, and any other articles on mercury. 
Lead, copper, tin, silver, and iron, all float on mercury. 
Gold sinks to the bottom. Why ? A fact the children 
should remember is that the heavier the liquid the greater 
the floating power. Therefore it is easier to swim in salt 
water than in fresh. And in the water of the Dead Sea, 
which is very salt, it is impossible to sink. 

Exp. 113. Here is another interesting experiment show- 
ing that the heavier liquid is the more buoyant. Place a 
new eg^ in fresh water, and it sinks to the bottom. The 
downward pressure of the Q^g caused by gravity is greater 
than the floating power of fresh water. Now put the Qg^ 
in strong brine and it floats near the surface of the liquid. 
Why ? The buoyancy of brine is a little more powerful 
than the pressure downwards due to gravity. Placed on 
mercury the q^^ seems to sink scarcely at all. Why ? 

Now take a lighter liquid, like spirits of wine, and it will 
be found on trial that cork or wood sinks deeper into this 
liquid than it does into water. Why ? 

III. Applications. 

The children will now be able to answer such questions as 
the following : — 

1. Why does a stone weigh less in water than it does in 
the air ? 

2. Why does a person find it difficult to stand upright 
in water sufficiently deep to cover the shoulders ? 

3. How is it that a man can float near the surface of 
water ? 

4. Why does ice form at the top, and not at the bottom 
of woter P 



94 OBJECT LESSONS. 

5. Why does oil float on water ? 

6. Why does cream rise upon milk P 



LESSON XI 

WEIGHT OF THE ATMOSPHERE. 

Articles for illustration : test-tube, gjass tube closed at one end 
30 inches long with small bore, mercury, water, and a couple of small 
cups. 

I. Weight of air. 

The teacher should refer to Lesson YL, page 83, and 
from this infer that the air, being attracted by the earth, 
must have weight. 

Next the teacher should describe how the air may actually 
be weighed. Exhaust* the air from a large bottle ; weigh 
it ; allow the air to enter, and re- weigh. The bottle full of 
air will be found to be considerably heavier than the empty 
bottle. 

A cubic foot of air weiyhs a little more than an ounce. 

But an ocean of air many miles in thickness covers and 
surrounds the earth. There is air on the top of the highest 
mountains, and men who go up four or five miles in balloons 
find air, or they could not live. As you will learn in 
another lesson, the air gets thinner and thinner as we ascend, 
but it probably reaches for forty or fifty miles or more 
before it comes to an end. 

II. Weight of a column of air. 

We have now to ask and answer the question, what is the 
weight — in others words, what is the pressure on the earth — 
of a column of air extending from the earth to its extreme 
limit above the earth ? 

• 6«e page 50. 



THIRD STAGE. 



95 



Exp. 114. Fill a test-tube with water 
the mouth under water in a shallow sauc i 
The test-tube is full of water. But water 
always runs down if it can. What keeps it up 
in the tube ? It is kept up by the pressure of 
the air on the water in the saucer. If we 
could take away* all the air from round about 
the tube the water would fall ; there would 
be nothing to keep it up. 



invert it with 
(Fig. 24). 




Fig. 24. 
So far this 



Exj). 115. Now instead of the test-tube let 
us use a tube open at both ends, only we 
cover one end tightly with a piece of wet 
bladder (Fig. 25). Now fill and raise as before, 
is the same experiment as the last ; but now prick a hole in 
the bladder. The water runs down, because the air has got 
at it and forced it down.f 

If we could use a tube about 33 feet long we should find 
that the pressure of the air would keep the 
tube full of water ; but make it about 34 feet 
long and the water would sink one foot in the 
tube. Hence the air can support a column 
of water of any length up to 33 feet, but no 
more. What do we learn from this ? That 
the weight of the air is sufficient to balance 
a column of water 33 feet high, but not a 
column 34 feet high. 

It is not easy to experiment with tubes so 
long, although such tubes have been made for 
the purpose of testing this weight or downward pressure of 
the air 

It is more convenient to use mercury. This metal is about 
13 J times as heavy as \7ater, and so we must divide 33 feet 




Fig. 25. 



• Tiiis-ean bo done under the receiver of an air-pump. 

t A cork, or the palm of the hand, may be used instead of the bladder. 



"96 OBJECT LESSONS. 

by 13 J to find the length of a column of mercury equal in 
weight to a column of water 33 feet high, both being held in 

tubes of the same diameter. It will be found to be 

between 29 and 30 inches. 

Exp. 116. Take a tube about 30 inches long — 
with small bore, because less mercury will be re- 
quired — closed at one end. Fill the tube by pour- 
ing in mercury. Place the finger firmly over the 
I open end. Invert and place in a small cup of 
$ mercury (Fig. 26). The column of mercury will 
^ be supported by the weight, or pressure down- 
wards of the atmosphere. 

Notice, we say the column of water is about 33 
feet long, and the column of mercury is about 30 
inches. You will learn the reason for this in a 
future lesson ; but 1 may now just tell you that 
the weight of the atmosphere is not always the 
same. It changes a little day by day, so that 
sometimes it will support a column of mercury 
31 inches high, and at other times perhaps only "-^9 inches, 
or even less. 

We can now easily find the actual weight of a column of 
air of any size, reaching from the sur- 
face of the earth to its utmost limit 
above. 

Take a tube square in section, and 
each side of the square measuring an 
inch (inside measure) (Fig. 27) ; we 
may say that the tube has a base of one 
square inch. Now the mercury whicli p. 07 

just fills a tube of this size and 30 inches 
long weighs about 15 lbs., and as this can be supported by a 
column of air of the same size, we conclude that the column 
of air weighs 15 lbs. It is more usual to say, The atmosphere 



ONE INCH 



THIRD STAGE. 97 

presses on every thing with a force equal to about 15 lbs. on the 
square inch. 

LESSON XII. 

PRESSURE OF THE ATMOSPHERE. 

Articles for illustration : boy's sucker, tumbler, water, and a thin 
card to cover mouth of tumbler. 



The teaclier should refer back to Lesson VIIL, page 51, 
and repeat one or two of the experiments to show that air 
presses in all directions. 

Secondly, he should refer to Lesson IX., page 91, where 
it is shown by experiment that, at the same depth, the pressure 
of water is equal in all directions. 

The atmosphere presses with equal force in all directions, when 
measured at the same distance above the level of the sea. 

The same law holds good in all fluids, including, of course, 
the air ; but we cannot prove it in the atmosphere by simple 
experiment because we cannot easily get to different heights. 

The law must be true at the surface of the earth. We have 
seen that the pressure of the air on every square inch is 
about 15 lbs., hence the pressure on a square foot will be 
more than a ton. The pres§ure, then, on the bottom of an 
" empty " bucket measuring a square foot will be over a 
ton. How, then, can we lift the bucket ? Just because the 
pressure upward is equal to the pressure downward, and 
we fail to feel the pressure of the air at all. Pressure side- 
ways, too, must balance or we could not stand upright. 

The pressure of the atmosphere on every child in the class 
is several tons ; but it is not felt because there is an 
equal pressure outwards from the air within the body. 

[If an air-pump can be obtained, the great pressure of the 

H 



OBJECT LESSONS. 



atmosphere can be demonstrated in a variety of ways. See 

Lesson XVIII., page 111.] 

If the air which surrounds the earth had the same density* 

everywhere, it would follow that, as we ascend, a given 

column would become just as much lighter as it is shorter ; 

but the air is by no means of the same density everywhere. 

It could not be, because it is very compressible, and the air 

above must squeeze the air below into less space by its 

weight. Therefore the air near the sea level is much 

heavier than the air at a great height. 

We may compare roughly a column of air to a column of 
bales of wool. The bales near the bot- 
tom will be squeezed, and made thinner 
than those above (Fig. 28). 

If we go to the top of a high mountain 
we leave the denser part of the air below 
us ; and a column of* air should weigh 
much less at the top than it does at the 
base of a mountain, and so it does. The 
weight is lessened much more rapidly 
than the length of the column above us. 
At the top of Snowdon the column with 
square inch section weighs about 13 lbs. 
instead of 15 lbs. At the top of Mont 
Blanc the column weighs only 7 J lbs 
This is shown by the height of a column 
of mercury which the air supports. 
For every thousand feet we ascend the column of mercury 

shortens by about one inch. But if the air were equally 

dense everywhere, we should have to ascend more than a 

mile before the mercury fell as much as this. 

On the other hand, if we descend into a mine the column 

of mercury lengthens one inch for every thousand feet . 

 'I'hickness, closeness, the same number of molecules occupying a similar 




Fig. 28. 



THIRD STAGE. 99 

LESSON XIII. 

THE BAROMETER. 

Articles for illustration : a narrow tube thirty-four incTies long, a sliallow 
cup, and enough mercury to till the tube and two-thirds of the cup. 



I. The weight of the atmosphere varies with the amount of 
water-vapour it contains. 

"Not only does the weight of the atmosphere vary accordmg 
to the height above the sea level at which it is measured, 
but it varies, as T have already told you, from day to day, 
and even from hour to hour, at the same place. To what is 
this variation due ? It is due to the variation in the quantity 
of vapour in the air. 

** The atmosphere is a mixture of certain gases and vapour ^ 
and you know that vapour is lighter than air. How do you 
know that vapour is lighter than air?" Because it ascends in 
the air, 

" Then which will be the heavier, dry air or a mixture of air 
and vapour ? " Lry air. 

" And how will the weight of the mixture vary ? " It 
will vary as the quantity of vapour in the atmosphere varies. 

*' When will the mixture be heaviest ? " When it contains 
the least amount of vapour. 

" And when will it be lightest ? " When it holds the largest 
quantity of vapour. 

" Now you can tell me when the mercury in our column will 
stand highest." When the atmosphere contains the least amount 
of vapour. 

" And when will the column be shortest ? " When the air 
contains the largest amount of vapour. 

*' The height of the column of mercury then shows two facts. 
What are they ? " The exact weight of the atmosphere at any 
given time / and whether there is little or much moisture in the 
atmosphere. 



100 



OBJECT LESSONS. 



II. Construction of barometer. 

Exp. 117. The teacher may now take a tube of narrow 
bore 32 or 33 inches long and closed at one end. Fill with 
mercury. Place the finger firmly over the open end, and 
invert in a small vessel of mercury (Fig. 29). 

Now call the attention of the children to what happens. 
The mercury falls, and there is a vacuum of two or three inches. 
Paste a narrow strip of paper on the upper part of the tube. 






Fig. 29. 



Fipr- 30. 



Fig. 31. 



Measure from the surface of the mercury in the vessel to the 
surface of the mercury in the tube. It measures, say 30 
inches. Mark ibis on the paper. Also ma^k 28, 29, and 
31 inches, and tell the children that, looking at the tube day 
after day, we shall find the column of mercury changing its 
height ; sometimes it will be as low as 28, and sometimes as 



THIEB STAGE. . : ; : 101 

high as 31 inches, but that its more ordinary . he JgiSt ift^ 
between 29 J and 30 1 inches. 

The children will see that the simple tube and vessel 
without support would be a very awkward instrument to 
stand in our houses, and the teacher may show how they 
are fixed in a frame. He may also show how the " wheel- 
barometer " works. [See Figs. 30 and 31.] 

III. The barometer an indicator of weather. 

We see, then, that the mercury in the glass tube measures 
the weight of the atmosphere. The word barometer means a 
measure of weight, and so we call our little apparatus a 
barometer. The barometer is a very useful instrument, 
because as it measures the weight of the atmosphere it tells 
us at the same time whether there is little or much moisture 
in it. Now much moisture means it is likely to rain, and 
little moisture means we shall probably have fine weather. 
That is, the barometer tells something of the kind of weather 
we are likely to have. When the mercury " falls " it 
betokens rainy weather, when it " rises " it points to fair 
weather. When the mercury falls very quickly it betokens 
a severe storm. 

LESSON XIV. 

THE SYRINGE. 

Articles for illustration : long glass tubes, one straight and one bent, 
with rather small bore ; a syringe of any kind. 



I. Its parts.* 

Consists of tube with small hole at one end, a piston-rod 
and a sucker. The teacher should take his specimen syringe 
to pieces and show its parts. 

* The teacher may construct his own syringe. Take a glass tube eight or 
nine inches long, with about a half-inch bore. Fix neatly into one end a 
short cylinder of wood with a small hole through the centre. Take a stiaight 
rod for piston, and bind worsted round one ena to make the sucker. 



102 



OBJECT LESSONS. 



<;^? 



17, IxH act) Oil. 

Uxp. 118. See that the piston-rod is pressed down. 
Place the point of the syringe beneath the surface 
of the water. Draw up the piston-rod ; the water 
follows the sucker. Press the rod back and the water 
is pushed out in a stream through the small hole. 

W/ri/ does the wafer enter the cylinder when the 
piston-rod is drawn back ? And why does it require 
a small hole, or holes, through which to force the 
icater. 

It is commonly said that the sucker sucks up, or 
draws up, the water ; but this cannot be, as I will 
show you. 

Exp. 119. Take this glass tube, place one end in 
this bottle of water and the other end in your mouth. 
Now *'8uck up'* the water through the tube. What 
makes the water come through the tube into your 
mouth? I will tell you. You "suck out" the 
air, and the water follows. But the water cannot 
Fig. 32. u3Qye Qf itself, something must force it up the tube. 

It is the air pressing on the water in the bottle. I will now 

prevent the air from pressing on the water in the bottle. I 

pass the tube through a hole 

I have made in this cork and 

cork the bottle. Both tube 

and cork must fit tightly. 

Now try and suck up the 

water. You cannot. Why ? 

Because although you re- 
move the air from the tube, 

the water cannot follow, there ^_ 

being no force to push it up. l^ig- 33. Fig. 34. 

Exp. 120. We may show the same thing another way 

H^^re is a bent tube. I partly fill it with water (see Fig. 35). 






THIRD STAGE. 103 

So long as I leave the short end open I can drink the 
water ; but when I place my thumb firmly over this end 
I can no longer get the water to ascend the 
longer leg. 

Precisely the same thing happens in the work- 
ing of the syringe as in the oottle when not 
corked. The sucker removes the air, or the 
greater part of it, from the cylinder, and the 
pressure of the air on the surface of the water in 
the vessel forces the water into the syringe. ^^^* ^^• 

Exp. 121. To answer the second question the teacher will 
fill a test-tube with water, and, placing a disc of paper over 
its mouth, invert. The upward pressure of the air keeps the 
water in its place. If I remove the paper I don't remove 
the pressure, and yet the water falls out. It falls oat because 
the gravity of its molecules is greater than their cohesion ; 
and when there is nothing to preserve the level surface of 
the water, the air breaks in and forces itself up among the 
particles, and actually turns the water out to take its place. 
Small holes are used in the syringe because they allow but a 
small water-surface for the air to act upon, and so the water 
does not run out freely unless pressed by the sucker. 

III. Its uses. 

Watering plants, cleansing their leaves. For cleansing 
our ears, &c. 

LESSON XV. 

THE COMMON PUMP. 

Articles for illustration : a glass model of a common station-pump •,'^ 
or the teacher may sketch on the blackboard (Fig. 39). 



I. Its parts. 
Like the syringe, the common pump consists of a cylinder 

* This can be purchased for about two shillings. 



104 



OBJECT LESSONS. 



and a piston with sucker ; but, unlike the syringe, it has little 
doors working on hinges, which we call valves. 

The teacher should, at the outset, explain the working of 





Fig. 36. 

valves, sketching the shapes of two or three of the most 

common on the blackboard (Fig. 36, a, b, c). 

The valve (Fig. 36b) is the one most often used in the 

common pump. It is called the bellows valve, because used 

in the common bellows. 

When you pull the hanidles of the bellows apart, as shown 

in Fig. 37, you make 
more room in the bel- 
lows, and the air forces 
the valve open and rushes 
in to fill up this space. 

When you push the 
handles of the bellows 
together, as in Fig. 38, 
you compress t^e air, 
and it closes the valve 
and rushes out in a stream 
through the nozzle. 
The teacher will next show the class the position of the 

valves in the model, or in the sketch. He should also draw 

attention to the facjt that the lower part of the cylinder, or 

the suction-pipe, is smaller than the upper part, commonly 

called the barrel. 

II. Its action. 
In Fig. 39a the pump handle is down, and consequently 



Fig. 37. 




Fig. 38. 



THIRD STAGE. 



105 



the sucker is as high as it can be raised. Both valves are 
closed. 

Raise the handle. The air between the sucker and the 
lower valve will be compressed, consequently it will force 
open the valve in the piston and escape above. When the 





Fig. 39. 

sucker is at its lowest point this valve will be again closed 
by the weight of the air above it. 

Press the handle down. The valve in the piston is kept 
closed by the downward pressure of the air, and the space 
between the piston and the lower valve will be deprived of 
the greater part of its air. The air in the suction-pipe, 
by its elasticity, will open the valve, and part of it will 
escape into the " empty " space in the barrel. It follows 
that the pressure of the thinner air on the water within 



106 OBTECT LESSONS. 

the suction-pipe will be less tlian the pressure outside, and 
then the water will be forced by the outside pressure up the 
suction-pipe until the pressure outside and inside are equal. 

Repeat the action with the handle, and the water will rise 
higher in the suction-pipe, till after a few strokes it forces 
open the lower valve and enters the barrel. 

When we raise the handle again the piston presses upon 
the water and forces it up through the valve in the piston 
(see Fig. 39b). The next down stroke actually lifts the 
water, and when sufficient is raised above the sucker it 
flows out through the spout. 

The teacher will now show the class that the principle of 
the pump is just the same as that of the syringe, or of 
" sucking " up water through a small tube. It is the 
weight of the outside air which presses up the water in the 
suction-pipe. 

What is the length of a column of water which the 
pressure of the atmosphere will sustain ? From 33 to 34 
feet. Then we cannot raise water with an ordinary pump 
from a greater depth than 33 or 34 feet. As a matter of 
fact we cannot raise it so high, because we fail to keep out 
all the air. The average is about 27 or 28 feet. 

III. Its uses. 

The teacher will give the uses, e.g. raising water from 
wells, draining mines, &c. 



LESSON XVI. 

FORCE-PUMPS. 



Articles for illnsl ration : — Models of force-pumps are mther expensive, 
and tlie teacher will probably have to substitute diagrams on the black- 
board. 

I. Force-pump. 
Fig. 40 represents a very simple form of the force-pump. 



THIRD STAGE. 



107 



cc 



^ 



Tlie action, while the piston is ascending, is like that of 
the common pump. In de- 
scending the action is different. 
There is no valve in the piston. 
This is placed somewhere in the 
discharge pipe which leaves 
the barrel near the bottom. 

In the diagram b represents 
the suction-pipe, c the barrel, 
p the piston ; D is the discharge 
pipe from the barrel, v and v' 
are valves opening upwards, cc 
is the ''condensing" chamber, 
and A is the discharge pipe 
into the air. 

When the piston is raised 
the water is forced up by the 
atmospheric pressure outside 








Ef 






■Er:: 


B 




^ 











i;_d 


^=-- 





Fio-. 40 




Fig. 41. 



into the suction-pipe, and thence, after a 
few strokes, into the barrel. When the 
piston is pressed down the valve v closes 
and the water is forced through the pipe d 
and into the chamber cc. This chamber at 
first is full of air ; but after a few strokes of 
the piston this air is compressed by the 
water forced in. 

The compressed air in its turn presses on 
the water and forces it out through the 
tube A — which is smaller than D — in a 
continuous stream. 

II. The lifting pump. 

The working of this pump can be seen 
at once from the diagram (Fig. 41). 



Water may be pressed up, or lifted, to almost any height 



108 



OBJECT LESSONS. 



in the small tube, provided sufficient force is exerted on the 
piston, and the machine is strong enough to bear the pressure. 

III. The fire-engine. 

The fire-engine is a kind of force-pump Fig. 42 represents 
the hand fire-engine. It is a douhle force-pump of the kind 




Fig. 42. 

shown in Fig. 40, and its action is precisely similar. The 
water is forced alternately from either cylinder into the 
central, or condensing chamber. 



LESSON XVII. 

THE SIPHON. 
Articles for illustration : bent glass tubes. [See cuts.] 



I. Experiments with water in bent tubes. 

Exp. 122. Take a narrow glass tube (say -ft- inch bore) 



THIRD STAGE. 



109 



about eight inches in length, and bend it as shown in Fig. 43. 
Fill the tube by immersing in water. Raise it gently out 
of the water; the water does not run out of 
either end so long as the tube is kept upright. 
Or the points of the fore-fingers may be placed 
over the ends whilst lifting out of the water. 



Exp. 123. I^ext take a wide tube bent in the 
same way, with legs of equal length. Place the 
open ends upwards and fill the tube with water, 
thin card over each end and invert (Fig. 44). 
as the legs are kept upright the water does 
out 




Fig. 43. 

Place a 
So long 
not run 



But if the tube be inclined either way the water runs 
A 





Fig. 44. 



Fig. 45. 



out of the lower end. When we incline the tube we make 
one leg longer than the other (Fig 45). 

II. Action of the siphon. 

{a.) Hoic is the water retained in both legs of the tube ivhen 
the legs are of the same length ? 

(b.) When tve make one kg longer than the other, why does 
the ivater run out at the lower end ? 

A few questions on the pressure of the air in every direc- 
tion will lead the children to see that the pressures upwards 



110 



OBJECT LESSONS. 



on B and c (Fig. 44) are equal. There is also a downward 
pressure on b and c, viz. the weight of water in each leg. 
And this will be equal when the legs are of equal length. 
[That is, of course, provided the tube is of the same bore 
throughout.] 

Suppose these pressures of the air upwards to be equal to 
a weight of 4 lbs. on each leg, and the pressure of the water 
downwards in each leg to be half a pound. Then this just 
amounts to the same thing as a pressure upwards on the 
water in each leg of 3J lbs., and the one pressure just 
balances the other. 



Exp. 124. Next suppose one leg to be twice the length 
of the other (Fig. 46). The pressures upward as before 
will be equal, say equal to a weight of 4 lbs. on the water in 
A each leg. But the pressure downward in the 

> longer leg will be double that of the other. If 

^ :J\ the water in the shorter leg weighs ^ lb., then 
the water in the longer leg weighs 1 lb. This 
amounts to the same thing as a pressure up- 
wards en B of 4 lbs. less ^ lb., and on c of 
4 lbs. less 1 lb., or 3J lbs. on b and 3 lbs. on c. 
And the pressure on b being greater than that 
on (;, the water is forced out at the lower end. 

?Fill any V-shaped tube having one leg 
longer than the other. Place the fingers over 
Fig. 46. the ends. Dip the short end into a vessel of 

water, and let the long end hang outside : the 
water will be taken from the basin in a continuous stream 
so lone: as the mouth of the short tube is below the surface 
of the water ; oi-, if the water be received in another vessel, 
until the surface of the water in both vessels occupies the 
same level. 

Any bent tube having one leg longer than the other is 
called a siphon. 



THIRD STAGE. Ill 

III. Use of the siphon. 

Used by brewers and wine and spirit merchants for 
emptying casks too heavy to be lifted. 

Can be used for carrying water any .distance from a higher 
to a lower level, passing over any elevations not higher than 




Fig 47. 

the length of a column of water which the atmosphere will 
sustain. What height is this ? 

The teacher may also suggest other uses of the siphon, 
such as taking the clear parts of a liquid from the thicker 
parts, leaving the muddy parts behind ; or taking a liquid 
from beneath the fat which may be floating on the top, 
leaving the fat behind. 



LESSON XVIII. 

THE AIR-PUMP. 
Articles for illustration : a diagram, and, if possible, an air-pump. 

I. Description. 

An air-pump is a machine for extracting the air from 
closed vessels. Fig. 48 represents a section of one of the 
more simple forms. 



112 



OBJECT LESSONS. 



R is the glass " receiver " from which the air has to be 
exhausted ; b is a brass cylinder, called the pump barrel, p 
is the piston, which is worked by the handle h attached to 
the rod r. c is a brass plate on which the receiver is made 




Fig. 48. 

to fit very accurately ; .s and t are valves opening upwards 
like small doors, s is in the piston itself, and t is near the 
bottom of the barrel. A is a screw which closes the tube d 
when necessary. 

II. Working. 

Exp. 125. Suppose -the piston to be descending. The 
valve t is closed and the comprossion of the air in the barrel 
will open the valve s and the enclosed air will escape. Now 
raise the piston. The pressure of the external air closes the 
valve s, and all the air above the piston will be forced out 
through the hole in the lid of the barrel through which the 
rod works. A vacuum would thus be made in the barrel, 
but the air in the receiver expands, opens the valve ty and 
fills the barrel. 

A. double stroke of the piston removes a portion of the air 
remaining in the receiver, because each time a vacuum is 
made by the piston the air in the receiver expands to fill it. 
This will go on until the tension of the air in the receiver 
is too feeble to raise the valve t. The receiver will never 



THIRD STAGE. 113 

become quite empty of air, although what it contains will 
be exceedingly rarefied. 

The air being rarefied, the pressure on the internal surface 
of the receiver is but little, while at the same time the 
pressure on the outside is 15 lbs. per square inch. . Hence 
the receiver will be fixed firmly by atmospheric pressure on 
the brass plate. 

The teacher may here explain that the action of the lungs 
and the tubes leading thereto from the mouth in "sucking" 
the air from a tube is just the action of an air-pump. The 
lungs are expanded, and the air in the tube expands and 
rushes in^ The tongue acts the part of the valve, and stops 
the mouth of the tube, whilst the air is expelled from the 
lungs, and the latter expand again. 

III. Its uses. 

The teacher will tell the children that the air-pump has 
many uses which will become apparent in future lessons. 
At present he may exhibit some experiments further illus- 
trating the pressure of the atmosphere. 

The following are suggested : — 

Exp. 126, Using a glass jar open at both ends as a 




Fig. 49. 

receiver (Fig. 49), the open end a may be covered with 

I 



114 



OBJECT LESSONS. 



sheet india-rubber, or soft bladder. As the air in the receiver 
is removed the eflPect of the atmospheric 
pressure on the bladder or india-rubber 
is very striking. If the palm of the 
hand is substituted for the bladder the 
pressure may he felt. 



Exp. 127. Two hollow half- spheres 
(Fig. 50) are made exactly to fit one 
anotber. One of the balf-spheres, b, is 
screwed, on to the plate of the air-pump, 
the other, a, is then placed firmly* on 
the top. A receiver is thus formed, 
which is exhausted by the pump. Turn 
the tap, T, to sbut out the external air 
and unscrew from the pump. fecrew 
on the handle h, and call upon a couple 
of scholars to exercise their muscles in 
trying to separate the half-spheres. 




Fig. 50. 



* It is exceedingly difficult to get brass and glass ground so perfectly true 
as to be air-tight when fitted together. A bit of lard spread over the 
surfaces will, however, remove all ^difficulty on that score. 




FOUETH STAGE. 



FOURTH STAGE. 



EFFECT OF 



LESSON I. 

HEAT ON BODIES (i). 
CONTRACTION. 



EXPANSION AND 



Articles for illustration : bladder, flask, bottle, glass tube, chalk, and 
nitric or sulphuric acid, thermometer. 




I. Gases. 

Exp. 128. Half fill a bladder with air or gas, and 
place in front of the fire. It begins to swell 
almost at once, and is soon quite full. Why ? 
Heat expands gases. 

On being removed from the fire tbe bladder 
slowly returns to its original size. Why ? Cold 
contracts gases. [The teacher should here explain 
that when we speak oicold we mean absence of heat.'] 

Exp. 129. Fill a flask or bottle with carbonic 
acid gas. Quickly insert cork stopper with tube, 
as in Fig. 51 ; invert, and plunge the other end 
into a bottle nearly full of coloured water. Apply 
the flame of a spirit-lamp to the flask of gas ; 
or hold the bottle with warm hands. Bubbles 
will be seen rising in the water from the tube. 
Why? Because the gas, being expanded by Fig. 5i. 
heat, forces its way through the water. 

Allow the flask to cool ; the water rises in the tube. Why ? 




118 OUJECT LESSONS. 

Because some of the gas has been forced out, and the re- 
mainder is compressed by the pressure of the air outside. 

II. Liquids. 

Fill a flask quite full of water, and heat it. The water 
runs over. Why ? 

Hold the ball of a thermometer in the hand, or breathe on 
it. The mercury ascends in the tube. Why ? 

Plunge the bulb in cold water. The column of mercury 
rapidly shortens. Why ? 

III. Solids. 

Exp. 130. Take a bar of iron which exactly fits into 
a ring. Heat the bar; it will no longer pass through. Why? 

Exp. 131. Or, take two brass tubes, one of which exactly 
fits into the other. Heat the smaller, it will no longer 
enter the larger. Why ? 

Plunge it in cold water. It fits again. Why ? 

These experimei.t < all show that when we add heat to 
bodies, whether solid, liquid, or gaseous, the heat makes 
them expand, and when we withdraw heat from bodies they 
all contract. 

With very slight help from the teacher the children will 
now be able to explain — 

(1) Why a kettle should not be quite filled before putting 
it on the fire to boil. 

(2) Why a glass often cracks when boiling water is poured 
into it suddenly. 

(3) Why the tire of a wheel is made a little too small, then 
made red hot before putting on, and why cold water is then 
poured on it. 

(4) Why the iron bolts which are used to fix the iron 
plates of ships are put in red hot. 

(5) Why in laying down the iron or steel rails in making 
Q railway the workmen do not fix the ends close together. 



FOURTH STAGE. 119 

(6) Why a glass stopper "fixed" in a bottle can readily 
be removed if we invert the bottle and plunge the neck into 
hot water. 

(7) Why chestnuts when being roasted often burst with a 
loud report and leap for some distance. 

(8) Why dry wood snaps on being burnt. 

(9) Why a shrivelled apple, on being roasted, becomes 
plump again. 

LESSON II. 

EFFECT OF HEAT ON BODIES (2).— LIQUEFACTION AND 
VAPORIZATION. 

Articles for illustration : any of the following — ice, sulphur, sealing- 
wax, tin, zinc, lead, glass, iron, mercury, iodine, alcohol, camphor, together 
with a test-tube and an iron spoon. 



I. Liquefaction. 

Heat expands solids, but it does more. It liquefies nearly 
all. 

The teacher may take any or all of the following solids 
and liquefy or melt or fuae them : — ice, sulphur, sealing-wax, 
tin, zinc, lead. The metals may be fused in an iron spoon. 

With sufiicient heat nearlj^ all bodies can be melted. 
Some bodies, such as iron, glass, sealing-wax, &c., soften 
before melting. The teacher may show how we take 
advantage of this circumstance in the case of iron to fashion 
various articles by hammering ; and in the case of sealing- 
wax by making an impression in the wax by means of a seal. 

II. Vaporization. 

Ex}), 132. Heat applied to ice changes it to water. Heat 
applied to water changes it to a gas or vapour, which we call 
steam. The process is called vaporization. Many solids 
may be changed to vapour. In addition to water the teacher 
may vaporize any or all of the following : — sulphur, mer- 



120 OBJECT LESSONS. 

cury, alcohol, iodine, camphor, and zinc. The vapour of 
iodine has a very characteristic colour when seen in a 
flask. The vapour of zinc burns with a bright green flame. 

The teacher will now lead the children to see that the 
state in which any matter exists, whether in solid, liquid, or 
vapour, depends entirely on heat. 

Further illustrations may also be given. Thus : — 

Palm oil is really a liquid oil in Africa (whence it comes). 
In our country it has the consistency of butter. 

Butter is almost a liquid oil in the hottest summer weather 
in this country. In winter it is quite hard. 

Olive oil, again, is a clear liquid in summer ; in winter it 
is solid. 

Mercury is a liquid in England. In the coldest regions, 
in winter it becomes solid. Like water, it is said to be frozen. 

Ether is ordinarily a liquid, but under a summer's sun it 
boils, and becomes a gas or vapour. 

We may say generally that — 

Heat added to a solid gives a liquid. 

Heat added to a liquid gives a vapour 

Heat subtracted from a gas gives a liquid. 

Heat subtracted from a liquid gives a solid. 



LESSON III. 
THE THERMOMETER. 



Articles for illustration : mercury, tube with very narrow bore, and a 
Fahrenheit's thermometer. 



I. Our senses are not exact measures of heat. 

Exp. 133. Take three basins ; in one basin put water 
at about 50° and in another water at about 90°. Direct a 
scholar to place one hand in the water in one basin and the 
other in the water in the other basin for a few seconds. 



FOURTH STAGE. 121 

Now pour both waters into the third basin, and let the 
scholar put both hands into the mixture. To one hand the 
water will seem warm, to the other it will feel cold. This 
shows that our sense of feeling is not a correct measure of 
heat. 

Again, the atmosphere at say 50° will feel cold to a per- 
son coming from a warm room ; but to a person emerging 
from an ice-house it will seem warm. Or again, marble, 
wood, and flannel may all be of the same temperature ; but to 
the hand the wood will feel colder than the flannel, and the 
marble will seem colder than the wood. 

II. How we measure heat. 

The amount of expansion which bodies undergo under 
varying temperatures is the best measure of heat. But we 
must select a substance which undergoes considerable ex- 
pansion, and which is not easily changed into another state 
by either heat or cold. Solids would not be suitable 
Why? They undergo too little expansion to be easily seen. 
Gases are not convenient. Why ? They are affected too 
much by atmospheric pressure, which is constantly changing. 

We select liquids. Will water answer well ? Why not? 
It changes to solid ice at a temperature not very low, and to 
steam at a temperature not very high. Spirits of wine 
answers well for very low temperatures, but changes to 
vapour at a lower temperature than water. Mercury is not 
easily frozen, and does not change to vapour till a high 
temperature is reached, and is therefore the substance best 
suited for measuring expansion, and thereby the amount of 
heat. 

III. The mercurial tiiermonieter. 

Take a capillary tube open at one end but with a bulb 
blown at the other. 

Exp. 134. To fill the tube. Heat the bulb. The air 



122 



OBJECT LESSONS. 



expands and part of it is expelled. Plunge the open end at 
once into mercury. As the air cools the pressure of the atmo- 
sphere without forces some of the mercury* up the tube into 
the bulb. 

Next heat the mercury in the bulb until it boils, when 
its vapour will drive out all the air arid fill the tube. 
Plunge the open end again into mercury. The bulb and 
tube will now be filled with mercury. Why ? 

Seal the open end, by melting the glass, before the mer- 
cury has time to cool. 

To graduate the thermometer^ viz. to mark the steps or 
degrees of heat. Put the thermometer in a vessel containing 
melting ice. The column of mercury falls because heat is 
withdrawn. When the column becomes stationary mark 
with a file on the tube the position of the top of the column 
of mercury. This point we call the melting point of ice, or, 
which is the same thing, the freezing 
point of water (Fig. 52). 

Next, suspend the instrument in 
steam rising from boiling water. The 
column rises because the heat expands 
the mercury. When it again becomes 
stationary mark the position of the top 
of the column. This point is called the 
boiling point of water (Fig. 53). 

We have now fixed two points on the 
tube corresponding to the boiling and 
freezing points of water respectively. 
Our next business is to divide the space 
between these two points into steps^ or 
grades. In one thermometer the freezing point is marked 
and the boiling point 100, and the space is divided there- 
fore into 100 steps. This thermometer is called the Centi- 
grade, viz. having a hundred steps, 

* Wc cannot pour mercury into a capillary tube. 



BO 

INQ 



J 




Fig. 52. Fig. 53. 



FOUiiTH iSTAGE. 



123 



The thermometer, most commonly used for household pur- 
poses in our country is called after its first maker, Fahren- 
heit. On this thermometer the freezing point 
is marked 32 and the boiling^ofnt 212. Thus 
the space between is divided into 180 parts or 
degrees. Other degrees are marked (if neces- 
sary) above 212 and below 32. The cipher (or 
zero) on Fahrenheit's thermometer was erro- 
neously supposed to represent the lowest tem- 
perature attainable. It is about the tempera- 
ture resulting from the melting of a mixture 
of snow, or crushed ice and salt. 

The teacher may show how the marks are 
made on the tube. It is coated with wax, and 
then scratches are made in tlie wax with the 
point of a needle. The tube is then placed 
in a solution (hydrofluoric acid) which eats into 
the glass, but does not affect the wax. 

He may also show how to find the tempera- 
ture on one scale that corresponds to a given 
temperature on the other. 

180° Fahr.rir 100° Cent. 
9° „ = 5° „ 



Fig. 54. 

CENTIGRADE 
THERMOMETER. 



And 1° Cent. = 



90 



Fahr. 



We must remember, however, that the number which 
represents a certain temperature on Fahrenheit's scale does 
not, as on a Centigrade scale, represent the number of 
degrees above freezing. It is 32° too many. 

Hence in changing from Fahr. to Cent, subtract 32 from 
the given number and multiply by a. And to change from 
Cent, to Fahr. multiply the given number by f and add 32. 

The degrees are usually marked on the '' frame " in which 
the tube and bulb are fixed (Fig. 54). 



124 OBJECT LESSONS. 

LESSON IV. 

FREEZING OF WATER. 

Articles for illustration : if possible a thermometer tube, and a freezing 
mixture ; a tlierniometer. 



The teacher will elicit from the children the following 
general laws given in preceding lessons. 

' 1. Bodies expand as they receive heat. That is, the 
molecules get farther apart, and the bodies become less dense. 

2. Bodies contract as heat is subtracted. That is, the mole- 
cules get closer together, and the bodies become more dense. 

3. That a denser body has more weight than an equal 
bulk of a less dense body. 

From this the children may be led to see — 

1. That as water in lakes, and ponds, and streams cools 
first near the surface, the particles become more dense 
and sink to the bottom, forcing up the less dense, because 
warmer, particles from below. In this way the whole body 
of water becomes cooled. 

2. That without some change, or deviation, or special 
exception, from the general law that bodies contract on the 
removal of heaty the process of cooling under a cold atmo- 
sphere would go on until the entire mass got just below 32°, 
when the whole would be changed to ice. 

3. That in frost}^ weather this would change all the water 
of our ponds, and streams, and shallow lakes to solid ice, 
killing the fish, and many other of its inhabitants, and 
milking it probable that the heat of summer would scarcely 
be sufficient to re-convert the whole of the ice to water. 

The teacher may now tell the children of the wonderful 
exception to the general law in the case of water. Water 
contracts and becomes heavier, bulk for bulk, on cooling, till 
it reaches about 40° Fah., that is about 8° above freezing 



FOURTH STAGE. 125 



point, and tl)en as it still further cools it expands till it gets 
below 32°, when it changes to ice.* A pint of water at 40° 
becomes l-j-'y pints of ice at 32°. 

The contraction and expansion of water in cooling from 
say 50° or 60° to 32° may be shown by using water instead 
of mercury in a thermometer tube, and placing the bulb 
and tube in a freezing mixture of ice and salt. 

Thus it is that ice always forms at the surface of water, 
and there remains as a kind of coating to keep off the 
cold winds from the water below. 

With a little help the children will now be able to 
explain — 

1 . Why a bottle filled with water and tightly corked will 
burst if placed in a freezing mixture. 

2. Why water-pipes frequently burst during frosty 
weather.f 

3. Why rocks and stones often split in winter. 

4. Hoio frosts pulverize the soil. 

5. Why shallow water freezes over very quickly in frosty 
weather. 

6. Why lakes of very deep water seldom freeze over, 
even in severe winters. 



LESSON V. 

BOILING OF WATER— CONVECTION. 

Articles for illustration : Florence flask, narrow tube, aniline solution, 
large test-tube, small lump of ice ; the spirit-lamp. 



Refer the children back to the process which goes on during 
the cooling of water from the surface. 

* Sea water does not freeze till cooled 4° or 6° below the freezing point of 
fiesh water. 

t Occasion may be taken to correct the conunon error that the pipes ajrs 
burst by the thaw. 



126 OBJECT LESSONS. 

The cooler particles sink down, the warmer particles rise 
to the top. Why ? 

I. Boiling. 

The same process goes on when we boil water. The particles 
below are heated ; they become, therefore, bulk for bulk, 
lighter and rise to the surface, and the cooler particles sink 
down, to be in their turn made warmer and lighter. 

After a time some of the water at the bottom becomes 
changed to steam. The steam rises in bells or bubbles, which 
burst as they become cooled in the cooler water above. 

As heat is still further applied the bells cannot be cooled 
sufficiently to burst in the water, and so they reach the top, 
where the steam escapes into the air. 

Soon the bells ascend to the surface in increasing numbers, 
creating more and more disturbance, and making the well- 
known appearance and noise of boiling. 

II. Convection. 

Exp. 135. The appearances above described may be 
readily demonstrated by boiling water over the flame of a 
spirit-lamp in a Florence flask. The upward and downward 
current may be prettily shown by introducing — before 
applying heat — a little deeply coloured aniline* solution to 
the bottom of the flask. 

This is done by means of a narrow tube used as a pipette. 
Stop the lower end with the forefinger of the left hand ; fill 
the tube ; press the forefinger of the right hand on the 
upper end ; remove the left hand forefinger. The solution 
is supported in the tube. How P 

Thrust the lower end of the tube to the bottom of the 

flask (Fig. 55) and remove the finger ; the solution flows 

out and colours the water at the bottom. 

 A few grains of cochineal thrown into the water will answer the same 
purpose. 



FOURTH STAGE. 



127 



Place the spirit-lamp under 
the flame may just q 
touch the middle of 
the bottom. Soon the 
coloured water imme- 
diately over the flame 
becomes heated and 
rises as an upward 
current through the 
colourless water. At 
the same time the 
cooler liquid at the 
sides begins to de- 
scend to take the 
place of that which 
rises, and in a short 
time the descending 
currents are made 



the flask, so that the point of 





Fig. 57. 



manifest to the eye by the 
colour in the water. 

In this way heat is conveyed, 
or carried y to all parts of the 
liquid ; and the process is called 
convection, viz. a carrying of heat. 

III. Water a had conductor of 
heat. 

Exp. 136. The teacher may 
now introduce an interesting ex- 
periment to show that the water 
does not carry heated particles 
downwards as well as upwards. 
Put a little ice at the bottom 



of a test-tube and keep it in position by a coil of wire. 



128 OBJECT LESSONS. 

Nearly fill tlie tube with cold water, and apply the flame of 
the spirit lamp at a little distance from the top (see Fig/ 
57). The water at the top may be made to boil, while the 
ice at the bottom remains unmelted. If ice is not to be 
obtained put a little aniline solution at the bottom. 

WTiy should we apply heat at the bottom of a vessel when 
we wish to boil water quickly ? 

Whp does the water sometimes boil in the spout of a 
kettle before the main body in the kettle boils ? 



LESSON VI. 

DISTILLATION. 

Articles for illustration : a small retort, Florence flask, lamp and 
stand. 



I. How vaponr and steam are condensed. Distillation. 

&p. 137. Place a cold glass over the flame of the spirit- 
lamp. Some of the vapour produced in burning condenses 
as water on the cold glass. 

Breathe on a piece of cold slate. The vapour from the 
breath condenses as water on the cold slate. 

Uoap. 138. Heat water in a retort, arranged as in Fig. 58. 
The steam passes down the long neck into a Florence 
flask, the latter standing in a basin of cold water. The heat 
from the spirit-lamp changes the water into steam, and the 
cold water in the basin changes the steam back again to 
water. This process is called disUllatmi. 

If ink be placed in the retort instead of clear water, it will 
be found that only pure water is distilled over. The other 
ingredients of the ink are left behind in the retort. Now 
put brine or a solution of sugar in the retort and distil. 
Again only pure water distils over, the salt or sugar is left 
behind. 



FOURTH STAGE. 



129 



When sea-water evaporates, all the salt is left behind, or 
the rain-water would be salt to the taste. 

Fresh water can be got out of sea-water by distillation, 
and many large ships now carry apparatus for the, purpose. 

g 




Fig. 58. 

Distilled water is the purest form of water, but is not 
pleasant to drink. 

II. The boiling point of liquids varies. 

Exp. 139. Heat spirits of wine in a Florence flask. When 
it boils insert a thermometer. It will be found that the 
top of the mercury stands at about 172° showing that 
this liquid boils and changes to vapour at 172°, or about 40° 
lower than water. Ether boils at about 95°. It can be boiled 
by placing it in the rays of the sun on almost any day in 
summer. Mercury boils at about 660°. 

III. Uses of distillation. 

Distillation is chiefly employed to get spirits from malt 
liquor, and brandy from wine. These liquids contain a 
mixture of alcohol and water. As alcohol distils over at 
about 172°, arrangements are so made that the mixture shall 



131) - OBJECT LESSONS. 

not be heated above 180° At this temperature the spirit 
distils over, leaving the water behind. As a matter of fact 
some of the water does distil over with the spirit, and it 
requires a second distillation to produce proof- spirit, a mix- 
ture of half water and half alcohol. 

Distillation is also used to separate the more volatile 
benzoline from the less volatile paraffin oil; and the latter 
again from the solid paraffin. 



LESSON VII. 

EFFECT OH PRESSURE ON THE BOILING PO'NT OF 

LIQUIDS. 

Articles for illustration : Florence flask and spirit-lamp. 



I. Diminished pressure lowers, increased pressure raises, the 
boiling point of liquids. 

Exp, 140. Half fill a Florence flask with water. Boil 
over the spirit-lamp ; the steam will drive out and replace all 
the air in the flask above the water. Remove the lamp. 
Cork the flask tightly, and invert as shown in Fig. 69. 
When the boiling ceases let cold water flow from a sponge 
over the flask, and the water commences to boil again. 

Now when the heat has been removed and cold water 
poured on the flask, the temperature of the water inside 
must be considerably below the usual boiling point of water, 
212° 

The explanation of the second boiling is simple. The 
space above the water was filled with steam, the application 
of cold water condensed it, so that the pressure on the 
water was diminished. 

From this we learn that under less pressure than that 
given by the ordinary atmosphere water boils at a lower 



FOURTH STAGE. 



131 



temperature. And the converse of this is true : if we increase 
the pressure we raise the boiling point. 
The same law holds good for all liquids. 



Exp. 141. Under the receiver of an air-pump (see 
page 112), the pressure on the 
water may be diminished almost 
to nothing, and water may be 
made to boil at such temperatures 
as 70° or 80°, or even lower. 

The presence of salts in solu- 
tion raises the boiling point. 

The teacher can show this by 
boiling brine or syrup, placing 
therein a suitable thermometer* 
to register the temperature. The 
boiling point is raised 10 or 12° 
Fah. 



II. Practical applications fol- 
lowing on the facts as now demon- 
strated. 




Fig. 59. 



1. In refining sugar, water has to be driven from the 
syrup by boiling. Now the boiling point of syrup is about 
220°, and at this temperature it is apt to get burned and 
discoloured. To avoid this the syrup is put into closed 
vessels, from which the air and vapour can be drawn off by 
a pump. In this way, by removing pressure the syrup is 
boiled at 150°, and the risk of burning is avoided. 

2. In the boiler of a steam-engine the pressure of the 
steam on the water is very great. If ten times as great as 
the ordinary pressure of the atmosphere, the boiling point 
rises to 360° Fah. 

3. The pressure of the atmosphere on high mountains, as 
* Viz., a thermometer which registers temperatures above 212*'. 



132 OBJECT LESSONS. 

we have seen, is considerably diminished, and water boils at 
a lower temperature. On the top of Mont Blanc, for 
instance, water boils at about 160°, and this temperature is 
not high enough to cook potatoes, or an egg. The potatoes 
will not get soft, and the egg will not harden. 



LESSON VIII. 

STEAM AND THE STEAM-ENGINE. 
Articles for illustration : diaOTam of steam-eujirine. 



I. Steam is highly elastic ; hence has great expansive force. 

The teacher may introduce this lesson by eliciting from 
the scholars any facts about steam — how it is produced, its 
chief properties, how it resembles air, and so on. 

He should next direct attention to the most important 
property of steam — its elasticity/ or expansive force, the 
property on which depends its use in the " steam-engine." 

The rush of steam from the steam-engine gives some 
notion of the great expansive force of steam. 

If water is boiled in a vessel closed quite tight with a 
cork or lid, either the cork or lid will be blown out, or the 
vessel will burst into pieces. Great iron boilers are some- 
times burst into thousands of fragments by the expansive 
power of steam. 

Under the ordinary pressure of the atmosphere a cubic 
foot of water when changed to steam occupies 1,700 cubic 
feet of space. That is, a cubic foot of water will produce 
sufficient steam under the ordinary pressure of the atmo- 
sphere — 15 lbs. to the square inch — to fill as nearly as 
possible a boiler 12 feet long, 12 feet wide, and 12 feet high. 

Suppose the top of the boiler could be so arranged as to 
work up and down like a square piston in a square box, 



FOURTH STAGE. 



133 



then at this volume it would remain stationary ; that is, 
there would be a pressure downwards on the lid equal to 
15 lbs. on every square inch, or nearly a ton on each square 
foot of surface, and there must be also a pressure upwards or 
expansive force in the steam of exactly the same power. Now 
if a ton weight be placed on each square foot of the lid the 
latter will be pressed half-way down the boiler and the 
volume of the steam will be one-half its former volume, but 
its expansive force is doubled, as we see from the fact that 
it supports double the weight. 

The same effect of doubling the expansive force is produced 
if we put double the amount of steam in our boiler. And, 
generally, we may say that the more steam we can get into a 
boiler the greater is the pressure or expansive force of the 
confined gas. 

If we heat water in a boiler there is no limit to the 
expansive force of the steam produced except the strength of 
the walls of the boiler itself. 




II. The steam-engine. 

The steam-engine is a machine constructed to utilize the 
expansive, or elastic force of steam. 

The teacher may illustrate the principle of the steam- 
engine by making sketches (Figs. 60 
and 61) on a blackboard, a d is a 
cylinder in which the piston, p, works 
up and down, or backwards and for- 
wards, but not quite to the ends of the 
cylinder; a and care tubes communi- 
cating with the air, but fitted with 
stop-cocks ; h and d are tubes commu- 
nicating with a boiler b. These tubes 
are also fitted with stop-cocks. 

Let us suppose all the stop-cocks to be closed, the piston 
to be at the bottom of the cylinder (Fig. 60), and the boiler 




134 



OBJECT LESSON'S. 



B to be full of compressed steam. Open a and d. The steam 
rushes in at c/ and forces the piston to the position it occupies 
in Fig. 61, the air being forced out at a. 

Now close a and d and open h and c. The greater part of 
the compressed steam at once rushes 
out at c, and more compressed steam 
entering at i, the piston is pressed 
back again to the position it occupies 
in Fig. 60, forcing out the remainder 
of the steam in the cylinder. If some 
heavy body be attached to the end of 
th 3 piston-rod, it will be pulled back- 
wards and forwards as the piston is 
pushed backwards and forwards by 
the steam. 

This is the essential part of every 
steam-engine — the pushing backwards 
and for tear ds of a piston in a cylinder 
hy the expansive force of steam. All the various and intricate 
parts of a steam-engine are so many mechanical contrivances 
to make the machine itself to open and shut the entrance and 
exit pipes, to render the motion uniform and of the kind re- 
quired, and to utilize as much of the steam-force as possible. 
Fig. 62 will serve to illustrate the working of a steam- 
engine in a very primitive form. 

s represents the steam-pipe leading from the boiler. It con- 
ducts the steam to the valve-chesty v c. In this chest the slide- 
valve V moves to and fro, and opens and closes alternately the 
pipes M and n leading to the cylinder c. When one passage 
is open the other is always closed. 

In Fig. 62 the tube n is open for the passage of steam into 
the cylinder from the boiler. The tube m is closed to the 
steam coming from the boiler, but is open through the 
interior of the valve v to the air at o. 

The expansive force of the steam presses the piston 




FOURTH STAGD. 



135 



onwards towards r ; motion is given to the crank g, and the 
shaft A is half rotated. 

When the piston is at e (Fig. 63) the valve closes the 
passtige N to the steam-pipe, but forms a passage for exit to 
the air. At the same time it opens the passage m, to allow 




Fig. 62. 



the steam to force the piston in the contrary direction 
The double movement of the piston rod once forward and 
once backward completes one revolution of the shaft. 

This rotation sets the crank 
H in motion, and by means of 
a rod the valve v is moved 
to and fro. The cranks are 
so arranged that the valve 
shall move in a direction con- 
trary/ to that of the piston. 

w^ (Fig. 62) is a large and 
heavy fly-wheel, which serves to keep the motion of the 
whole regular and uniform. By means of a belt passing 
over this wheel motion may be communicated to any 
machinery desirable. 




Fig. 63. 



136 OBJECT LESSONa 

LESSON IX. 

CONDUCTION OF HEAT.— CLOTHING. 

Articles for illustration : metal wires or rods, glass rod, sealing-wax, 
and any of the conductors, bad conductoi-s, and non-conductors mentioned 
below. 

We have seen in a previous lesson one method by which 
heat is carried in liquids, viz. by convection ; but the mole- 
cules of a solid are not free to move, and so cannot carry 
heat by convection, 

I. Conduction. 

Exp. 142. Take a piece of copper wire four or five inches 
long, and hold one end in the flame of the spirit-lamp, 
Soon it gets too hot at the other end to be held in the hand. 
What does this teach us ? That the heat travels along the 
wire ; hut that it occupies a certain time in doing so. 

Exp. 143. Try a glass rod in one hand, and the wire 
in the other. You can hold the glass rod an inch or two off 
from the flame even after you have been obliged to drop the 
wire. What do we learn from this ? We learn that the copper 
icire conveys or conducts the heat more quickly than the glass. 

Heat, when it travels through bodies in this way, is said 
to be conducted. Those bodies which conduct heat quickly 
are called good conductors. Those which conduct heat slowly 
are called had conductors. Some conduct so little heat as to 
oe named non-conductors. 

II. Conductors. 

The teacher may now direct the scholars to test the con- 
ducting power of various substances, such as other metal 
wires, mercury in a narrow tube,* water in a test-tube, 
sealing-wax, wood, stone, earthenware, cork, leather, wool, 
&c., &o. 

• See Fig. 67, page 127. 





FOURTH STAGE. 




13; 


Arrange in three 


classes, writing names 


on the black 


board, thus: — 








Good Conductors. 


Bad Conductors. 


Non-Conductora. 


silver 


stone 




wool 


copper 


brick 




fur 


gold 


earthenware 




silk 


brass 


glass 




hair 


tin 


wood 




feather^ 


iron 


sealing-wax 




cork 




leather 




india-rubber 




linen 




air 




cotton 






Special attention 


should be called to 


air 


as a non-con- 


ducting body. 









III. Special uses of good and bad conductors. 

Vessels used for cooking purposes, for melting or boiling, 
need to be good conductors to let the heat pass through 
quickly. 

Tools used hot, such as soldering-irons, branding-irons, 
&c., must have non-conducting handles. These are generally 
made of wood. 

Clothing, especially for winter use, should be made of non- 
conductors, so that the heat of the body may not escape too 
quickly into the cold air without. 



The teacher may now get answers to such questions as 
the following: — 

1. Wh^ are fireproof safes made with double walls having 
the space between filled with sand, or some other bad 
conductor ? 

2. Whj/ do people wrap ice in flannel, or cover it with 
sawdust during hot weather ? 

3. Why are windows in cold countries made double ? 



138 OBJECT LESSONS. 

4. Two substances, say wool and marble, have the aarru, 
temperature^ as tested by a thermometer. When they are 
colder than the hand, why does the marble feel colder to the 
touch than the wool ? When they are hotter than the 
hand why does the marble feel hotter than the wool ? 

5. Why does a stone hearth feel colder than a carpet, or 
hearthrug ? 

6. ^Vhy are woollen or paper pads used for holding the 
handles of kettles ? 

7. Why are wooden handles fixed to coffee-pots ? 

8. Why are woollen and fur garments worn in cold 
weather ? 



LESSON X. 

RADIATION OF HEAT.— RADIATORS. 

Articles for illustration : rod of iron, three pieces of tin-plate [see 
below], three tin cans [see below]. 



I. Radiation. 

Bxp. 144. Heat a rod of iron — the poker for instance — 
red hot. 

Let children hold their hands near the poker ; the heat is 
felt in every direction at some distance from the poker. Or 
the teacher can show that a match will take fire when 
brought near without touching the poker. 

How does the heat get from the poker to the hand or the 
match ? Not by conduction^ for air is a bad conductor. And 
besides, it has been found by experiment that heat passes 
from one body to another quite as well in a vacuum. Again, 
heat can pass through bodies not in contact. For instance, 
the heat of the sun passes through our windows and warms 
our rooms. 

We .cannot tell for certain how the heat passes across 
from the warmer to the cooler body ; but we are quite sure 



FOURTH STAGE. 139 

that, like light, it passes in straight lines, or rays as they are 
usually called. The process of communicating heat from 
one body to another by rays is called radiation. The radiated 
heat which is taken up by other bodies is said to be absorbed. 

In the experiment the heat radiated from the red-hot 
poker is absorbed by other bodies around, and the process 
goes on until the poker and the other bodies in the room are 
of the same temperature. 

The teacher may show how heat is radiated from a body 
by sticking pins or needles into a ball of worsted, taking 
care that they radiate from the centre of the ball. 

II. Good and bad radiators. 

Exp. 145. Take three pieces of tin-plate, one bright and 
clean, another rusty and therefore rough, and paste brown 
paper over the third, or cover it with lamp-black. Place 
them out in the sun. or in front of a fire at some distance 
from it. The rusty and the covered plate will in a few 
minutes feel hotter than the polished plate. What do we 
learn from this ? That bodies having dark or rough surfaces 
absorb heat more quickly than bodies with polished surfaces. 

Exp. 146. Next take three "tin" cans of the same 
size. Cover one on the outside with brown paper, the 
second with lamp-black, leaving the third clean and bright. 
Fill each with warm water of the same temperature, and set 
them on wool mats, or several thicknesses of brown paper, 
and at some distance apart, and of course away from the fire. 
In the course of ten minutes or so test the water with the 
thermometer. The water in the bright vessel will be hotter 
than that in either of the others. What does this experi- 
ment teach us ? That rough and dark surfaces radiate heat 
more quickly than bright surfaces. 

And from the two experiments we learn that good radiators 
are good absorbers, and that bad radiators are haddbsorlevs. 



140 OBJECT LESSONS. 

Dark clothing absorbs and radiates heat more rapidly 
than light-coloured clothing. In very cold countries some 
animals, such as hares and foxes, change their colour in the 
winter and turn white. The advantage is that the heat of 
their bodies does not pass away so quickly through a white 
coat. 

Absorption and radiation are going on around us con- 
tinually. The earth absorbs the heat of the sun's rays during 
the day, and radiates it at night. As - the days in 
summer are much longer than the nights, there is more time 
for absorption than for radiation ; and as the days in winter 
are shorter than the summer days, there is less time for 
absorption in winter than for radiation, and thus absorption 
and radiation of heat account in part for the difference in 
summer and winter temperatures. 



LESSON XI. 

HOW HEAT AFFECTS THE ABSORPTION OF WATERY 
VAPOUR BY THE ATMOSPHERE. 

Articles for illustration : alum, water, spirit-lamp. 



The children are familiar with the fact that water, in the 
shape of vapour, is always present in the atmosphere, and 
that the amount is constantly varying, the pressure or 
weight of the atmosphere varying with it, as shown by the 
barometer. 

The aim of this lesson will be to show (1) that the 
warrner the air the more moisture it can absorb ; and (2) that the 
capacity for absorption increases in a greater ratio than the 
increase of temperature. 

I. The warmer the air, the more moisture it can absorb. 
Exp, 147. Dissolve alum in cold water till the water 



FOURTH STAGE. 141 

can hold no more. The water is said to be saturated. Heat 
the solution to about 120°. It can absorb more alum, but 
presently becomes saturated again. Heat the water to boil- 
ing point. It requires more alum again to make a saturated 
solution at the boiling point. Now allow the solution to 
cool. As the water cools some of the alum comes out and 
falls to the bottom or clings in pretty crystals to any rough 
surface. On the further cooling more alum comes out, until 
at freezing point the water holds very much less alum in 
solution than at higher temperatures. 

If we heat a saturated solution of alum at say 60°, to 
any temperature above 60° and then cool it again, no alum 
comes out before we arrive again at 60°, when, if we cool 
still more, a portion of the solid is deposited. 

Now air acts with regard to watery vapour in a way very 
similar to the action of water on alum. At any specified 
temperature the air can hold any portion of vapour up to a 
certain amount. When it is quite full and can absorb no 
more it is said to be saturated. If we increase the tempera- 
ture we enable the air to take up more vapour, but again 
its power of absorption is limited. The process of absorption 
stops as soon as the air becomes saturated. It is quite clear, 
then, that for every degree of temperature the air has a dif- 
ferent point of saturation. 

Just as with the solution of alum, when saturated air 
becomes cooled some of the water is forced or squeezed out. 

II. The capacity for absorption increases in a greater ratio 
than the increase of temperature. 

To make this very clear the teacher should take such an 
example as the following : — 

Suppose we have a cubical box 5 feet in the side. The 
air in this box at 32° Fahr., will hold about an ounce of 
water in the shape of vapour. That "is, an ounce of water- 
vapour will saturate 125 cubic feet of air at 32° Fahr, 



142 OBJECT LESSONS. 

Now if the temperature of the air be raised say 30° Fahr.* 
the air in the box can hold double the amount of moisture. 
viz. two ounces. Eaise another 30°, viz. to 92° Fahr., it can 
hold double the amount it held at 62° Fahr., that is, it can 
hold four ounces, and so on. For every 30° of added tem- 
perature the capacity for holding tcater is doubled. As a matter 
of fact, however, air is not often saturated. 

Given 125 cubic feet of air at 92°, holding two ounces 
of vapour, we may cool this air down to 62°, that is to the 
point of saturation with two ounces, before any water will be 
pressed out. If we cool still further some of the vapour 
will be given off, and will be seen as dew or fog, or in some 
other form. 

The children may learn from this lesson why water " dries 
up ** or changes to vapour more quickly on a day when the 
air is dry than when the air is humid or moist ; and why it 
changes to vapour more rapidly, as a rule, in summer than in 
winter. The further the atmosphere is from the point of 
saturation, and the higher its temperature, the more water- 
vapour can it absorb. 

With a little help the children will now be able to answer 
such questions as the following : — 

1. Where may we expect the air to be always nearest to 
its point of saturation ? 

2. Why are south-west winds in England moist and 
humid ? 

3. In what parts of the earth does the atmosphere hold 
the largest amount of vapour, and why ? 

4. What is the cause of the general complaint of the dry- 
ness of the air in rooms heated by stoves, or furnaces ? 

5. Why do people often put saucers containing water 
over stoves? 

• Aiithoiities differ somewhat both as to this number and to the amount 
of vapour which water can hold at 32" Fahr. 



FOURTH STAGE. 143 

LESSON XII. 

HEAT THE CAUSE OF MOTION IN THE AIR. 
Articles for illustration : glass trough (or substitute), oil, candla 



I. Recapitulation. . 

The teacher should first elicit from the children the two 
causes of variation in the weight of any given volume of 
the atmosphere, viz. heat and the amount of water-vapour 
present. Thus a cubic foot of warm air has less weight than 
a cubic foot of colder air, and a cubic foot of moist air has 
less weight than a cubic foot of dry air. 

Secondly, he should get from the children what must be 
the result when there is a mass of warm air in one place and 
cold air in another; or a mass of warm moist air in one 
place and cold dry air in another ; and generally that if 
there is lighter air in one part, and heavier in another, the 
colder air will rush in, and forcing up the lighter air occupy 
its place. 

Exp. 148. He may illustrate as follows : divide a 
glass trough in the centre with a fitted piece of cork ; fill 
one side with oil and the other with coloured-water and 
then remove the partition. The heavier water presses up 
the lighter oil and partly takes its place, the oil at the same 
time taking up some of the space at first occupied by the 
water. 

There are, in fact, two currents. The heavier water flows 
one way and the lighter oil the other. Much in the same 
way motion in the air is brought about 

II. Action of stoves and open fires. 

In warming a room the cold air is constantly pushing up 
the warmer air, and the upper part of the room consequently 
is always warmer than the part near the floor. 



144 OBJECT LESSONS. 

The teacTier may show the upward current of warm air 
from a lamp or a stove-pipe by holding in it some light 
substance, such as a feather. 

When there is a fire in the room the air is ever pushing 
towards it. There is a current of light because hot air 
going up the chimney, and air is coming in from every 
point, from every door and window, crack and crevice, to 
supply the place of that which escapes up the chimne5\ 

Hold a lighted candle near the firepLice ; the flame bends 
towards the fire. Hold it near the bottom of the door and 
it is blown inwards. 

Some of the warm air of the room, too, escapes through 
any opening near the ceiling, the top of the door for 
instance. Open, for about an inch, the door, of a room 
in which the air is considerably warmer than the atmosphere 
outside, and hold the flame of a candle near the top, it is 
blown outwards. Now hold it near the bottom, it is blown 
inwards. Why ? 

In an open fireplace some of the air of the room goes up 
the chimney above and not through the fire ; this is colder 
than the air which goes through the fire, and the current 
up the chimney therefore is not very rapid. But if you 
prevent this cooler air from going up by placing a 
" blower '* above the bars of the grate all the air must go 
through the fire. This makes the air hotter, and the 
current up the chimney becomes much more rapid. 

As we shall learn by and by, the more free is the supply 
of air to the fire, the more briskly the fire burns. This 
explains why a blower makes the fire burn more fiercely. 

III. Winds. 

The land and sea breezes will serve to illustrate how heat 
brings about winds. 

These winds blow only in warm regions of the earth and, 
a£> their name implies, near the sea. They are caused by 



FOURTH STAGE. 146 

the difference in the absorbing and radiating powers of land 
and water. 

In the evening, after a warm day, the land radiates its 
heat much more quickly than the water, and the air above 
the land therefore becomes cooled much more quickly than 
the air above the water. It follows that the cold air rushes 
from the land to displace the warmer air above the water. 
In the evening, therefore, the breeze is from the land. 

In the morning the air over the land becomes warmed 
quicker than the air over the water, and a breeze from the 
sea sets in.* 



LESSON XIII. 

DEW AND HOAR-FROST. 



I. Formation of Dew. 

From what has already been learnt about the radiation 
and absorption of heat by solids, and the absorption of vapour 
by the air, the teacher will be able to elicit from the 
children all the salient points in connection with the forma- 
tion of dew. 

He should refer first of all to the formation of water on 
the outside of a glass of cold water when brought into a 
warm room, to the formation of water on the windows of our 
houses, on the windows of closed carriages, and on the walls 
of our houses when a thaw sets in. The explanation is in 
all cases the same : the cold objects cool the air close to them 
till the point of saturation is reached, and then on further 
cooling some of the vapour is deposited on the cold body. 

We have seen that air absorbs and radiates heat very slowly. 

But many of the objects on the surface of the earth absorb 

and radiate very quickly. 

* It is a matter for the discretion of the teacher whether or not to pursu 
this subject iurther at this stage. 

L 



146 OBJECT LESSONS. 

On warm sunny days the earth and all the objects on its 
surface absorb heat. But when the cool evening comes this 
heat is radiated into the air, and the warmed air rises and 
cooler air from above takes its place. This goes on until 
the air in contact with the now cold bodies gets cooled so 
far that it fails to retain all the moisture which it held when 
warmer, and the moisture — squeezed out as it were — is 
formed on the cool bodies in little drops, which we call d£w, 

II. How the formation of dew is promoted or retarded. 

This is the general explanation of the formation of dew ; 
but many circumstances assist or retard its formation, which 
it will be interesting to note. 

1. Dew is formed most abundantly when a fine, clear, cool 
night follows a hot sunny day. [Such days are most 
common in autumn.] On a clear cool night the earth 
radiates its heat very freely. 

2. Little or no dew is formed on a cloudy or windy night. 
Clouds radiate heat back again and prevent its going into the 
cold atmosphere above. Winds keep the air mixed so that 
the lower stratum is not kept near the earth sufficiently long 
to be cooled below its point of saturation. 

3. Dew is deposited freely on grass, or on wool, or mats ; 
while little or none is deposited on stones, or on the gravel- 
walk. Grass and wool absorb, and hence radiate heat more 
freely than the stones and gravel. 

4. Dew is never deposited under cover. The deposit of 
dew under trees is very slight. The covering above radiates 
the heat back again. 

5. When the air is brought into contact with bodies cooled 
below 32° Fahr., the moisture as it forms on the cold bodies 
becomes frozen, and hoar-frost is the result. 

6. A very slight covering — even paper or muslin — serves 
to protect shrubs and plants from frost by preventing or 
retarding the radiation of beat from the plant. 



FOURTH STAGE. 147 

7. WheD the air is very full of moisture, and the night 
has been very calm and the radiation consequently very 
abundant, the chill is so rapid that the vapour is condensed 
more quickly than it can be deposited, and a mist or fog is 
formed. This mist prevents any further radiation of heat 
from the earth. 



LESSON XIV. 

RAIN, SNOW, HAIL, SLEET. 



I. How rain is termed. 

The teacher will refer to Lesson YIII. of this stage for the 
general law on which the formation of rain depends, viz. 
that the capacity of air for moisture increases or decreases in a 
greater ratio than the temperature. 

Suppose we have three cubical boxes of air each 5 feet in 
the side, and therefore containing each 125 cubic feet of air. 
We will further suppose that the air in each is saturated with 
moisture, the first at a temperature of 32° Fahr., the second 
at 62°, and the third at 92° 

If the air at 32° contains an ounce of water- vapour, then the 
air at 62° contains two ounces and the air at 92° contains/o^^r 
ounces. That is, in the three boxes of air, containing in all 
375 cubic feet, there are seven ounces of water. 

Let us agree to mix the air in these three boxes. Then 
we shall have 375 cubic feet of air at 62° [because 62° is the 
average of 32°, 62°, and 92°.] 

How much water will 375 cubic feet of air contain at 62°? 
Clearly six ounces, because 125 cubic feet can hold only two 
ounces. 

But we have seen that, before the three boxes are mixed, 
they contain together seven ounces, hence when mixed they give 



149 OBJECT LESSONS. 

up one ounce. Tliia one ounce is squeezed out, because there 
is no room for it, and it becomes first a mist and then rain. 

It must not be supposed that air is always saturated with 
water- vapour. This is far from being the case, or rain would 
be constantly falling at every slight change of temperature. 
But air at varying temperatures and holding varying 
amounts of water- vapour is being constantly mixed by winds, 
and whenever the mixture becomes cooled below its point 
of saturation, the excess forms mists (clouds); on further 
cooling the tiny particles of mist join together and form rain. 
But if the mist becomes cooled below 32° Fahr., then it is 
frozen into snow. 

II. duestions on interesting facts connected with tlie formation 
of rain. 

1. WTiy does rain fall in drops ? Because the particles 
attract each other, and those that are near combine and form 
drops. 

2. How is it that the cold night does not always cause 
rain ? Because the air is not always near saturation, and it 
can thus be chilled, and yet hold its vapour. 

3. What is snow ? Condensed vapour frozen by contact 
with air below 32*^. 

4. What is hail ? Hail is rain frozen by passing through 
a stratum of air below 32° Fahr. in its descent. 

5. How is sleet formed? Sleet is formed when snow 
in its descent passes through a bed of air above 32° 
Fahr. The snow is partially thawed and falls in a half- 
melted state. 

6 Why is there no snow in summer time? Snow is 
formed in the upper regions of the atmosphere as well in 
summer as in winter, but in summer it becomes melted in 
its descent through warm air. When rain falls in the valleys 
in Switzerland in summer, the tops and sides of the moun- 
tains often receive a coating of snow. 



rOUKTH STAGE. 149 

LESSON XV. 

SPECIFIC HEAT. 

Articles for illnstration : 4-oz. pieces of lead and iron, 4 ozs. of mer- 
cury, a small tin vessel, a thermometer, and some apparatus for boiling 
water. 



I. What is specific heat? 

Exp. 149. Take equal weights, say 4 ozs., of lead, iron, 
and mercury, and heat them for some time in boiling water. 
[The mercury may be held in a test-tube.] 

The three metals will have their temperatures raised to 
212^. Next take three vessels, each containing say 4 ozs. 
of water at the ordinary temperature of the room, say t>5>^, 
and transfer the metals to these vessels. Each of the solids 
will, of course, raise the temperature of the water ; but they 
will not raise it equally. The lead will raise it least, the 
mercury will raise it a little higher than the lead, and the 
iron considerably more than either. 

We may now add 4 ozs. of water at 212" to 4 ozs. at 55^, 
and we shall find that the hot water raises the temperature 
of the cold far more even than the iron. 

What do we learn from this experiment ? 

We learn that some bodies can hold more heat than others. 
The iron evidently retained more of the heat got from the 
boiling water than the lead, for it gave more to the cold 
water. 

If lead takes up less heat than the iron, we should expect 
that it will take less time to reach a certain temperature than 
iron. And this is so. 

Exp. 150. Take the lead first ; put it in a tin vessel and 
hold it over boiling water till it reaches a temperature of say 
180°. [To test the temperature, keep the bulb of a ther- 
mometer touching it.] Note the time. Now test the iron 



150 OBJECT LESSONS. 

in the same way and note the time. As the iron requires 
more heat to raise it to 180*^, it takes considerably more time. 
The teacher may also test the converse by noting the time 
it takes each metal to cool. The lead holding less heat cools 
first. 

Exp. 151. "We can show that some bodies hold more heat 
than others in another way. 

Take an ounce of water at 112°, and an ounce at 40°, we 
get two ounces at 76°, as may be shown by the thermometer. 

Now take an ounce of mercury at 112° and an ounce of 
water at 40° ; we have a mixture of two ounces, but the 
temperature will be only about 42°. The hot w^ater raised 
the cold through 36°, while the hot mercury raised it only 
about 2"*. 

Bodies, then, differ in their power of taking in and holding 
heat, and the amount of heat which a given weight of a body 
takes in to raise through a given range of temperature is 
called the specific heat of the body. A pound weight, and one 
degree are taken as the units. 

II. The importance of water having a high specific heat. 

The ocean covers four-fifths of the earth's surface, in some 
places to the depth of several miles, and this forms an 
enormous storehouse of heat. It takes up an immense 
quantity of heat without rising much in temperature, and 
yields it up again when required, without itself being lowered 
much in temperature. 

The great specific heat of water is therefore the chief 
agent in equalizing the temperature of the globe. 



FOURTH STAGE. J 51 

LESSON XVI. - 

LATENT HEAT. 

Articles for illustration : Florence-flask, spirit-lamp, lumps of ice, 
thermometer. 



I. What is latent heat ? 

Exp, 152. Take a Florence flask, half fill with water, 
and heat to boiling point over the spirit-lamp. Note 
the rise of temperature by setting a thermometer in the 
flask. The mercury rises steadily in the column till it 
stands at 212°. At this point it is stationary, and no amount 
of heat further applied under the ordinary pressure of the 
atmosphere will make the water rise above 212°. 

What then becomes of the heat we continue to apply 
during the boiling of water ? We answer this question by 
asking another. What change does the continued applica- 
tion of heat to water at 212° bring about ? Clearly the 
change in the state of water from liquid to gas. The heaty 
then, is used up in the process of changing water from the 
liquid to the gaseous state. But the steam is no hotter than 
the water. The heat seems to disappear ; at any rate it has 
no efiect on the thermometer, and hence we call it hidden, or 
latent heat. 

Exp. 153. Now half fill a vessel with cold water, and put 
in a few lumps of ice. With a thermometer again note 
the change of temperature. The column of mercury sinks 
gradually to 32°, where it remains until all the ice is melted. 
Even on the application of heat, the water shows no increase 
of temperature until the ice has disappeared. 

If ice or snow at, say 20°, be placed over a fire, the ther- 
mometer will show an increase of temperature till the 
mercury reaches 32°, but there it will stand so long as ice 



152 OBJECT LESSONS. 

or snow remains ; but when all the snow and ice are melted, 
the temperature gradually rises till it reaches 212°. 

Here, again, much heat is consumed in the process of 
changing water from the solid to the liquid state, and the 
heat used up does not affect the thermometer. It is hidden 
away or made latent, 

IL Latent heat of water. 

The teacher can next give the children an idea of how 
much heat is used up, or made latent, in changing ice to 
water, and water to steam. 

Exp. 154. Mix an ounce of water at 32° with an ounce at 
174°, and we have two ounces of water at 103° ; but mix an 
ounce of pounded ice at 32° with an ounce of water at 174°, 
and we get two ounces of water at 32^ ; that is, no less than 
142° of heat have been taken from the ounce of water to melt 
the ice. Of course, a body in falling through 142° of tem- 
perature must give out just as much heat as it takes in in 
rising through 142° of temperature. 

We may say, then, that the amount of heat required to 
melt a pound of ice is equal to that required to raise a pound 
of water through 142°, or equal to that required to raise 
142 lbs. of water through 1°. 

Thus the latent heat of water is said to be 142°. 

III. Latent heat of steam. 

The children will tiave noticed, probably, how much longer 
it takes water to " boil away," viz. change to steam, than it 
does to raise it to the boiling point from zero — about five 
times as long. The teacher may show this fact by experi- 
ment ; but an experiment to show the absolute amount of 
heat made latent in the change of water to steam will 
probably have to be described. 

An ounce of steam at 212° — in other words, an ounce of 
water changed to steam — if passed into 298 ounces of cold 



FOURTH STAGE. 153 

water will raise its temperature 1^. That is, the latent heat 
of steam is 298°. In this case we have recovered the heat 
which was latent. 

Different bodies vary very much in the amount of heat 
they make latent on passing from solid to liquid, or liquid to 
gas. Thus the latent heat of alcohol is less than one-half 
that of water, while that of ether is less than one- sixth. 

IV. The advantages we derive from the high latent heat ol 
water and steam. 

1. It takes a considerable quantity of heat to melt ice, and 
hence it takes a considerable time to complete the change. 

If it were not so the winter ice and the snow of the 
mountains and high valleys would melt too quickly, produc- 
ing overwhelming torrents and floods. 

2. Similarly in the case of steam, if it were generated too 
quickly, we should be much more liable to dangerous 
explosions. 

The teacher may with advantage still further enlarge on 
the special properties of water as tending to prevent sudden 
changes of temperature. 



LESSON XVII. 

COOLING BODIES AND FREEZING MIXTURES. 

Articles for illustration : small quantity of ether, snow or ice, am- 
monium nitrate and chloride, and thermometer. 



This lesson consists of interesting applications of principles 
enunciated in former lessons ; and, given the facts as shown 
by experiment, the reasons may be elicited by questioning. 

I. Cooling bodies. 
Exp, 155. Pour a little ether on the palm of the hand ; 



154 OBJECT LESSONS. 

it quickly evaporates and produces a painful sensation of 
cold.* 

** In what state was the ether when I poured it into Tom's 
hand ? " In a liquid state. 

** In what state is it now it has gone away into the air ? " 
In the state of vapour. 

'' To change a liquid to a gas or vapour, what is neces- 
sary?" Heat. 

*' Where does the liquid ether obtain its heat ? " From 
the hand. 

"Then what makes the hand feel cold?" The heat is 
taken away to change the liquid ether to vapour ofitJier. 

On this experiment the teacher may deal with the 
following : — 

1. Why we put aromatic vinegar and water on the head 
when it feels hot and feverish. 

2. How perspiration cools the body. 

3. Why ladies use fans in hot rooms. 

4. Why a windy day feels colder than a still one when 
the temperature, as shown by the thermometer, is the same. 

5. How a shower of rain cools the atmosphere. 

6. Why sprinkling a floor with water cools a room. 

II. Freezing mixtures. 

Exp. 156. Make a mixture of three-parts by weight of 
snow or pounded ice, and one part of crushed common salt. 
The two solids liquefy, and if a small bottle of water is 
placed in the mixture the water will become fmzen, even if 
the mixture is placed in front of the fire.f If tested with the 
thermometer the temperature will be found many degrees 
below freezing point. 

• Put a drop or two of water on half an ounce of carbon disulphide in a 
shallow vessel. Place in current of air. The water chnnges to ice. 

t Make a mixtur*i (^f two parts by weight of pulverised ammonium nitrato 
and one part of Hmnionium chloride, and dissolve in three parts of wat'-r. 
Stir the mixtures with a test-tube containing a little water. The water in the 
test-tube will be Irozen, 



FOURTH STAGE 156 

** In what state were the snow and salt before mixing ? " 
In the solid state. 

" What change was brought about on mixing P " They 
were changed to a liquid. 

" What was necessary to produce the change ? " Heat. 

" Whence could the bodies obtain their heat ? " Only 
from themselves and the vessel in which placed. 

" How can you show that the heat was abstracted from 
the bodies themselves ? " Because they became much colder. 

" If you put your finger first in the freezing mixture and 
then into some snow at about 32°, which will feel the warmer, 
and why ? " The snow, because none of its heat has been taken 
away. 

On this experiment the teacher may ask : — 

1. Why people should not throw salt on the pavement in 
frosty weather to melt the snow or ice. 

2. Why the air often feels cold and chilly when a thaw 
sets in. 

3. Why the temperature often rises after a fall of snow. 



LESSON XVIII. 

SPECIFIC GRAVITY OF SOLIDS. 

Articles for illustration : balance, and specimens of same size for 
weighing, a short tube of lead with solid cylinder to fit exactly, or an 
equivalent. 

The teacher will refer to the lessons on gravity and buoy- 
ancy of liquids as an introduction to this lesson, and then 
tell the class that it is very convenient for us to be able to 
compare the weights of equal volumes of different bodies. 

I. Standard of comparison. 

To be able to compare the weights of bodies we must 
make some one body the standard for comparisou. 



156 OBJECT LESSONS. 

Take pieces of, say, wood, cork, and lead of equal bulk, 
and ask the children to compare their weights. They will 
say at once that lead is heavier than wood, and wood than 
cork, but Jioic much heavier they cannot say. 

Distilled water at a temperature of 39°* is the standard of 
comparison used in this country. 

IT, How to compare the weight of any solid with the weigh tof 
an equal bulk of water. 

To show this roughly, but, at the same time, very clearly, 
the teacher must arrange to measure a bulk of water equal 
to the bulk of the solid selected for experiment. 

For instance, a half inch lead tube two or three inches long, 
firmly closed at one end with a piece of hard wood. A solid 
cylinder of lead to fill this tube exactly may be made by 
pouring in molten lead, or a small earthenware vessel may 
be filled with melted sealing wax, or a solid glass rod may 
be found to fit with tolerable exactness into a glass tube. 

Example 163. Suppose we take lead for the experiment. 

Drop the solid cylinder of lead by means of a piece of 
thread gently down into a tumbler filled with water to the 
brim, and collect the displaced water which runs over into a 
vessel below. 

Pour the collected water into the lead tube ; it very nearly 
fills it. It would quite fill it were it not that we lost some 
on the outside of the tumbler and on the basin below. Now 
weigh this water. Say it weighs 1 oz. 

Now weigh the lead, first in the ordinary way. It will 
weigh a little over 11 ozs. Now weigh it in water. It 
weighs a little over 10 ozs. That is, it loses a weight of 
1 oz., or the weight of the water (Unplaced, 

[For method of weighing in water (see Fig. 63). a is a 

weight equal to the scale-pan which has been removed.] 

* At this temperature water is at its maximum density ; that is, it is heayier 
«t thia temperature tlian at any other. 



FOURTH STAGE. 



157 



From these experiments we learn — 

1. That when placed in water a solid displaces a volume 
of water equal to its own volume. 

2. That a solid loses weight when weighed in water. 




Fig. 64. 

3. That the weight of water displaced is equal to the 
difference between the weight of the solid in air and in water. 

In the case of the lead the weight of the lead in air was 
a little over 11 ozs., and the weight of a volume of water 
equal to that of the lead was 1 oz. Clearly therefore lead 
is a little over eleven times the weight of water. And this is 
called its oicn iveight compared icith watery or its specific gravity. 

If glass had been used instead of lead, we should have 
found its specific gravity to be 2 J. That is, glass is two and 
a half times as heavy as water. Platinum, the heaviest of 
metals, is twenty-two times, and gold nineteen times, as heavy 
as water. Hard oak is a little heavier than water, the other 
common English hard woods a little lighter than water. 
Cork has a specific gravity of J. 

If the children are sufiiciently advanced the teacher may 
from the above experiments deduce the rule for finding tho 
specific gravity of solids heavier than water. Divide the 
weight in air hy the loss of iveight in water. The quotient is 
the specific gravity. 



SUPPLEMEISTTARY LESSONS ON PROPORTION. 

In Stage V. many of the lessons, especially those on 
machines, require a clear understanding oi proportion. 

If the Stage coincides with Standard VI., as is most 
likely, it may be desirable to rationalise the proportion that 
comes under the head of arithmetic. 

A sense of proportion is also one of the most important 
developments of human faculty. Morals (science of what is 
due) depend largely on it. ^Esthetics are scarcely anything 
else than a fine sense of proportion, e.g.^ in architecture, music, 
&c. The comfort of social life depends on " give and 
take,'* and this is taught only by an inherent sense of 
proportion. 

For these reasons the subject ought to have a more 
prominent place in school teaching. And as in its most 
elementary form proportion appeals to the eye and to 
other senses, its apprehension may be greatly assisted by 
object lessons. 



LESSON I. 

Articles for illustration : Two shades of red and two sTiarles of blue 
colours ; blocks ol wood, or substitute, weighing respectively 6 lbs., 3 lbs., 
2 lbs., and 1 lb. 



1 Comparison of Colours. 

Set before the class two patches of some colour — rod, for 
instance — one light and one dark. Mark them A and b. 



SUPPLEMENTARY LESSONS. 159 

" What colour is the piece marked a ? " Red. " And 
the piece marked b ? " Red. 

" Do you see any difference in the shade of colour in the 
two pieces ?" Yes, that marlied b is darker than the one 
marked a. 

Next take two shades of another colour, say blue, and 
mark titeic c and d. 

" What dixterence do you note in these two colours which 
I have marked c and d ? '* d is darker than c. 

** Well, now can you tell me how much darker b is than 
A, or D than c ? " No. 

" No, because you have no means of exactly measuring 
colours." 

" Can you tell me whether d is as much darker than c as 
B is darker than a ? No, and for the same reas£>n — you have 
no means of exactly measuring colours. If you could say 
that B was twice as dark as a, and that d was twice as dark as 
c, then you could answer. Yes, D is as much darker than c as 
B is darker than a." 

" We will now take another sort of comparison which you 



II, Comparison of Weight. 

Take four blocks of wood, weighing respectively 6 lbs., 
3 lbs., 2 lbs. and 1 lb. [Of course, weights of any more con- 
venient substances may be substituted. ] 

Direct a boy to compare as best he can, by holding in his 
hand, the weights of the larger pieces, and then of the 
smaller pieces. Now weigh the blocks, and lead the children 
to compare their weights. Thus, the weight of the first 
block is twice the weight of the second, and the weight of 
the third block is twice the weight of the fourth. 

m. :Ratio. 

"What difference or relation is there between the first 



160 OBJECT LESSONS. 

block and the second as to their weights ? " The first ia 
twice (or 2 times) tJie second. 

** The number 2 then represents the relation between 6 lbs. 
and 3 lbs., and we call 2 the ratio of 6 lbs. to 3 lbs." 

" Now what is the ratio of 2 lbs. to 1 lb. ? Clearly 2, because 
the first is ttcice the second. That is, the ratio of 6 lbs. to 
3 lbs. is equal to the ratio of 2 lbs. to 1 lb. 

The teacher may now explain (1) that to find the ratio of 
two numbers we always divide the first number by the 
second. The quotient is the ratio, (2) That relation can 
exist only between things of the same kind. There can 
be no ratio between lbs. and feet, or between cats and 
shillings.* 

Questions such as the following will fix these ideas in the 
minds of the scholars. 

1. What ratio is there between — 



2. 



(1) 12 lbs. and 6 lbs. ? 

(2) 6 lbs. and 12 lbs. ? 

(3) 3d. and9d.? 

(4) 8d. and 2d. ? 

(5) £10 and £1 ? 
(G) £2 and £9 ? 


Ana 

» 
» 

99 


. 2 

i 

J 
4 

10 


s the ratio of — 






(1) 5 pks. to 2 pks. 

(2) 19 yds. to 57 yds. 

(3) 21 horses to 7 horses 

(4) 16 feet to 6 feet 

(5) J inch to I inch 

(6) 3 cows to £5 


Ans. 

99 


2 

i 
3 

H 

2 
Impossible. 


• fcJoe note, page 162, 







SUPPLEMENTARY LESSONS. 



161 



LESSON II. 

I. Pnrther Illustrations of ratio. 

Draw squares on the blackboard, the first, A, 2 feet in the 
side, and divide it into 6-inch squares ; the second, b, and 



A 






U 



the third, c, each 1 foot in the side, and divide into 6-inch 
squares ; and the third a square 6 inches in the side. 

The scholars will see at a glance that the square a is four 
times the area of the square b ; and also that the square c is 
four times the area of the square d. 

That is, the ratio of a to B is 4, and the ratio of c to d is 4. 

In other words, we may say that the ratio of a to b is the 
same as the ratio of c to d, or there is an equality of ratios. 

Take another example. 

Draw three equal squares, and then nine of same sizf> 
arranging them as in Figs, a and b. 



A 














B 










1 

























In this example clearly the ratio of A to b is f or J. 
Now draw two lines and divide as in c and d. 



162 OBJECT LESSONS. 

C D 



4 12 

In the second example tlie ratio of c to d is -pj or J. 
That is, the ratios in each case are the same. Here is 
again an equality of ratios, 

II. Proportion. 

Now, when there is equality of ratios, the numbers which 
represent area, or weight, or length, or anything else, are 
said to be in proportion. 

If 10 men can dig 15 acres in a certain time, 5 men, 
working at the same rate will dig 7^ acres in the same 
time ; the quantities of work done are in proportion to the 
number of men employed. 

The ratio of 15 acres and 7^ acres is 2, and the ratio of 10 
men and 5 men is 2. 

This may be written — 

15 acres is to 7^ acres as 10 men is to 5 men. 

Or in short — 

Acres Acres Men Men 

15 : 7i :: 10. : 5 

Again, if 7 men earn £3 3s. (viz. 9s. each) in a given time, 

6 men should earn £2 14s. (viz. 9s. each) in the same time. 

The ratio of 6cJs. to 548. is f ^ = -J, and the ratio of 7 men 

to 6 men is -J. 

8. 8. Men Men 

or 63 : 54 :: 7 : 6 

Now suppose the 6 men receive only £1 78. (or 48. 6d. 
each) for their work, this would be unfair, it would be out oj 
proportion ; work and pay should be on the same proportion. 

The teacher may show this ns follows : — 

The ratio of 63s. to 27s. is \ or 2^, and the ratio of 7 men 
to 6 men is i or H. 



SUPPLEMENTARY LESSONS. 163 

There is no equality of ratios, and hence there is no pro- 
portion. 



LESSON III. 

I. Terms. 

" What is the ratio of 3 to 2 ? *' f or li. 
" What is the ratio of 6 to 4 ? " f or li. 
The ratios being equal, we have this proportion — • 

3 : 2 :: 6 : 4 

Tell the children that the numbers which form the pro- 
portion are called termSy and they are named \st, 2nd, ord, 
and 4ith, in the order in which they are placed. 

Thus — 1st term : 2nd term \\ 3rd term : 4th term. 

The 1st and 4th are called extreme terms, and the 2nd 
and 3rd are the mean terms. 



II. The product of the extremes equals the product of the 
means. 

The teacher may show this by taking any number of 
actual examples, or by some such method as the follow- 
ing :— 

Draw lines a, b, c, d. 

I — i i i A I ' ' '  '  

I t I B I I ' « « D 

Thus A represents 3 units, b 2, c 6, and d 4. 
Then a : b I : c : d, because 3 : 2 : : 6 : 4. 
Next draw two parallelograms with sides a and d and 
B c respectively. 



164 



OBJECT LESSONS, 



B 

I I I 



The parallelogram a D is equal in area to the parallelo- 
gram B c. Note that the length of the sides a and d are 
the two extremes in the above proportion, and b and c the 
two means. 

Show that these figures are equal in area, each containing 
12 equal squares. 

The parallelogram formed by the lines which represent 
the extremes has the same area as the parallelogram formed 
by the lines which represent the meam. 

3 X 4 = 12 and 6 X 2 = 12 



Where proportion exists the product of the extremes is equal 
to the product of the means. 

An example. 

If 24 yds. of cloth cost 96s., what must I pay for 19 yds. 
it the same rate ? 
Here the first ratio is — 

24 yds. : 19 yds. =^^ 

And the second ratio is — 



968. : a;s.=^ 



96 



24 96 
And rji =■ —y or the terms would not be in proportion, 
xy X 



SUPPLEMENTARY LESSONS. 165 

We may write the proportion — 

24 : 19 :: 96 : a? 

Or 19 : 24: :: X : 96 
And 24 iZJ = 96 X 19 
. ^ _ 96 X 19 _ ^. 

III. Given three terms of a proportion, to find the fourth term. 
Take the proportion 3:4 : : 6:8. 

(1.) To find the ^rs^ term. Put x to represent it. 

Theniz? : 4 :: 6 : 8 
„ 8 a? = 24 
X = 3 

(2.) To find second term — 

3 : ip :: 6 : 8 

6X — 24: 

ip = 4 
(3.) To find third term— 

3 : 4: :: X : S 

4x = 24 
a; = 6 

(4.) To find fourth terra — 

3 : 4 :: 6 : 0? 

3X = 24: 
X = S 



LESSON lY. 



The teacher may further illustrate the idea of proportion 
in a variety of ways. 

The following are sugggestions ; — 



166 OBJECT LESSONS. 

(1.) Rates are paid in proportion to the yearly value of 
the house. 

Value of Value of 

large house I small house 1 1 large rate • small rate 

£100 : £20 :: £5 : £i 

(2.) The value of men's work being equal, wages are paid 
in proportion to time worked. 

longer time : smaller time 11 larger wage : smaller wage 
20dys. : 5dys. :: £5 : £1J 

(3.) We say men are well or ill proportioned. What does 
this mean ? We have an ideal in our minds which pleases 
the eye, and with this we compare. Say, for instance, that 
a 6-foot man has an arm which measures 32 inches, and this 
to the eye is a pleasing ratio (72 ins. : 32 ins). 

Then a man measuring 5 feet 3 inches (or 63 ins.), should 
have an arm 28 inches long to be in the same pleasing 
ratio ; — 

For 72 ins. : 32 ins. :: 63 ins. : 28 ins. 

Suppose the arm of second man to measure 30 inches, then 
we should say it is too long for the body or the body too 
short for the arm ; the lengths of the body and the arms 
are not well proportioned, 

(4.) This kind of comparison is of constant recurrence. 

(a.) Size of clothing must be proportioned to the size 
of the wearer. 

(b,) Work assigned must be proportioned to the strengh 
of the worker. 

(c.) The strength of different parts of the bodies of 
animals is proportioned to the work they have 
to do. The neck of the elephant, for instance, 
is proportioned to the size and weight of the 
head it has to carry, and the work the trunk has 
to perform. 



SUPPLEMENTARY LESSONS. 167 

{d.) The empire of man over the brute force of the 
lower animals is proportioned not to his strength, 
but to the knowledge he possesses of their re- 
spective constitutions. 

{e.) In architecture there must be a just proportion 
between the parts. 

(/.) In sculpture, and in painting, there must be a 
just proportion of the several parts to one 
another and the whole. 

If further elucidation is necessary, the proportional com- 
passes will be a good subject for another lesson. 




FIFTH STAGE. 



FIFTH STAGE. 



LESSON I* 
FORCE. 



Articles for illustration : a glass tube and pith ball and a piece of silk, 
or a magnet and iron tilings. 



I. Definition of force, 

Exp. 164. Rub a dry glass rod briskly witli a warm 
silk handkerchief, and then present one end to a pith* ball 
suspended by means of a silk thread. The ball is at first 
drawn towards the rod, but after touching it is pushed away. 

Exp. 165. Or, strew a few iron filings on a sheet of 
paper placed over a bar, or horseshoe, magnet. The filings 
arrange themselves in lines diverging from the ends of the 
magnet. 

Now what causes the pith ball, or the iron filings to move ? 
To this question we can give no satisfactory answer. We 
know that the objects move, and we are quite certain there 
must be a cause; but there our knowledge ceases. This 
cause we call 2i force. 

When a body moves we know there must be a cause, and 
when a body in motion is brought to rest we know there 
must be a cause. When there is a change in the motion of a 
body there must be a cause. When a body is held in a 
particular position we know there must be a cause. 
* Cut from the pith of a branch of the elder-tree. 



172 OBJECT LESSONS. 

We attribute motion, and rest, and change, and position to 
some force. 

Force is that trhich can produce, change, or destroy motion. 

II. Kinds of forces previously described. 

Having demonstrated in this simple way what we mean 
when we speak of force, the teacher will assist the children 
to recall to their minds such forces as have been described in 
preceding lessons. These are : — 

1. Force of cohesion. 

2. Force of adhesion. 

3. Force of capillary attraction. 

4. Force of gravity. 

A few questions will serve to refresh the memory on the 
chief points connected with cohesion, adhesion, and capillary 
attraction ; but before proceeding to deal with another 
force — the chemical force — it will be well now to explain 
the phenomenon of gravitation a little more fully. 

III. Attraction of gravitation. 

It is a law of nature, so far as we know, that all bodies, 
great and small, attract each other ; and, if they are free to 
move, will move towards each other with increasing swift- 
ness until they meet. 

Thus (Fig. 65) a and b are balls of equal size and 

A c B 



weight. Hence they attract with equal force, and would 
meet at c, a point midway between them, if there were no 
preventing cause. 

But the fact is the earth attracts the balls, and being so 
many times larger attracts with so much greater force that 



FIFTH STAGE. 173 

the balls have no power to move except towards the earth. 
And this is the case generally. It is not that the earth 
simply draws all bodies on its surface to itself ; but that all 
bodies pull the earth towards themselves just as the earth 
pulls them. Only that the earth is so many millions of 
times larger than the largest body on its surface, that the 
effect of the pull of the latter cannot be felt or measured. 
It is as though an elephant were pulling at one end of a rope 
and a fly at the other. 

[It may be interesting to the children to learn the 
rapidity or velocity of a body moving towards the earth. It 
moves through 16 feet in the first second : for every suc- 
ceeding second it moves with a greater velocity. In the 
second second it travels 3 X 16= 48 feet, in the third second 
5 X 16 = 80 feet, in the fourth second? X 16 = 112 feet, 
and so on. If a stone be dropped from the top of a tower 
and it occupies two seconds in falling, the height is 2 x 2 
X 16. If three seconds 3 X 3 X 16. If four seconds 4 X 
4 X 16, and soon.] 



LESSON II. 

CHEMICAL ATTRACTION. 



Articles for illustration : the articles will, of course, depend on the 
experiments selected. The first experiment is the most striking, but it 
requires a little care. 



Exp. 166. Place "flowers'* of sulphur in a small flask, 
and drop in a few bright copper shavings. Heat the mix- 
ture over the spirit-lamp ; but place the whole apparatus 
in a pan or tub with a little water at the bottom in case the 
flask should break. 

Heat gently. The sulphur melts, then blackens and 
boils. The copper now becomes red hot, when the lamp 




^^^ 



174 OBJECT LESSONS. 

may be removed. The copper and sulphur unite together to 
forji an entirely new substance, giving 
off intense light and heat in the pro- 
cess. 

&p. 167. On a small bit of 
phosphorus* place a few grains of 
iodine. The two combine and burn 
with a smoky flame. 

IJxp. 168. Pass carbonic acid gas 

through lime - water (see Lesson 

Fig. 65. XIII., page 60). The gas combines 

with the lime in the water to make chalk, which sinks to the 

bottom as a white powder. 

Uxp. 169. Put a small piece of the metal sodium on water 
in a broad shallow vessel. The metal becomes spherical 
in shape and runs about over the water but gets less 
and less until it finally disappears. The metal takes some- 
thing from the water, of which we shall learn more in a 
future lesson ; and forms with it a new substance which 
dissolves in the water. 

Uxp 170. Mix ammonia gas with hydrochloric acid gas 
(see Lesson VII. , page 49) ; a new substance is formed quite 
different from either of the gases. 

From one or more of these experiments, or from some 
similar experiments, the teacher will lead the children to 
see that some substances have such an attraction for each 
other that when placed together they unite to form a 
separate and distinct substance. Here, then, we have 
another force, which is called chemical affinity, or chemical 
attraction. When placed close together under favourable 
circumstances many substances unite to form others. 

In the first experiment heat had to be applied to start the 

combination of the sulphur and copper, and we may say 

* Phosphorus should always )>e kept undf^r water. When a small piece 
has t » be cut oflF hold the stick with wet blottiuti -paper. 



FIFTH STAGE. 175 

generally that heat brings about and promotes the chemical 
combination of substances. 

Sometimes, however, heat will separate the substances 
which are joined to form another substance. 

Exp. 171. Heat a little of the red oxide of mercury in a 
test-tube. A gas is given off, whose presence is detected 
by the re-kindling of a glowing chip of wood placed in the 
tube, and liquid mercury is left behind. 



LESSON III. 

ELEMENTS AND COMPOUNDS. 

Articles for ilhistration : dioxide of manganese, hydrochloric acid, 
copper foil, or powdered antimony. 



I. Elements and compoxinds compared with letters and words. 

The children will most readily grasp the meaning of 
elements and compounds in the language of chemistry by a 
comparison with the letters and words of our written language. 

Take a page of any book ; no matter how many words it 
contains, there can be no more than twenty-six letters in the 
page. So of the whole book ; there may be thousands of 
words, but they are all built up of two or more of the 
twenty- six letters of the alphabet. 

Again, some of these letters are used very often, others very 
seldom. For instance the letters, o, a, e, &c., are constantly 
recurring, whereas the letters 2, a?, and q, occur but seldom. 

Again, the same letters arranged in different order make 
different words. 

Lastly, words vary in the number of letters which form 
them, from two or three up to a dozen or more ; but the larger 
number of words are formed of the smaller number of letters. 



176 OBJECT LESSONS. 

Now if we cal! the letters of the alphabet the elemonts of 
our written language, we may call words the compounds y 
because they are built up or compounded of the letters. 

The various substances — solid, liquid, and gaseous — which 
form nature's great book — the world — are like words. 
They are, for the most part, built up of simple substances 
which cannot, at present at least, be divided into any other 
substances. These are the letters, or the elements. 

Letters are joined to form icords; so elements combine to 
form cotnpomids. There are twenty- six letters in the alpha- 
bet. Nature's alphabet consists of about sixty- four elements. 

The letters a and /may stand alone; some of the elements, 
too, stand alone. They exist free and uncombined, as mercury 
and sulphur. 

Some of the letters are in common use, others occur but 
rarely ; so of the elements some are very common, others 
are very scarce and are but seldom seen. 

Some of our words are built of two letters, others of 
three, four, or five, and so on ; so some of the compounds 
consist of two elements, others of three, others of five, six, 
or seven, and so on. 

Many of our words of two or more syllables are made up 
of shorter words ; and so new compounds may be formed by 
the union of other compounds. 

The compounds bear no likeness whatever to the elements 
of which they are composed. Instance the experiment with 
sulphur and copper filings in the last lesson ; or the invisible 
gas and the liquid metal which were got from a red powder. 

II. The very common elements. 

The teacher should make a table of the very common 

elements for future reference.* 

* Elkmbnts. 

Oases Non-:Metallio MetaUio 

oxygen ciirbon iron copper 

nitrogen sulphur lead gold 

hydrogen tin silver 

chlorine mercury rioc 



FIFTH STAGE. 



177 



II. Illustrations. 

Further to illustrate the formation of compounds having 
properties absolutely different from their combining elements, 
the teacher may make the following experiment. 

1. To make chlorine gas. 

Fit up apparatus as shown in Fig. 66. Put in a little 
Q 




Fig. 66. 

strong hydrochloric acid and rinse it round the flask. 
Remove the cork, and put in a few lumps of dioxide of 
manganese. 

Pour in through the funnel strong hydrochloric acid suf- 
ficient to cover the manganese. Heat gently. Collect the 
gas in a dry bottle. It cannot be conveniently collected 
over water because water readily dissolves it. When the 
bottle is full, which may be seen by the greenish yellow 
colour of the gas, remove the flask from the room and cover 
the jar quickly with a greased glass plate, as the gas is 
very irritating when breathed. 

Now we have a jar of chlorine gas, one of the elements. 
It is a heavy gas. We know this because it displaces the 
9,ir in the jar. It has a greenish yellow colour. 



178 OBJECT LESSONS. 

Remove the cover and insert a leaf or two of " Dutcli " 
gold-leaf, or thin copper leaf ; the leaf ignites and bums with 
a smoky flame. If powdered antimony he thrown in bright 
sparks will appear. 

In the first case the compound, chloride of copper, is 
formed ; and, in the second case, chloride of antimony ; and 
these bear no likeness whatever to the elements of which 
they are made. When substances unite chemically they 
produce new bodies entirely different from themselves. If 
possible show one of these "salts" — chloride of copper or 
antimony — for the purpose of comparing the properties of 
a compound and of its elements. 



LESSON IV. 

THE AIR A MIXTURE OF GASES. 

AiyriCLES for illustration : apparatus and materials for making oxygen, 
piece of thin iron wire, and a lump of lead or other heavy body. 



I. Oxygen. 

Exp. 172. Heat potassium chlorate, to which about one- 
fourth of its weight of black oxide of manganese has been 
added, in a retort ; and, after allowing sufiicient time for the 
air to be expelled, collect the gas which comes ofi*. 

The colourless gas which we collect is called oxygen. It 
is heavier than air for we can collect it as we collected 
chlorine by displacement. 

But its most important property is that bodies burn more 
readily in it than in air. 

Exp. 173. Ignite a splinter of wood, extinguish the flame, 
leaving only a red spark. Plunge into a jar of oxygen, 
and the splinter bursts into flame at once, and burns more 
brightly and much more freely than in air. 

Exp, 174. Take a fresh bottle of gas and pour in a littld 



FIFTH STAGE. 



179 



water just to cover tte bottom. Coil about a foot or eigbteen 
inches of very fine iron wire round a piece of glass tubing, 
so as to form a spiral. Remove the wire from 
the rod, fix one end in a cork which just fits 
the neck of the bottle, and to the other end fix 
a bit of wax taper. 

Ignite the taper and immerse the whole in 
the oxygen (Fig. 67). The taper ignites the 
wire, and beautiful bright sparks fly ofi" in every 
direction. 

If convenient the teacher may give other 
experiments to show this special property of 
comhustion. 




Fig. 67. 

supporting 



II. Nitrogen. 

Exp. 175. Fix a short piece of wax taper on a lump of 
lead and place in a shallow vessel. Pour in sufiicient water 
just to cover the lead. Ignite the taper and cover it with a 
bottle inverted, as in the cut (Fig. 68). 

The taper burns brightly for a short time, but gradually 
gets dim, flickers and dies out. At the same time the water 
rises, filling about one-fifth of the bottle 





Fig. 68. 



Fig. 69. 



Slip the bottle ofi* the lead into the water, remove the lead, 
and slide a piece of greased glass over the mouth of the 
bottle and invert (Fig. 69). 



180 OBJECT LESSONS. 

If a lighted splinter of wood is now plunged into the 
bottle, the flame is immediately extinguished. 

The teacher will now explain that we burned away all the 
oxygen, and that there is still a gas left behind. This gas 
we call nitrogen. , 

The active property of oxygen should next be compared 
with the negative property of nitrogen, and the quantity, 
by volume, of oxygen and nitrogen in the air should be 
deduced from the quantity of nitrogen — four-fifths — left in 
the bottle. 

III. Composition of the atmosphere. 

Lastly, the teacher will recall to the minds of the children 
that there are also small quantities of carbonic acid gas* in 
the air ; and a larger, but varying, quantity of aqueous 
vapour. These gases are all mixed together in the atmo- 
sphere as we can mix sand and sugar. They are not 
chemically combined, like mercury and oxygen in the red 
oxide of mercury. 

A mixture may be distinguished from a compound by the 
fact that each element in the mixture keeps manifestly its 
own properties, and does not merge them in those of a 
different substance. The oxygen of the air is diluted in 
nitrogen like brandy in water. But it keeps its properties 
just the same. 



LESSON V. 

WATER A COMPOUND OF TWO GASES. 

Articles for illustration : apparatus and materials for making hydrogen 
gas. 

I. Hydrogen. 

Exp. 176. Put some pieces of granulated zinc in the 
401116 (Fig. 70). Cover the zinc with water and add 
* About four parts in one thousand* 




FIFTH STAGE. 181 

sulphuric acid through the funnel. Hydrogen is given off, 
and issues through the short glass tube.* At first it is 
mixed with air. Allow a few minutes for all the 
air to be expelled, f and then ignite the hydrogen. 

The gas burns with a blue flame which gives 
off great heat, but very little light. 

Hydrogen may be collected by displacement, 
only in this case the bottle must be inverted and 
the gas poured upwards, because of its exceeding 
lightness. Hydrogen has about one-fourteenth 
the weight of air. V^^^^^ 

Fill a bottle in this way and thrust up into it ^^^' *^' 
a lighted taper. The gas ignites at the mouth of the bottle, but 
the taper is extinguished when it is surrounded by the gas. 

The teacher should here compare the different action of a 
bottle of oxygen gas. Hydrogen burns when in contact 
with the air or with oxygen ; but puts out a lighted taper 
plunged into it. Oxygen does not burn at all ; but a taper 
plunged into it burns more brightly than in air. 

We may say, then, that oxygen supports combustion, but 
is not itself combustible ; and that hydrogen is itself com- 
bustible, but does not support combustion. 

II. Water. 

Hold over the flame a cold dry glass vessel. It soon 
becomes covered with moisture. Why is this ? The 
burning of the hydrogen is just a union of the elements, 
oxygen and hydrogen, to form the compound water. This 
water rises as steam, and condenses on the cold glass. 

The teacher may illustrate the intense chemical citfracHon 
which exists between oxygen and hydrogen by exploding a 
mixture in a soda-water bottle. 

* The tube is drawn out to a fine point or jet. 

t N.B. — A mixture of air and hydrogen explodes, and hence a lighted 
taper should never be brought near until we are quite sure the whole of the 
air has been forced out of the bottle. 



182 OBJECT LESSONS. 

Exp. 177. Fill the bottle with water, and over the jmeti- 
matic trough allow hydrogen to expel two- thirds of the water, 
and oxygen the remaining one-third. The bottle will then 
contain a mixture of hydrogen and oxygen in the pro- 
portion of two to one. Wrap a duster round the bottle, 
remove from the water, and apply quickly a lighted taper. 
A smart explosion follows. The gases combine, and form 
water.* 

LESSON VI. 

COMBUSTION. 

Articles for illustration : rust of iron, and charcoal or other articles to 
show combustion. 



I. Meaning of combustion. 

We saw in the last lesson how freely oxygen and hydro- 
gen unite, and that in the act of combination they give off 
heat and light. Bodies which, when they combine, give off 
light and heat are said to undergo combustion. Or to put it 
in another way, we give the name combustion to the union 
of two or moi 3 substances when in the act of combination 
they give off light and heat. 

We say that hydrogen burns, but it burns only when it 
can combine with oxygen, as in the air. It would not burn 
in a bottle of carbonic acid gas for instance. Oxygen does 
not burn in the air, because it cannot combine with itself, 
or with the nitrogen mixed with it in the air. But oxygen 
causes combustion in an atmosphere of hydrogen, just as 
hydrogen bums in oxygen. 

In nature, however, oxygen is found free everywhere, 
whilst hydrogen is never found uncombined, and so it comes 
about that when we speak of combustion in common lan- 
guage, we always mean the combination of some body or 

* If the teacher can command the apparatus for decomposina: water, he 
will be able still better to show the composition of water. 



FIFlH STAGE. 183 

other with oxygen. Oxygen, therefore, is said to support 
combustion. 

Sometimes in chemistry the word combustion is used in a 
wider sense. Thus the oxygen of the air combines with iron, 
and forms a reddish-brown powder which we call rust, and 
the iron is said to be slowly consumed, although it gives off 
no light and no appreciable heat. 

[Show the rust of iron and compare its properties with 
those of its elements, iron and oxygen.] 

Again, the oxygen of the air taken into the body through 
the lungs combines with the waste matter, and in the act of 
combination gives off heat. This combust ion of the waste 
matter is the source of all the heat of the body. 

II. The common products of combustion. 

We have said that charcoal, or carbon, is an element. 
What do we mean when we say that charcoal burns ? We 
mean that it combines with the oxygen of the air, and that in 
the act of combination it gives off heat and light. And what 
new compound does it form? If we burn charcoal in oxygen 
over lime-water we shall find that the water becomes milky, 
showing that the compound is the carbonic acid gas de- 
scribed in a former lesson. You may remember that carbonic 
acid gas is harmful to breathe in large quantities, and some- 
times people have committed suicide by shutting themselves 
in a close room, and breathing the fumes of burning charcoal. 

A large part of our coal, and wood, and oil, and coal gas 
consists of carbon, and hence wherever these are burned car- 
bonic acid gas is made. 

When hydrogen was burned we saw that water was made, 
and a large part of our fuel and lights being composed of 
hydrogen in combination with carbon, or with carbon and 
oxygen, whenever we have a fire or a light from oil or gas 
we are manufacturing water. Carbonic acid gas and water, 
then, are the two chief products of combustion. 



184 



OBJECT LESSONS. 



LESbON VII. 

THE CHEMISTRY OF A CANDLE. 
Articles for illustration : a candle, and stem of a " clay " tobacco-pipe. 



i. The candle. 

EePer to Second Stage, Lesson XV., page 64, on candles. 
Question as to what substances are used in the manufac- 
ture of candles. 

Tell the children that all fats and oils are made up of 
various compounds of carbon and hydrogen, or of carbon, 
hydrogen, and a smaller quantity of oxygen. The common 
fats classed under the name of tallow contain the three 
elements. Paraffin and paraffin oil contain no oxygen. The 

wick is made up of carbon, hydrogen, 
and oxygen, with a little earthy 
matter. 

II. How a candle burns. 

Light the candle ; call attention to 
the cup of melted tallow, then to the 
ascent of the liquid fat up the wick 
by capillary attraction. The children 
may see the flow upwards. When 
the liquid fat reaches the flame it is 
changed by the heat to gas. It is 
the gas which burns. Show this by 
putting one end of the stem of a "clay" pipe into the 
centre of the flame (Fig. 71). A portion of tho gas escapes 
through the pipe and may be ignited at the other end. 

III. Structure of the fiame. 
Now look at a stead// candle-flame* very carefully side- 

• The candle may be placed in a wide chimney -glass, but of course open to 
the uir below. 




Fig. 71. 



FIFTH STAGE. 185 

ways. In tlie inside a dark zone is easily detected. This 
is simply a zone, as we have shown, of unburnt tallow-gas. 
Next, and surrounding this central zone, is the very bright 
or luminous zone. Here, for the most part, the hydrogen of 
the gas combines with the oxygen of the air. This chemical 
union produces an intense heat, which causes the tiny 
particles of carbon to glow, and in fact produce the light. 
Outside this light -producing zone there is a more abundant 
supply of oxygen, and combustion is complete. This outside 
zone is therefore very hot, but yields less light. 

The candle-flame then consists of three parts : a dark 
central zone of gas to which the oxygen of the air cannot 
penetrate, and which therefore is not burning ; a second 
or light-producing zone enveloping the first, where some 
oxygen penetrates, and where the particles of carbon are 
raised to white heat before themselves undergoing complete 
combustion ; and a third, or heat zone, again enveloping the 
luminous zone, where combustion is complete. 

We can show the threefold structure of the flame in another 
way. Press a sheet of white paper, held horizontally, into the 
flame of a candle almost down to the wick. 
Retain in that position for a second or two. .di ^J& iiiK 
Remove and note the efiect on the paper. i^H^^^^^fer 
A black ring of carbon, in the shape of '^^^^^^HHl 
fine soot, is shown ; outside this there is =i^^^^^^^Ri'' 
another ring of lighter shade where less '"=l|i^^i|f ■' 
carbon is deposited, and within the ring a Fig. 72. 

light deposit of soot is shown. 

This deposit of a dark and two light rings of carbon is 
easily explained. The dark ring corresponds to the lumin- 
ous zone where there is abundance of carbon at a white 
heat. The outer light ring corresponds to the heat zone, 
where combustion is more complete and consequently there 
is less carbon to deposit. 

The carbon within the dark ring is deposited as we press 



186 OBJECT LESSONS. 

the paper down through the flame. That the inner zone 
deposits no carbon may be shown by directing the jet from 
the pipe (Fig. 72) on to a sheet of paper. 



LESSON VIII. 

ELECTRIC AND MAGNETIC FORCES. 

Articles for illustration: strips of copper and zinc, piece of copper wire, 
sulphuric acid, iron filings, a magnet, a thread of silk untwisted and a 
sewing needle. 



I. Electric force. 

Exp. 178. Take a strip of zinc plate about 4 inches long 
and an inch in width and place in a glass (Fig. 73) containing 
dilute sulphuric acid. 

Direct the children to notice the result. There is formed 
a collection of bubbles on the surface of the zinc. These 
break away, rise to the surface of the liquid, and are dispelled 
in the air. Then other bubbles take their places. Now 
what causes these bubbles, and of what gas do they consist ? 

Refer back to the lesson on 
hydrogen. How is hydrogen 
prepared ? These bubbles then 
are hydrogen gas. We can 
collect and burn them. 

Next put a strip of copper 
Fig. 73. Fig. 74. of similar size into the dilute 

acid and without touching the 
zinc. Are bubbles formed on the copper ? No, only on the 
zinc. Now lean the strips of zinc and copper against each 
other as in Fig. 74, and note the result. Torrents of bubbles 
rise from the copper, and but very few from the zinc. It will 
be found after awhile that the zinc is worn away, but that 
the copper is left intact, notwithstanding that the bubbles 
rise from the copper. 





FIFTH STAGE. 



187 




Fig. 75. 



The same effect follows when the plates are connected 
by a wire instead of being placed in contact. Break the 
wire, and bubbles no longer rise from 
the copper. 

We may conclude from these expe- 
riments that there must be some con- 
nection between the metals to bring 
about the particular action we have 
noted. Unless the wires are connected 
the particular action does not occur; 
hence it seems that some influence is 
exerted by the metals upon one another through the wire. 
Exp. 179. Suspend a magnetized sewing needle* by a 
fine untwisted silk thread. The needle 
will point practically north and south. 
Now carry the wire connecting the 
plates under and parallel to the needle 
as in Fig. 76. The needle turns on its 
axis and tends to place itself at a right 
angle to the wire. 

Here, then, we have another force 
which we have not before considered. 
It is called the electric force, and is 
closely allied to another force referred 
to in Lesson I. of this stage — the 
magnetic force. 




Fig. 76. 



II. Electric and magnetic forces connected. 

Exp. 180. Take a magnet and plunge it into iron filings. 
Note the result. 

Now wrap a piece of paper round a piece of thick iron 
wire — a six-inch French nail will answer very well — leaving 
the ends free, and then wind around it twenty or thirty 

* To make a magnet of the needle, draw it several times acioss one end of 
a magnet from end to end and always in the same direction, not backwards 
and forwards. 



18^ OBJECT LESSONS. 

turns of copper wire, keeping the coils from ton chin g each 
other. Connect the ends of the iron wire with the zinc and 
copper plates, and plunge one end of the wire into iron 
filings. The wire has becomes a magnet which attracts the 
filings just as the magnet did. The electric force in the 
wire imparts to the iron another force — a magnetic force 
— precisely similar to the force exercised by the magnet. 
When the contact between the plates is broken the wire 
ceases to act as a magnet. Its force is gone. 

It is sufficient for our purpose here to show that there are 
two other forces of nature very closely allied beyond those 
already described. It is of course in the discretion of the 
teacher to pursue the subject further as opportunity may 
ofier. 



LESSON IX. 
CENTRE OF GRAVITY. 



Articles for ilhistration : water-bottle, couple :f forks, a few wheat- 
straws without flaw, and some blocks of wood. 



The teacher should introduce the subject of this lesson by 
a reference to Lesson I. on gravity, viz. pressure down- 
wards, or weight. Place say a pound weight in the hand of 
one of the scholars. He experiences a pressure downwards. 
What is this pressure ? A pound. Now with what force 
must he press upwards to keep the weight from falling ? 
Clearly one pound. 

I, To find the centre of gravity. 

Exp. 181. Now balance a slate horizontally on the thumb 
of the left hand. What force does the thumb exert in an 
upward direction ? A force equal to the pressure of the 
slate downwards, viz. its weight. 



FIFTH STAGE. 189 

In the case of the slate every particle of it presses- down- 
ward in a perpendicular direction (Fig 77). 




Fig. 77. 

The pressure upwards is collected in one point, the top of 
the thumb, and this point supports the whole weight of the 
slate. We can imagine the whole weight of the slate to be 
collected at the point supported by the thumb, for when that 
point is supported the whole slate is supported. If the 
thumb be placed on any other point the slate is not balanced, 
and falls to the ground. We may call the point where the 
whole weight of the slate is supported the centre of weight, or 
as it is more commonly called, the centre of gravity. 

In the case of the slate, which is of regular geometrical 
form, we can find the point by drawing lines diagonally from 
corner to corner. The point where the lines cross is the 
centre of gravity. 

In this case we have not considered the thickness of the 
slate ; but suppose the slate to have a thickness of half an inch, 
then the exact centre of weight will be a quarter of an inch 
from the surface, but in a vertical line above the point sup- 
ported by the thumb. If, therefore, we wish to support any 
body, we must be careful to apply the support directly under 
or above the ce^itre of gravity. 

Another method of finding the centre of gravity of a body 
is by suspension. Take the slate once more. Suspend it 
from any point, say a corner. Draw a perpendicular line, 
found by improvising a plumb-line of a piece of fine twine 
and a weight. Suspend from another point, say a second 
corner, and draw a second perpendicular line. The point 



190 



OBJECT LESSONS. 



where the lines cross will be the centre of gravity ; and if a 
fine hole were drilled through the slate at this point it 




Fig. 78. 

might be suspended in a horizontal position by means of a 

piece of string (Fig. 78). 

A bottle of water may be supported by a single bent 

straw (Fig. 79). The straw is bent before being placed in the 
bottle, so that when the bottle is lifted 
the centre of gravity is displaced and 
brought directly under the point of 
suspension. 

II. When the centre of gravity of any 
body is supported, that body cannot falL 

Take two blocks of wood cut as in 
Fig. 80. In (1) the line of direction 
A B falls within the base of support ; 
hence the centre of gravity is sup- 
ported and the block does not fall. 

In (2) the line of direction c d falls 
without the base of support, and the 
block cannot stand in the position indi- 
cated. 

A picture of the " leaning tower of Pisa *' might here be 
exhibited. And it might here be explained that the reason 
why the tower has stood safely for hundreds of years is that, 
as in the case of block (1) Fig. 80, a perpendicular line from 
the centre of gravity fulls within the base. 




Fig. 79. 



FIFTH STAGE. 



191 




The teacher should now assist the children in pointing 

out how men and animals are continually shifting the 

position of their centres of gravity 

to bring them within their bases 

of support. A man in carrying a 

weight on his back leans forward, 

a nurse carrying a baby leans 

backward ; a man carrying a 

bucket in one hand leans toward 

the other, and stretches out the 

other arm to preserve his balance. 

It must be borne in mind that the base of support is not 

necessarily limited to that part of the under surface of a body 

which rest on its support. Thus the base of support of a 

man resting on two feet includes not only the space actually 

covered by the feet, but also the space between the feet. 
One other point with reference to the centre of gravity 
remains to be considered, viz. that the loiver 
the centre of gravity lies the more stable is the 
body ; that is, the less easily can the body be 
overturned. 

It is not easy to balance a lead pencil on 
the point of the finger, but if weights be at- 
tached as in Fig. 81 the centre of gravity is 
brought below the point of support, and the 
pencil is supported without difficulty. 

In ships and boats the centre of gravity is 
brought as low as possible by putting heavy 
ballast in the bottem. 

Exp. 182. A curious and interesting ex- 
periment to further illustrate this fact may 
be arranged as in Fig. 82. 
Fix two forks in a cork, and into the bottom of the cork 
insert a sewing needle. On the neck of a bottle place a coin 




192 



OBJECT LESSONS. 



— a half-crown or a penny. Balance the forks as shown in 
the figure. The contrivance may be made to revolve with- 
out destroying the equilibrium. The 
centre of gravity here is somewhere in 
an exact line with the needle but con- 
siderably below it. This line we have 
called the line of direction. 




The teacher may now ask such test 
questions as the following : — 

1. Why does a person in rising from 
a chair bend forward ? 

2. Why is it more difficult to over- 
throw a man when he is standing with 

his feet some distance apart than when his feet are placed 
close together ? 

3. Is there any advantage in turning out the toes when 
we walk ? 

4. A man stands with one side close to a perpendicular 
wall; why cannot he hold up the other leg from the ground ? 

5. Why cannot a man standing with his back and feet 
close to a wall stoop to pick up anything in front ? 

6. How far may a wall be made to lean and yet stand 
securely ? , 



To make the following lessons on Simple Machines or the 
Mechanical Powers effective, the teacher will need working 
models. Their being rough and commonplace will not 
detract from their value as working illustrations. 



LESSON X. 
LEVERS AND THEIR USES. I. 



The teacher should introduce the subject of the simple 



FIFTH STAGE. 



193 



machines by questioning the children on such of them as 
they must have seen in their everyday rambles. The pulley 
used for raising building materials on to the scaffold ; the 
ladder up and down which casks are rolled, or pushed into 
or from waggons ; levers used for lifting bodies too heavy for 
the unaided arms, and so on. 



I. First order of lever. 

The model lever* should next be introduced, and its parts 
described. 

The pin on which the lever turns is called the /ule?^um, f. 
The body to be raised 
is called the iceight, 
w, and the force ap- 
plied at the other 
end of the lever to 
raise the weight is 
called the power , p. 




Fig. 84. 



The parts of the lever on each side of the fulcrum are called 
the arms. 

Under the teacher's guidance, the children should discover 
for themselves all the facts useful for them to know about 
the lever. 

1. Place equal weights, one at the end of each arm. Note 
the result. They balance. What advantage is there in a 
lever so arranged ? Suppose the weight to be 30 lbs., and 
we have to raise it one foot. 30 lbs. would be heavy for a 
boy to lift. But if fastened to one end of the lever he might 
sit on the other, and so raise the weight without exertion. 



* The lever may consist of a bar of stiff wood about 4 feet 
in lengt h through which several holes are drilled, as in Fig. 
84. The stand may consist of a hase made of inch deal board 
15 inches by 8 or 9, and an upright made of 4-inch quar- 
tering. A groove mnst be cut in the quartering at top to 
allow of free motion to the bar, Fig. 83, a. A hole is drilled 
through for the pin on which the lever turns, Fig. 83, b. 



u 


o 



A B 

Fig. 83. 



194 OBJECT LESSONS. 

The advantage is in changing the direction of the force. In 
pushing or pulling downwards we are assisted by the weight 
of our bodies. 

2. Remove the weight, take out the pin and shift the lever 
one foot to the right, the pin to go through the hole marked 

2 in the Fig. One arm will now be 1 foot long, and the 
other 3 feet. Now make the weight 3 lbs., and attach 
1 lb. to the end of the long arm. Again note the result. 
1 lb. at the end of the long arm rather overbalances 3 lbs. 
at the end of the short arm. If the lever had no weight it 
would exactly balance. 

3. Arrange the lever so that the long arm shall measure 

3 feet 6 inches, and the short arm 6 inches. It will be 
found that 1 lb. at the end of the long arm will overbalance 
7 lbs. at the end of the short arm. 

From these experiments the children will readily g/asp 
this principle, that the longer the ^^ power-arm " 18 in comparison 
mth the *' weight-arm/* the greater is the weight we can raise 
mth a given power. 

The teacher may also go a step beyond this, and show that 
— neglecting the weight of the lever — the weight multiplied 
into its distance from the fulcrum is equal to the power multiplied 
into its distance f rot n the fulcrum. 

Or weight : power : : length of power arm : length of 
weight arm. 

II. Examples. 

Common examples of this kind of lever may now be 
brought under review. 

1. Ordinary scales for weighing. In this machine the 
arms are of equal length, and a pound on one side balances a 
pound on the other. 

2. The child's see-saw. Note, if one child is heavier than 
the other, how the arms have to be arranged. 

8. The common steel-yard (Fig. 85), another machine for 



FIFTH STAGE. 



195 



weighing goods, is a lever of the kind described, but with 
one arm longer than the other. Question on the proportion 
between* p and w in the steel- yard. 




w^W^ 



Fig. 85. 

4. Another example of the use of the lever is illustrated 
in Fig. 86. 

5. The poker used in stirring the fire, the small trucks 
used by porters at railway 

stations for picking up ,^,^.==*^1^^^ ..^^^^^^^^^^^\ 

luggage, the ordinary 
pump - handle, and the 
claw - hammer are other 
examples of this lever. 

Lastly, the children 
should be informed that the particular arrangement of the 
lever in which the power is at one end, the weight at the 
other, and the fulcrum between the two, is called a lever of 
the first class, or first order. 




LESSON XI. 
LEVERS AND THEIR USES.— 2. 



I. Lever of the Second Order. 
We now proceed to consider the lever used in another way. 
Shift the lever so that the fulcrum is at one end (Fig. 87), 



196 



OBJECT LESSONS. 



and the power p at the other. If a pound weight be placed 
on the end of the lever at p it is quite clear that, to keep the 
lever in its horizontal position, we must pull upwards with a 
force of 1 lb. in addition to the force required to support the 
lever. 

Now shift the weight to the centre of the bar. In this 




Fig. 88. 



Fig. 89. 



position the upward force at p must be half a pound, as the 
fulcrum supports the other half. This may be tested by- 
passing a line from p over a small pulley suspended from 
above, and aflBxing a half-pound weight * (Fig. 88). 

Again, shift the weight to the distance of one foot from 
the fulcrum (Fig. 89). What upward force must be exerted 
at P to support the weight in this position ? 

Suppose for the moment the power to be applied at the 
centre of the lever at p' ; in that position, omitting the 
weight of the lever, the upward force must be half a pound. 
But a force at p equal to a quarter of a pound acting in an 
opposite direction, will balance a force at p' of half a pound. 
Hence a quarter pound force at p will support lib. at a dis- 
tance of one foot from the fulcrum. 

Allowing a little for the weight of the lever, experiment 
will show ^he truth of this statement. 

* Homo extra weight will be required to support the lever itself. 



FIFTH STAGE. 197 

From the above experiments the teacher will again be able 
to deduce — 

1. That the lo'ifjcr the power-arm is in proportio)i to the 
weight-arm, the greater is the weight we can raise tcit/i^a given 
power. ' 

2. That the weight multiplied by its distance from the fulcrum 
is equal to the potver multijMed hy its distance from the fulcrum. 

II. Examples. 
Common examples of this lever : — 

1. The wheelbarrow. The wheel is the fulcrum, the 
load in the barrow the weight, and the man lifting the handles 
the power. 

2. A boat oar. The water is the fulcrum, the boat the 
weight, while the power is applied by the hand. 

3. A chopping- knife turning on a fulcrum at one end is 
another example. 

4. Nutcrackers and cork- squeezers are double levers of 
this kind. 

The children may now be 
told that when a lever is so 
arranged that the fulcrum is 
at one end, the power at the 
other, and the weight in the 
middle, it is said to be of the Fig. 90. 

second class or order. 

Fig. 90 shows how a lever of the second class is some- 
times used. 

LESSON XII. 

LEVERS AND THEIR USES.— 3. 




I. Lever of the Third Order. 

We now come to a lever having a different arrangement 
again. It is called the lever of the third class or order. 



198 OBJECT LESSONS. 

The fulcrum is at one end of the lever as in the second 
kind ; but in this case the weight is at the other end, and the 
power is applied somewhere between the fulcrum and the 
weight jFig. 91). 

It will be found on trial, viz. by passing the string by 
which the power is appKed over a pulley, that if the power 

p' p 



w 

Fig. 91. 

is applied in the centre of the lever, it will require a 2 lb. 
weight to support 1 lb. at the end of the lever. 

The teacher may show that this must be the case from the 
lever of the second kind. Suppose the weight w of 1 lb. to 
be arranged to pull the lever upwards, then it will balance 
a weight of 2 lbs. at p. Or, which is the same thing, a 
power equal to 2 lbs. in weight applied at p in one direction 
will balance a weight of 1 lb. applied at w in the opposite 
direction. 

In the same way the teacher may show that it will 
require a power equal to 4 lbs. applied at p' to support a 
weight of 1 lb. at w. 

The second and third kinds of lever are identical, except 
that the power in one case takes the place of the weight 
in the other. 

In the lever of the third kind it will be seen that the 
power is always greater than the weight. Where, then, 
is the advantage ? The advantage is that the weight is 
moved through a greater space, and of course at greater 
speed, than the power. This may be shown by experiment. 

In this case also the poicer multiplied into its distance from 
the fulcrum is equal to the weight multiplied into its distance 
from the fulcrum. 



FIFTH STAGE. 199 

II. Examples. 

Levers of this kind are less common than the others. A 
good illustration will be to direct a boy to hold a weight in 
a long-handled shovel or in a spade. 

One hand, usually the right, acts as the fulcrum, the 
other hand furnishes the power which *lifts or balances the 
weight* 

A man raising a board, a pole, or a ladder with one end 
pressed against the bottom of a wall or held by a second 
person is another example which may be illustrated in the 
schoolroom. [Note, when the raising commences the lever 
is of the second kind, and so remains until the man gets 
below the centre of gravity of the article he is raising.] 

In the mechanism of the human body there are many 
examples of levers of the third kind. The arm may be 
taken as the most simple for illustration. The socket of 




the elbow joint forms the fulcrum, the biceps muscle is the 
power, and the weight is the forearm and anything sup- 
ported by it. 

A pair of tongs is an example of a double lever of this 
kind. 

* A fishing-rod held with both hands forms a similar example. 



200 



OBJECT LESSONS. 



LESSON XIII. 
THE PULLEY.* 



I. Single Fixed Pulley. 

From the lever of the first kind to the pulley is an easy 
transition. 

Fasten a short lever (see foot-note), to work on a screw 
as a pivot on the side of the pulley-stand (Fig. 94) Show 
how the lever works. The arms are of equal lengths and the 
height and power must be equal. There will be just as hard 
a pull in one cord as in the other This pull or strain is 
usually described as the tension. 

Now substitute for the lever a disc of deal board having 
a diameter equal to the whole length of the lever (Fig. 95). 



F© 





Fig. 94. 

It turns on the screw as a fulcrum, as in the case of the lever. 
Fasten the cords to the disc at a and b. This machine is 
nothing more nor less than a lever of the first kind. 



 The side of a deal table or an old box stood on end 
and with the lid removed, will serve as a pulley-stjind. 
Insei-t a couple of screws on which to hang the pulleys or 
fasten the cord. 




Fig. 93. 



PII'TH STAGE. 



201 




Fig. 95. 



Next take a longer cord and pass it over the pulley. The 
necessity for the groove round the circumference is at once 
apparent. By this apparatus 
we can raise the weight as 
high, as the pulley is fixed, 
but as the arms of the lever 
are of equal lengtli the power 
must equal the weight. That 
is, if we want to raise 1 cwt. 
we must pull down with a 
force of 1 cwt. 

This revolving lever with 
arms of equal length is called 
a pulley. 

Show the scholars that the 
advantages of this pulley are 
(1) direction in the application of the power, and (2) we can 

raise the weight to any desired 
height. 

II. A Fixed and Movahle Pulley. 

The teacher will now fit up a 
couple of pulleys,* one fixed and 
one movable as in Fig. 96, and 
then proceed to show the advan- 
tage of this system of pulleys. 
Suppose the weight w^ to be 8 
lbs. The cord a b passes round 
the movable pulley and supports 
the weight ; hence the tension 
or strain in each part of the 
cord is the same. What is 
^'^' ^^- this tension ? To find this we 

refer back to a lever of the second kind. 

* Block pulleys may be purchased at quite a small cost. 




202 



OBJECT LESSONS.- 




Fig. 97. 



We may suppose two props (Fig. 97) to support a weight 
of 8 lbs. between them. Clearly each prop will support half 

the weight, viz. 4 lbs. Just 
in the same way each part 
of the cord supports half the 
weight, and the tension is 
4 lbs. 

The tension of the cord 
marked b is 4 lbs., and the 
tension in the cord c must 
also be 4 lb., for in the fixed pulley we have seen that we gain 
no mechanical advantage ; the weight and the power are equal. 
The teacher will deduce from the above that, when there 
are two pulleys, one fixed and one movable, the weight is 
double the power — that is, to raise 2 cwt. a power equal to 
1 c*Wt. only is required. In practice this is not quite true, for 
we have to take into account the weight 
of the movable pulley and the cord. 

1X1. Three or more Pulleys. 

The children may next be shown by 
a blackboard sketch (Fig. 98), the ad- 
vantage of using a greater number of 
pulleys. 

Suppose the cord a has a tension, or 
pull downwards, of 1 cwt. ; then b and 

will each pull upwards with a force of 

1 cwt. each, viz. pulley No. 1 will be 
supporting a weight of 2 cwt. 

The tension in F and d being 2 cwt., 
the pulley No. 2 will support a weight 
of 4 cwt. 

In the same way pulley No. 3 will support a weight of 8 cwt. 

For each movable pulley the weight capable of being raised 
is doubled. 




FIFTH STAGE. 



203 



In practice a considerable portion of the power is expended 
in raising the pulleys and in overcoming friction.* 

The teacher may lastly show that here, as in the lever, 
what we gain in weight we lose in space and time. 



LESSON XIY. 

THE WHEEL AND AXLE. 



The principle of the wheel and axle, a combination of the 
lever and the pulley, may be shown by using the disc of the 
last lesson (see Fig. 99). 

Fix the weight by the cord at a, and attach a strip of wood 




at B. By trial find out what weight suspended at the end of 
the long arm f p of the lever will support a given weight, 
w, at the end of the short arm a f. 

At present our machine is a lever of the first kind. 

Given that our wheel is arranged to work horizontally, 
and the groove widened to take a number of coils of the cord, 
and the cord lengthened to the extent required, we have the 
wheel and axle as used on board ship for raising the anchor 
(Fig. 100). 

» See Lesson XVIII. 



204 



OBJECT LESSORS. 



Show the scholars how this is practically a large wheel and 
a small one, the circumference of the larger wheel being the 




Fig. 100. 




Fig. 101. 



path made by the end of the spokes, and the part of the axle 
which receives the cord the smaller wheel. 



We may use a wheel with a groove instead of the spokes, 
and then, placed in a perpendicular direction (Fig. 101), we 
see the machine as used in another way. 

It would not be difficult for the teacher to illustrate this 
by a rough model* as shown in Fig. 102 and to show that if 




the radius of the larger wheel is 3 inches and of the smaller 
1 inch, a weight of 1 lb. placed at p will balance a weight of 
3 lbs. placed at w. 

Note that the cord round the large wheel or axle is wound 
in a contrary direction to that round the small axle 

 Ono largo and one small cotton-reel fastened together answer very well. 



FIFTH STAGE. 



205 



Sometimes instead of the large 
This is the case in the common 
u'indlass for raising water out of 
wells. (See Fig. 103.) 

The teacher will show here 
again that the centre of the axle 
is the fulcrum, the radius of the 
axle the short arm, and the 
radius of the circle described by 
the handle the long arm of the 
lever, and from these calculate 
the power requisite to raise a 
given weight. 



axle a handle 




LESSON XV. 

THE INCLINED PLANE. 



The teacher may introduce this lesson by some questions 
on the way in which brewers' men raise the barrels into 
their waggons, and on the comparative amount of force 
required to carry a load of equal weight up a hill with a 
gentle slope, or up another with a steep slope, walking at 
the same pace in each case. 

To carry a load up the gentle ascent will be the easier task. 
Why, and by how much ? 

To answer these questions in the most satisfactory way, 
the teacher must construct an inclined plane and affix a 
pulley at the top (Fig. 104). 

It will be found on trial that a weight of, say 5 lbs., acting 
downwards from the pulley will support a weight of 10 lbs. 
on the inclined plane when (1) the string from the weight to 
the pulley is parallel to the plane, and (2), when the length 
of the plane a h is double the height a c. 

Wq should have expected this to be the case from the 



206 



OBJECT LESSONS. 



principle that "what is gained in weight is lost in space and 
time.** Here the weight has to travel over twice the length it 
would have to travel over supposing it to be lifted perpendi- 
cularly from c to A. 

When the length of the plane is three times the height, 




Fig. 104. 

the power required to balance the weight will be one-third 
the weight ; when the plane is four times the height, the 
power required is one-fourth the weight, and so on. 

In actual practice, to raise a weight on the inclined plane 
the power must be somewhat greater than the proportion 
here given, because there is friction to overcome. It will be 
sufficient if the scholars thoroughly master the fact that 
the longer we make the inclined plane the easier it is to 
raise a given weight. 

When very heavy casks have to be rolled up an inclined 
plane another contrivance is sometimes adopted. Two ropes 
are fastened at the top of the plane, then laid along the 
plane to the bottom and passed round the cask and doubled 
back to the top of the plane. This may be imitated in 
a small inclined plane by passing a couple of pieces of twine 
round a piece of lead tube. 

In this case the teacher will show that the power has to 
move through twice the distance, and that consequently 
twice the weight can be raised by a given power. The 
apparatus constitutes in fact a single movable pulley, and 



FIFTH SXAGE. 



207 



the tension in each cord is half that of the tension in a 
single cord as in Fig. 104. 

Tico ropes are used for convenience ; they keep the cask in 
proper position for rolling, but no further advantage is 
gained over the use of one rope. 



LESSON XVI. 

THE WEDGE. 



Take a piece of wood cut in the shape of an inclined plane, 
and show how it can be used for raising a weight on the 
table by pushing the plane under the weight instead of 
pushing or pulling the weight up the inclined plane (Fig. 






Fig. 105. Fig. 106. Fig. 107. 

105) ; in fact, making the inclined plane a movable instead 
of a fixed machine. 

Secondly take another inclined plane of the same shape 
and size as the former ; place the two base to base (Fig. 
106), and use them to raise the weight just in the same way 
as with the single inclined plane. 

Tell the children that the double inclined plane is called 
a wedge. Show its use for splitting wood by a sketch on 
the blackboard (Fig. 107). 



208 OBJECT LESSONS. 

The power applied in the case of the wedge is not a 
simple pull or a push, but a sharp blow with a hammer or 
mallet, the force of which we cannot measure ; * neither in 
most cases can we measure the force of resistance to the 
entry of the wedge. Hence, all the teacher need to impress 
on the children is that we gain a great mechanical advan- 
tage by using this machine, as we do in using others. 

The teacher will lastly refer to common forms of the 
wedge and their uses, such as a knife, a nail, the prongs of a 
digging fork, a ploughshare, a needle, a pin, &c. 



LESSON XVII. 
THE SCREW. 



The screw, like the wedge, is a movable inclined plane ; 
but in this case the inclined plane is wrapped round a 
cylinder. 

This can be shown very readily. 

Take a piece of white flexible cardboard. Cut from it a 
long and low inclined plane. Colour the edge of 
the plane, and wrap round a cylinder (Fig. 108). 
[The size of the cylinder must depend to some 
extent on the length of the inclined plane, but the 
whole should be as large as possible.] 

The children will be able to see from this experi- 
ment the likeness of the coloured line formed by the 
coloured edge of the plane to the thread of the 
screw. 

Next take a large screw of some kind (Fig. 109) 
Fig. 108. — wood or iron — and let the children trace the easy 
ascent of some supposed living insect round and round the 

• Lot r = power, and it rosistiincp, /> = length of half the hack of the 
wedgo, / =:length ot wedge ; then p ; k J ; A ; /, 




FIFTH SI AGE. 



209 



cylinder, but up and up the inclined plane from the bot- 
tom to the top. 

Refer to spiral staircases constructed on a similar plan. 

It is not often that the inclined plane wound round a 
cylinder is used as a fixed machine for raising bodies 



on 




the contrary, it is nearly always used as a movable machine 
for penetration, or for squeezing and holding together. 

In ** presses" for pressing very hard, long and powerful 
levers are employed to turn the screw. Fig. 110 represents 




Fig. 110. 

a copying- press with two levers. 

The pressure in a machine of this sort is very great. By 
looking at one turn of the screw unrolled we shall get some 
idea of the power (Fig. 111). 



210 OBJECT LESSONS. 

Let the height of the inclined plane, or, which is the same 
thing, the distance between one thread and the next, be 
one-eighth of an inch, and the length of the inclined plane, 
viz. the circumference of the cylinder, be three inches or 



Fin;. 11] 

twenty-four eighths ; then the weight (in this case the 
pressure downward) will be twenty-four times the power. 

The children will understand that when we use a lever to 
turn the screw, the force with which the screw presses down- 
wards will be immensely increased, and they may be told, 
just to illustrate the power of presses, that the machine is 
now practically an inclined plane whose height is the 
distance between the threads and whose length is the cir- 
cumference of the circle described by the lever. 

Let this circumference be 6 feet, viz. 72 inches, or 576 
eighths of inches, and the distance between the threads one- 
eighth of an inch ; then a pressure of 10 lbs. at the end of 
the lever would produce, if there were no frietion, a pressure 
below the screw of 5760 lbs., or more than 2^ tons. Prac- 
tically the pressure is much less. 

The common screw for joining wood or iron work, the 
corkscrew, and gimlet, are all examples of the " screw/* 



LESSON XVIII. 

FRICTION. 



In the preceding lessons on the simple machines the term 
friction has been once or twice used. What is friction ? 
And what effect has friction on bodies moving one over 
another ? 



FIFTH STAGE. 211 

Take a block of rough wood and call on one of the scholars 
to press it along over a rough board. The boy has to exert 
considerable force to slide the block along. Now let him 
slide a polished block along a polished table. The force 
required is much less. Over smooth ice or a sheet of glass 
it would be less still. 

The resistance which the moving block meets with from 
the surface on which it moves is called /nc^^OM. 

Boys cannot slide any distance on a road, however level, 
but they can make long slides on the ice. Why ? The 
friction is less on the ice. It is difficult to walk on ice ; we 
are liable to slip and fall because the friction is slight. But 
strew ashes or sand over the ice, the friction is increased and 
we can walk at ease. In frosty weather sand is thrown over 
the streets to keep the horses from falling. 

In taking heavy loads in waggons down steep hills, the 
driver fastens an iron shoe on one wheel to keep it from 
turning round. The friction between the shoe and the road 
is great. It prevents the waggon from moving so freely and 
eases the horse. Sometimes " breaks " are applied to carriage 
wheels. The friction between the break and the wheel 
hinders the wheel from turning so freely. 

When masons want to move large blocks of stone, they 




Fig. 112. 

put rollers under them to lessen the friction (Fig. 112). 
The teacher may illustrate this by putting a heavy body or 
two or three " round " rulers. 



212 OBJECT LESSONS. 

It is the friction between the road and the wheel which 
makes the wheel turn. Take the railway train, for instance. 
The steam moves the piston, and the piston, by means of 
complicated machinery, moves the large " driving wheels ; " 
but if there were no friction between the wheel {>nd the iron 
rail the wheel would simply turn round and ndt roll for- 
ward. The wheels would not bite, as people say. 

Sometimes in frosty weather it is difficult to start or stop 
the train because the rails are so slippery — that is, there is 
little friction — and sand is thrown on the metals to increase 
the friction. The wheel, of course, turns on an axle, and 
here we want no friction, because all friction here tends to 
hinder motion, and so the axles are kept well greased or 
oiled. So of carriages and waggons, that part of the axle 
on which the wheel turns is kept well greased to lessen the 
friction. 

Friction is not confined to solid substances. Rocks over 
which water constantly flows are worn away by friction. It 
is by friction that the wind raises the waves on the water, 
and it is by friction that the air causes a corn-field to assume 
a wavy motion. 

Wherever there is motion on the earth it is more or les8 
retarded by friction. 



INDEX. 



Absorbent, 9. 
Adhesion, 77, 172. 
Adhesive, me ming of, 17. 

Substances, 17. 
Air, lesson on, 48. 

Pressure of, 50. 

Elasticity of, 52. 

A mixtm-e of gases, 178. 
Ail-pump, lesson on. 111. 
Alcohol. 16. 
Alum, 15. 
Atmosphere, weight of, 94. 

Pressure of, 97. 

Affected by heat, 140. 

Composition of, 180. 

B 

Balloons, 68. 
Barometer, 99. 

Construction of, 100. 
Benzine, 16. 
Benzoline. 63. 
Blotting-paper filter, 12. 
Bodies, light, heavy, 8. 
Boiling of water, 125. 
Bricks, 24. 
Brittle, 20, 82. 



Camphor, 16. 
Candles, 64. 

Chemistry of, 184. 

Common dip, 64. 

Mould, 64. 
Capillary attmction, 79, 172. 
Carbonic acid, lesson on, 60, 183. 
Cement. 18. 
Centre of gravity, 188. 
Chalk, 10. 
Charcoal filter, 12. 
Chemical attraction, 173. 

Experiments on, 174. 
China. 24. 
Clay, 24. 
Clothing, 137. 
Clouds, 43. 
Coal-g.is, 56. 
Cohesion, 75, 172. 
Colours, compiaison of, 159. 
Combustion, 182. 

Common products of, 183. 
Conduction of heat, lo6. 
Conductors, 136. 

Good and bad, 137. 
Contraction, 117. 
Convection, 126. 
Cooling of bodies, 153. 

Copper wire, 20. 
Cork, 22. 



Dew, 40, 145. 
Diamond, 19. 
Distillation, 128. 
Ductile metals, 26, HU. 

E 

Earthenware, 24. 
Earth filter, 12. 
Elastic, meaning of, 21, 82. 
Electric force, 186. 

And magnetic, 187. 
Elements and compounds, 175. 

Common, 176. 
Evap )ration, 40. 
Expansion, 117. 

F 
Filter, a blotting-paper, 12. 

A charcoal, 12. 

Earth, 12. 

Sponge, 11. 

Use of, 13. 
Firedamp, 5''. 
Five -engine, the, 108. 
Flexible, 20. 82. 
Fo.-, 41. 

Force, definition of, 171. 
Force-pump, 106. 
Freezing mixtures, 151. 
Freezing of water, 124. 
Friction, 210. 
Fusion, 25. 



Oas, lesson on, 54, 76. 

Glass, 6. 

Glue, 17. 

Gold, 19, 26. 

Gravity, force of, 83, 172. 

Centre of, 1H8. 
Gravitation, attraction of, 172. 
Gunpowder, 31. 



Hail, 46, 147. 

Hard and soft, 18. 

Hardness, 81. 

Heat cause of motion in air, 143. 

Heavy and light, meanmg ot, 8. 

Hoar frost, 115. 

Hydrogen, 180. 



Ice, 45, 46. 

India-rubber, 22, 23. 
lion, 8. 



Latent heat, 152 



214 



INDEX. 



Latent heat of water, 152. 

Of steam, 152. 
Lead, 26. 

Lesson on, 27. 

Alloys of, 29. 
Lever, the, and its tises, 192. 

First order, 19.3. 

Second order, 195. 

Third order, 197. 
Lifting pump, 177. 
Light and heavy, 8. 
Lime, 16. 
Liquefaction, 119. 
Liquids, 3, 55, 76, 118. 

Buoyancy of, 92. 

Pressure in, I., 87. 

Pressure in, II., 90. 
Lucifer matches, 31. 



Malleable, 26, 1 
Mercury, 17. 

Lesson on, 
Metals, 19, 26. 
Mis% 44. 
Molecule."!, 78. 

Of water, g 
Mortar, 18. 



46. 



N 



Naphtha, 17, 69. 
Nitrogen, 179. 
Non-absorbents, lesson on, 13. 



Oxygen, 178. 



Paraffin oil, lesson on, G2 

Particles, minute, 3. 

Pewter, SO. 

Plane, the inclined, 205. 

Plaster of Paris, 17. 

ri.istic, 23. 

Porous, 9. 

Bodies, 11. 
Pressure, effect of, on boiling-point oi 

liquids, i;30. 
" Properties " of bodies, 5. 

Shape, size, colour, 6. 
Proportion, lessons on, 158—167. 

Terms of 163. 
Pulley, the single fixed, 200. 

Fixed and movable, '201. 

Three or more, 202. 
Pump, the common, 103. 
Putty, 14 



Radiation, 138. 

Badiators, good and bad, 1C9. 



Rain, 44, 147. 
Eatio, 159. 

Illustrations of, 161. 

Terms of, 168. 
Jlnshlight, 64. 

8 

Bait, 10, 14, 15, 25, 37. 

Manufacture of, 15. 
Saltpetre, 16. 
Screw, the, 208. 
Siphon, the, 108. 
Sleet, 148. 
Snow, 45, 46, 148. 
Soap-bubbles, 65. 

What they teach, 66. 
Soft and hard, 18. 
Solids, proi)erties of, 3, 76, 81,11C 

Specific gravity o^ 165. 
Soluble, 14. 
Solvents, 15. 
Specific heat, 149. 
Sponge, 6, 7, 9, 23. 
Sponge-filter, 11, 
States of matter, 76. 
Steam, 44, 132. 
Steam-engine, the, 133. 
Steel, 19. 

Stoves and ot)en fires, 143. 
Sulphur, lesson on, 30. 
Substances soluble in water, l^J 
Sugar, 25. 37. 

Manufacture of, 15. 
Syringe, the, 101. 



Tar, lesson on, 58. 
Tenacious metals, 27, 82. 
Thermometer, the, 120. 

Mercurial, 121. 
Tough, 20. 
Tui-pentine, 10. 
Twine, 10. 



Vaporization, 119. 
Vapour, 40. 



W 



Water, 16. 

Lesson on, 86. 

A solvent, 16, 87. 

Finds, its level, 86. 

Compound of two gases, ISO. 

Distilled, 39. 
Wedge, the, 207. 
Weiglit. lesson on, 7, 83, 8i. 

Compariiion of, 169. 
Whilebone, 22. 
Wheel and axle, 2C3. 
Winds, 144. 
Wood, a 



THE END. 



RICKS' NATURAL HISTORY OBJECT 

LESSONS. 



PART I.— PLANTS AND ANIMALS. 



Chapter. 

I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

X. 

XI. 

XII. 

XIII. 

XIV. 

•XV. 

XVI. 

XVII. 





Chapter. 


Introduction. 


XVIII. 


A Typical Plant. 


XIX. 


General Classification of Plants. 


XX. 


Minute Structure of Plants. 


XXI. 


Roots and their Functions. 


XXII. 


Stems and their Uses. 


XXIIl. 


Leaves and Buds. 




Flowers, their Parts and Uses. 


XXV. 


Fruits and Seeds. 


XXVI. 


The Palm Trees. 




Cereals, the Sugar-Cane, etc. 


xxvn. 


Starches. 




Oils and Fats. 


XXVIIL 


Gums, Resins, Gum-Resins, etc. 


XXIX. 


Cotton, Hemp, Flax, Jute. 


XXX. 


Paper. 


XXXI. 


Bleaching and Dyeing. 





Tea, Coffee and Chocolate. 

Spices. 

Opium, Quinine and Camphor. 

Indigo, Oak-Galls, etc. 

Classification of Animals, 
and XXIV. Classification of 
Vertebrata. 

Classification of Invertebrata. 

Coverings of Vertebrate Ani- 
mals. 

The Bony Skeleton and its 
Modifications. 

Teeth, — Varieties and Uses. 

Tongues. 

Tails and their Uses. 

The Principal Internal Organs 
of Animals. 



PART II. —SPECIMEN LESSONS, 



Lhsson. 

I. Paws and Claws. 

II. Cocoa-Nut. 

III. Cotton and Wool. 

IV. An Egg. 

V. Acorn and Hazel-Nut. 

VI. Milk. 

VII. Onion, Turnip, Carrot. 

VIII. Cat and Dog. 

IX. Down. 

X. A Quill Feather. 

XI. Gutta Percha. 

XII. Leaves. 
XIII., XIV. and XV. Starch. 

XVI. The Horse. 

XVII. The Cow and the Sheep. 

XVIII. Honey and Wax. 

XIX. Ivory. 

XX. and XXI. Seeds and Seedlings. 

XXII. Olive Oil. 

XXI I I. Liber. 

XXIV. Mammals and Birds. 
XXV. Reptiles and Fishes. 

XXVI. Mammals. 

XXVII. Chewing the Cud. 

XXVI 1 1. Horns and their Uses. 

XXIX. Parts of a Flower. 

XXX. Birds' Nests. 

XXXI. The Hedgehog. 

XXXII. Whale Oil. 

XXXIIL Leather. 



Lesson. 

XXXIV. The Mole. 
XXXV. Cotton. 

XXXVI. Vertebrata and Invertebrata. 
XXXVII. The Cockroach. 
XXXVIII. The Earthworm. 
XXXIX. Spider's Threads. 
XL. Bleaching. 

XLI. The Rat and His Relatives. 
XLII. Beaks of Birds. 
XLIII. andXLIV. Snakes. 
XLV. andXLVI. Fishes. 
XLVII. Insects — Form and Structure. 
XLVIII. Insects — Benefits and Injuries. 
XLIX. Insects, — Metamorphosis. 
L. Insects, Legs and Feet. 
.LI. Insect and Spider. 
LII.-LIV. Legs and Feet,— Mammals. 
LV.-LVI. Legs and Feet, — Birds. 
LVII. Flour. 
LVIII. The Frog. 
LIX. The Frog,— Life History. 
LX. and LXI. Eggs. 
LXII. Snails. 
LXI II. Snails — Whelk and Periwinkle. 
LXIV. Snails. 

LXV. The Amoeba and Foraminifera 
LXVI. The Hydra. 
LXVII. Sea Anemones and Corals. 
LXVIII. Plant Factories. 



352 pages. 



121 illustrations and seven plates. Cloth, 1^1.50. 
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T^atural History Object Lessons, a Manual for Teachers. 

By Gno Ricks, Inspector of Schools, 1 
tail price, i -50. 

Guides for Science- Teaching. 

Published under the auspices of the Boston Society of Natural History. For 
teachers wlio desire to practically instruct classes in Natural History, and designed to supply 
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History, 25 cts. XIV. Bovvditch's Hints for Teachers 

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30 cts. XV. Clai'p's Observations on Common 

VI. Hyatt's Mollusca, 30 cts. Minerals, 30 cts. 

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Elementary Course in Practical Zoology. 

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The Teaching of Geology. 

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Astronomical Lantern ami How to Find the Stars. 

By Ri'.v. Jamks I'KKFNtAN Ci.akke. I ntcudcd to familiarize studcuts with thc constel- 
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