REESE LIBRARY 
 
 ^ 
 
 OF THE 
 
 ^ UNIVERSITY OF CALIFORNIA- 
 
 Received^ 
 Accessions No.^^^j^ 
 
CLASS-BOOK 
 
 OF 
 
 ELEMENTARY MECHANICS. 
 
I'RINTED BY PHILIP AND SON, LIVERPOOL. 
 
CLASS-BOOK 
 
 ELEMENTAET MECHANICS. 
 
 tfntrolwctton to Natural 
 
 ADAPTED TO 
 
 THE REQUIREMENTS OF THE REVISED NEW CODE. 
 
 PART I. MATTER. 
 
 WILLIAM HEWITT, B.Sc. 
 
 
 SCIENCE DEMONSTRATOR FOR THE LIVERPOOL SCHOOL BOARD. 
 
 LIBRARY 
 
 UNIVERSITY OF 
 
 CALIFORNIA. 
 
 LONDON: 
 GEORGE PHILIP & SON, 32 FLEET STREET; 
 
 LIVERPOOL: CAXTON BUILDINGS, SOUTH JOHN STREET, 
 AND 49 & 51 SOUTH CASTLE STREET. 
 
 l88o. 
 
PREFACE. 
 
 SOME three years ago the Liverpool School Board resolved to 
 introduce experimental science teaching into their schools, and 
 the writer was appointed to organise a scheme of demonstrations 
 in accordance with this resolution. The subject selected for 
 demonstration in the boys' schools was elementary Natural 
 Philosophy, or Mechanics as defined in Schedule IY. of the 
 Revised Code. The experience gained in the practical carrying 
 out of that scheme has led to the production of the present 
 work. It is primarily intended as a lesson book to be used by 
 the scholars in the intervals between the experimental demon- 
 strations. The difficulty hitherto has been to get the children 
 to express, in anything like precise language, the ideas which 
 the experiments have suggested to their minds. It is hoped 
 that a careful reading of the following lessons week by week, 
 supplemented by the comments and explanations of the teacher, 
 may go a long way towards removing this difficulty The 
 exercises at the end of each lesson are believed to constitute a 
 valuable feature of the book if properly used. They deal for 
 the most part with practical applications of the principles set 
 forth in the preceding lesson, and have been specially designed 
 to stimulate the observing and reasoning faculties of the 
 scholars. Each exercise, in fact, might furnish a theme for con- 
 versation between a teacher and his class. It is universally 
 acknowledged that education of this kind, where the scholars 
 
VI PREFACE. 
 
 themselves are made to do the main part of the work, the 
 teacher guiding their efforts and occasionally assisting them, is 
 the best education in the true sense of the word. 
 
 Although the book has been arranged to accompany a special 
 series of demonstrations, it is also adapted for use as a class- 
 book in any school where the subject is taught. It would be a 
 very easy matter for an intelligent teacher to devise numerous 
 simple experiments, in addition to those incidentally referred to 
 in the book, to illustrate each lesson. A very small outlay 
 would provide all that was really necessary in the way of appa- 
 ratus, and indeed a vast number of illustrations and experiments 
 might be shown by the aid of such simple objects as are to be 
 found in any house or schoolroom. 
 
 The present part deals with MATTER, and embraces the sub- 
 jects of the first stage of Mechanics as defined by the Revised 
 Code. A second part, dealing with FORCE, and embracing the 
 second and third stages of the subject, is in preparation. 
 
 W. H. 
 LIVERPOOL, April 1880. 
 
CONTENTS. 
 
 LESSON PAOB 
 
 1. MATTER 1 
 
 2. THE THREE STATES OF MATTER 2 
 
 3. SOLIDS 4 
 
 4. STRUCTURE OF SOLIDS . . ,* * , .6 
 6. HARDNESS OF SOLIDS . . , . . . * 9 
 
 6. EFFECTS OF HEAT UPON SOLIDS . , . . .11 
 
 7. PROPERTIES OF SOLIDS 13 
 
 8. MEASUREMENT OF LENGTH 15 
 
 9. MEASUREMENT OF AREA AND VOLUME. PLANE SUR- 
 
 FACES 17 
 
 10. LIQUIDS ,.20 
 
 11. SURFACE OF LIQUIDS ....... 23 
 
 12. CAPILLARY PHENOMENA OF LIQUIDS .... 26 
 
 13. PRESSURE OF LIQUIDS 27 
 
 14. SPECIFIC GRAVITY OF LIQUIDS 30 
 
 15. BUOYANCY OF LIQUIDS. SPECIFIC GRAVITY OF SOLIDS . 33 
 
 16. FLOATING BODIES . ...' . .36 
 
 17. EFFECTS OF HEAT UPON LIQUIDS 39 
 
 18. CONVERSION OF LIQUIDS INTO SOLIDS AND GASES , . 41 
 
 19. PROPERTIES OF LIQUIDS ....... 44 
 
 20. GASES 46 
 
 21. THE AIR PUMP 48 
 
 22. PRESSURE OF THE AIR ,.,,... 50 
 
viii CONTENTS. 
 
 LESSON PAGE 
 
 23. THE BAROMETER . 52 
 
 24. THE SIPHON . 56 
 
 25. PUMPS 58 
 
 26. EFFECTS OF HEAT UPON GASES . . . . 61 
 
 27. PROPERTIES OF GASES 63 
 
 28. GENERAL RELATIONS BETWEEN SOLIDS, LIQUIDS, AND 
 
 GASES . 65 
 
 29. POROSITY AND COMPRESSIBILITY 67 
 
 30. ELASTICITY .69 
 
 31. TENACITY, DUCTILITY, AND MALLEABILITY . . .71 
 
 32. MEASUREMENT OF TIME .73 
 
 33. INSTRUMENTS FOR MEASURING TIME . . . .76 
 
 34. MEASUREMENT OF VELOCITY . . . . . .79 
 
 EXAMINATION QUESTIONS 83 
 
L I B B A R V 
 
 UNIVERSITY OF 
 
 CALIFORNIA. 
 
 x 
 
 CLASS-BOOK OF ELEMENTARY MECHANICS. 
 LESSON 1. 
 
 MATTER. 
 
 That which we can perceive by our senses to exist is called 
 " matter." Substance, and thing, are words sometimes used instead 
 of matter. 
 
 We are provided with five senses, i.e., five means 'of finding out 
 something about the substances by which we are surrounded. These 
 senses are termed sight, smell, taste, touch, and hearing. By 
 the sense of sight, for example, we ascertain that the sun is bright, 
 that leaves are green, and that water is transparent. By the 
 sense of smell, we perceive that there is a difference between 
 ammonia and water, though to the sense of sight both are alike. 
 By means of the sense of taste, we are able to class some substances 
 as sweet, others as bitter, and others again as sour, &c. The sense of 
 touch enables us to distinguish between rough and smooth sub- 
 stances, and to ascertain whether the edge of a knife is sharp or 
 blunt. The sense of hearing sometimes informs us that the wind is 
 blowing, (i.e., that the substance we call air is in motion), and that 
 the waves are dashing on the beach. 
 
 Let us suppose that we enter a room into which coal-gas is 
 escaping. We become aware of the presence of that particular kind 
 of matter, in the first instance, probably, by the sense of smell. We 
 discover the exact place where it is escaping, perhaps by hearing a 
 slight hissing noise. We make sure of it by holding our hand over 
 the hole, so as to feel the current (or wind) of gas. And possibly we 
 make one more experiment to satisfy ourselves that it is really coal- 
 gas, by cautiously putting a light to it, to see if it will burn. 
 
 Each substance has certain characters or properties, by which we 
 can distinguish it from other substances ; and these properties we 
 ascertain by the use of our senses. Thus a marble has the properties 
 of roundness, smoothness, greyness (or some other colour), incom- 
 bustibility, &c. In order to describe a substance, we mention some 
 of its most important properties. 
 
 All kinds of matter have several properties in common, of which 
 
 A 
 
2 THE THREE STATES OF MATTER. 
 
 three of the most important are these, extension, divisibility, 
 weight. By saying that any portion of matter has extension, is 
 simply meant that it takes up some room, i.e., extends over a certain 
 space. By divisibility is meant the property of being divided ; 
 and no one has yet seen a piece of matter so small that it cannot 
 ^possibly be divided into still smaller pieces. A drop of water on the 
 'point of a needle, is large enough to contain thousands of very small 
 animals swimming about in it. A small lump of sugar put into a 
 larce pail full of water is broken up into pieces too small to be seen, 
 and some of it is present in the smallest drop of water we can take out 
 of the pail. Weight is a property which we shall consider in a future 
 lesson ; for the present, it will be sufficient to know that when we 
 say a body has the property of weight, we mean that it presses down 
 towards the earth. With a good balance, and with care, every sub- 
 stance that we are acquainted with can be weighed, even such sub- 
 stances as the air and coal-gas which are invisible. There are other 
 properties common to all matter, with which we shall become ac- 
 quainted as we proceed with these lessons. 
 
 We have learned that our senses inform us of the existence of 
 matter, and that there are many different kinds of matter ; each 
 kind possessing certain properties. Some of these properties we can 
 ascertain by observation alone, others can only be discovered by ex- 
 periment, that is by doing certain things to make these properties 
 plain and apparent. 
 
 EXERCISES. 
 
 1. What properties of glass can be ascertained by the sense of sight, touch, 
 
 and taste respectively ? 
 
 2. By what properties may coal-gas be distinguished from air? 
 
 3. "What are the properties of iron and of copper respectively ? 
 
 4. Name three bodies to illustrate each one of the following properties : (1) 
 
 transparency ; (2) sweetness ; (3) redness j (4) smoothness ; (5) round- 
 ness ; (6) brittleness. 
 
 5. Describe the properties of a good knife. 
 
 LESSON 2. 
 
 THE THREE STATES OF MATTER. 
 
 In the preceding lesson, we learned that there were many 
 different kinds of matter, distinguishable by their properties ; for 
 example, red matter, green matter, sweet matter, &c. We soon learn, 
 however, to group together all these different kinds of matter into 
 three great classes, which we call respectively solids, liquids, and 
 gases. Matter occurs in three states or conditions, viz., in the 
 condition of a solid, a liquid, or a gas. 
 
THE THREE STATES OF MATTER. 3 
 
 In the solid state, the parts of which a portion of matter consists 
 are relatively fixed, and therefore the solid retains its shape if left 
 to itself. 
 
 In the liquid state the parts are not fixed, but can move about 
 among themselves ; the shape changes according to the shape of 
 the vessel which contains the liquid. Hence it is necessary to sup- 
 port a liquid round the sides, otherwise it will flow or spread out 
 into a thin sheet. 
 
 A gas is still more changeable than a liquid, since, if not pre- 
 vented by being enclosed on every side, it will spread out, or flow, in 
 every direction. A gas will take not only any shape, but also any 
 size. 
 
 We can state the distinction between these three states of matter 
 very shortly, by saying that a solid retains both its size and its 
 shape : a liquid retains its size, but will change its shape : a 
 gas will change both its size and its shape. 
 
 Some kinds of matter are occasionally found assuming each of 
 these three states. Thus water is usually met with in the form of a 
 liquid, which must be enclosed round the sides by a vessel, if we 
 wish to retain it in any particular shape and place. Cold, however, 
 changes liquid water into the solid which we call ice, in which the 
 parts are so firmly held together that we see it lying about in lumps. 
 Heat effects another change, for it converts liquid water into the gas 
 which we call steam. The same is true of solid butter and other 
 kinds of fat, which are converted by heat into liquids. It appears, 
 therefore, that the particular state in which we find a certain portion 
 of matter at any time, depends altogether on the temperature or 
 degree of heat. 
 
 Liquids and gases have many important properties in common, 
 and are sometimes classed together under the one name fluids. 
 This term is applied to them because they will flow, that is, will 
 spread out of themselves, if not supported round the sides. The 
 great difference, however, between them is this, that while it is very 
 difficult to alter the size of a liquid, even a little, a small quantity 
 of gas will fill out a very large space, and a large quantity of gas can 
 readily be forced into a very small space. 
 
 EXERCISES. 
 
 1. Name ten solids, ten liquids, and as many gases as you can. 
 
 2. Name ten fluids. Explain the meaning of the term fluid. 
 
 3. Mention several substances which are sometimes liquids and sometimes 
 
 solids. What causes them to change from solids into liquids ? 
 
 4. State the properties of ice which distinguish it from water. 
 
 5. The water of the sea rises into the air in the form of vapour, and usually 
 
 descends again in the form of rain or snow. Explain the causes which 
 thus change the state of the water. 
 
4 SOLIDS. 
 
 LESSON a 
 
 SOLIDS. 
 
 Matter, as we have seen, may take one or other of three forms, 
 viz., the solid, liquid, or gaseous form, and we have learned some- 
 what of the differences which exist between these forms. We must 
 next take each form by itself, and study its characters. 
 
 The particles (or very small parts) of which we know every solid 
 body is composed, are held together by some power or force, and 
 to this force the name of cohesion has been given. In most solids 
 this cohesion is strong, as we find when we try to separate one por- 
 tion of a solid from another. But its strength varies in different 
 solids, for it is much easier to tear apart a thin wire of lead, than a 
 wire of steel of the same thickness. And in many solids the force 
 of cohesion varies, according as we try to break the solid in various 
 ways. A strip of glass, for example, would support a heavy weight 
 hung from it lengthwise, while it would snap across at once if we 
 tried to bend it. Substances like glass, which readily snap when 
 we try to bend them, are said to be brittle. Substances which can 
 be bent in various ways without breaking, are said to be tough. 
 Were it not for cohesion, there would be no such thing as a solid 
 body ; matter would be broken up into the smallest possible pieces. 
 Most of the other properties which belong specially to solids, are 
 due to the fact that in solid bodies generally the cohesion is 
 strong. 
 
 There is one other fact about cohesion in solids which is very in- 
 teresting, viz., that when once the cohesion is destroyed, as by 
 powdering the solid, it cannot easily be restored. A drop of water 
 can be divided into two or more drops, and these when brought 
 together again will at once unite to form one large drop as at first. 
 The powder of broken glass will not unite again, even though 
 pressed together very strongly ; and the cohesion can only be restored 
 by melting the solid, i.e., by first turning it into a liquid. And this 
 is the case with most, if not all solids. 
 
 A solid body retains its shape if left to itself, and does not need 
 to be supported round the sides as a liquid does. The shape of a 
 solid can be altered more or less by pressure on it, and some solids 
 are so soft that they can be moulded into any shape. Lead is a 
 familiar example of such solids ; sponge, bread, clay, and putty are 
 other examples. It is easy to understand why sponge and bread 
 can have their shape so easily changed, since, besides being soft, 
 they are porous, that is, full of small holes or pores. Clay and 
 putty again are moulded so easily, because they really consist of fine 
 powders mixed with a liquid, viz., water in one case, and oil in the 
 other. A powder is a collection of very small solids. 
 
 The size of a solid body is definite or fixed, so long as the body is 
 
SOLIDS. 5 
 
 not acted on by any great force. But by the action of a powerful 
 force, all solid bodies can be slightly compressed, or forced into a 
 smaller space. As a general rule, we must remember that solids 
 are difficult to be compressed, and can only be compressed 
 very little. 
 
 It' a block of wood rest on a table, and between the wood and the 
 table a sheet of paper be placed, the paper will of course be pressed 
 down by the wood. If now a weight be set on the top of the wood, 
 the paper will be pressed down still more, for the pressure of the 
 weight is carried through the wood, or the wood is said to transmit 
 the pressure. If a second piece of paper rested against the side of 
 the. wood, the pressure of the weight would have no effect on this 
 piece of paper, for the wood transmits the pressure only in one 
 direction. And this is one of the most important properties of solids, 
 that they transmit pressure, but only in one direction, viz., 
 the direction in which the pressure acts on them. To take one more 
 example of this important property: if some one were to push against 
 one side of a table, the effect of the push would be felt at the oppo- 
 site side, but not in any other part of the table. 
 
 A number of solids together, especially if they are small and 
 round as marbles or shot, or better still, if they form fine powder 
 as sand or flour, transmit pressure in a somewhat different 
 manner from that in which one solid does. In a bag of flour or a 
 bag of sand, we may see that there is great pressure not only on the 
 bottom, but also on the sides of the bag. And if we set a weight 
 on the bag, or press on it with our hands, we may see that the 
 pressure is transmitted laterally, as well as in a straight line. When 
 we come to consider liquids, we shall see that they also transmit 
 pressure laterally, but much more perfectly than these masses of 
 small solids. 
 
 EXERCISES. 
 
 1. When is a body said to be brittle ? Name several brittle bodies, and 
 
 several bodies which have the quality opposite to brittleness. 
 
 2. Explain why you can put your finger into sand, and not into a piece of 
 
 iron. 
 
 3. Why does a paper bag containing flour often burst at the sides ? 
 
 4. Define the words cohesion, transmission, laterally, compression. 
 
 5. Name two very important differences between solids and liquids. 
 
 6. If a piece of iron were ground into fine powder, how could the cohesion^ 
 
 be restored, so as to form the whole into one solid lump 
 
STRUCTURE OF SOLIDS. 
 
 LESSON 4. 
 
 STRUCTURE OF SOLIDS. 
 
 A piece of sugar, or a drop of water, is built up of a vast number 
 of very small portions, or particles, and it is for this reason that we 
 are able to divide each of those bodies. As we expressed the fact 
 in a former lesson, these bodies, in common with all others, have 
 the property of divisibility. The smallest portion of sugar, or of 
 water, which could possibly be obtained, is called a molecule, 
 which really means a little piece. And the same is the case with 
 all other solids, and with liquids and gases. We may therefore 
 make this general statement : All bodies are built up of mole- 
 cules. In the course of these lessons, we shall frequently have to 
 speak of the molecules of which bodies are composed. If we under- 
 stand clearly this idea of the constitution of matter, we shall find 
 that we are able to explain many facts, which would otherwise be 
 very difficult to explain. 
 
 When we come to examine the manner in which solids are built 
 up, we find great differences in various solids. And as this is a 
 very important matter in many respects, we must examine it some- 
 what in detail. If we take a quantity of the solid known as alum 
 and crush it into powder, we shall find on putting some of this 
 powder into a quantity of cold water, that all or part of it will 
 dissolve and disappear "in the liquid. Having dissolved as much 
 alum as possible in the water while cold, if we then heat it, we 
 shall find that the water will dissolve a still 
 further quantity. If the hot water be satu- 
 rated with alum i.e., made to dissolve as 
 much alum as possible and be then poured 
 out into a saucer or other vessel and set aside 
 to cool, a further observation may be made. 
 As the water cools, a portion of the alum will 
 separate from it in the form of a solid sub- 
 stance, or rather in the form of many solids, 
 which are small at first but gradually increase 
 Flg - l * in size. Examination of these solids soon 
 
 reveals the fact, that they are all more or less of one shape, viz., 
 a four-sided pyramid, or perhaps like two ordinary pyramids 
 attached by their bases (fig. 1). 
 
 And further, however dirty the solution may be, these solids, 
 at any rate while they are small, will be found clean, almost 
 transparent, with bright smooth faces and sharp edges. Such bodies 
 are called crystals, and the alum is said to be crystallised. 
 
 Many other bodies behave in a similar manner, separating from 
 the liquid in which they have been dissolved in the form of crystals. 
 The crystals of the other substances would not be of the same shape 
 
STRUCTURE OF SOLIDS. 7 
 
 as those of alum, each different substance having a more or less dif- 
 ferent form of crystal. But they would agree with the alum crystals 
 in having smooth shining faces, sharp edges, and in being, as a rule, 
 nearly pure. Crystallised sugar, sugar candy, and saltpetre are 
 familiar examples of crystallised bodies. A large number of crystals 
 of different substances are found in caves, in mines, in crevices of 
 rocks, &c., and some of these have been formed from solution, like 
 the crystals of alum. In museums we may see examples of many 
 of these crystallised minerals, such as gold, diamonds, quartz or rock 
 crystal, sulphur, several kinds of " spar," and the ores of many 
 metals. Very beautiful substances they usually are, and well worth 
 looking at carefully. So constant is the form of each particular 
 mineral, that experienced persons are able to distinguish one mineral 
 from another by its form alone. 
 
 The bottom of the vessel containing the alum solution, if left for 
 a day or two, will be found covered with a cake of alum, from which 
 cake, on its upper surface, project parts of crystals. But in the great 
 mass of it the crystals are so entangled together, that it is almost, if 
 not quite, impossible to distinguish one crystal from another. Bodies 
 which are evidently composed of crystals, but in which the crystals 
 are not easily distinguishable, owing to their being so mixed up 
 with each other, are said to be crystalline, Marbleaud loaf sugar 
 are good examples of crystalline bodies. The small sparkling faces 
 which are seen in these substances are faces of crystals, but the 
 crystals are all so matted together, that none have had the oppor- 
 tunity of growing large and distinct, 
 
 When a substance is dissolved, its molecules are separated from 
 each other by the action of the liquid, and being so small are 
 invisible. In many cases, as the liquid cools, or as it is slowly eva- 
 porated, cohesion draws these separated molecules together again, 
 forming them into one or more solids. The molecules, however, do 
 not come together in any order or position, but fit into each other so 
 orderly and neatly, as to build up the regularly formed body which 
 we term a crystal. So that a crystal may be said to be a solid 
 having a regular form, and composed of molecules which are 
 arranged in a definite order. Two important consequences follow 
 from this, the first of which is, that since particles of dirt or other 
 substances would not fit properly in the building up of the crystal, 
 these are usually left behind in the liquid, and the crystals are ob- 
 tained almost pure. And if they are dissolved and crystallised again 
 several times, they will at last be obtained quite pure. Common 
 w r ashing soda is crystallised, to separate it from the dirty substances 
 which become mixed with it in the process of manufacture ; and 
 most of the solid substances sold by druggists are crystallised in 
 order to purify them. The second consequence is, that a crystal will 
 cut or split more easily in some directions than in others ; just as 
 it is easy to drive a nail into the side of a wall between the bricks, 
 while it would be extremely difficult to drive it down into the wall 
 from the top. The molecules in a crystal correspond to the bricks 
 
8 STRUCTURE OF SOLIDS. 
 
 in the wall ; and a crystallised substance differs from an un crystal- 
 lised one, just in the same manner as a neatly-built wall differs from 
 a confused heap of bricks. The men who have to cut diamonds and 
 other precious stones into various shapes, take advantage of this pro- 
 perty of cleavage or splitting, and always endeavour to arrange their 
 work so as to cut the mineral in the easiest direction. 
 
 There is another way in which crystals of some substances may 
 be formed, viz., by first melting the substance so as to turn it into 
 a liquid, and then allowing it to cool slowly. In this way crystals 
 of sulphur and some metals are formed. 
 
 Snow consists of small crystals of solid water, or ice, which often 
 
 have most beautiful shapes 
 (fig. 2). These can be seen 
 very distinctly by observing 
 some snowflakes, caught on a 
 dark surface say a black coat 
 during a snow-storm. 
 
 Substances like clay, putty, 
 butter, and lead, which can 
 be cut as easily in one direc- 
 g ' 2 * tion as in another, and which 
 
 do not under ordinary circumstances take any definite shape, are 
 called amorphous or shapeless bodies. 
 
 There are other names used in addition to those already men- 
 tioned, in describing the structure of solids. Thus, solids which can 
 readily be split into splinters, threads, or fibres, are called fibrous 
 solids, and of these we have examples in wood, rhubarb, and lean 
 meat. Paper is a fibrous body, and is seen under the microscope 
 to consist of a number of small fibres crossing each other in all 
 directions. When a piece of paper, such as that used for blotting 
 paper, is torn, these fibres may be seen at the torn edges. 
 
 Another term sometimes applied to certain solids is granular, 
 which means composed of grains, or small rounded pieces. Sand- 
 stone is a rock composed of grains of sand, and is therefore said to 
 be granular. 
 
 EXERCISES. 
 
 1. What is a crystal ? Name a number of common substances which are 
 
 crystallised. 
 
 2. What is the advantage of crystallising solids ? 
 
 3. Sketch the form of a crystal of alum, and of sugar. 
 
 4. Explain why wood will split more easily in one direction than in another. 
 
 5. In what way does the arrangement of the fibres differ in the following 
 
 fibrous substances paper, rope, calico ? 
 
HARDNESS OF SOLIDS. 9 
 
 LESSON 5. 
 
 HARDNESS OF SOLIDS. 
 
 We can, without any difficulty, plunge our finger into water or 
 other liquids, because the cohesion is so small that the parts readily 
 separate to allow our finger to pass between them. But on attempt- 
 ing to penetrate a piece of stone in a similar manner, we meet with 
 a great resistance, owing to the greater cohesion of the solid stone. 
 We usually express this fact by saying that the stone is hard. Cer- 
 tain solids yield somewhat to the finger, and are dinted, as india- 
 rubber, or easily penetrated, as butter, and these we distinguish as 
 soft. Different solids have various degrees of hardness, as we should 
 expect after having learned that the force of cohesion varies so much 
 among them. A better way of testing the hardness of a solid than 
 to push it with our finger, is to take a sharp edged, or sharp pointed, 
 instrument, such as our finger nail, or a knife, or a pen, and 
 endeavour to cut or scratch the body. If with a steel knife we can 
 cut the " blacklead" and the wood of a pencil, and if neither wood 
 nor blacklead will cut, or even scratch, steel, it is plain that steel 
 must be harder than either of those two substances. Similarly, if 
 we can scratch lead with our finger nail, and if lead will not scratch 
 our nail, it must be that our finger nail is harder than lead. Once 
 more, glass is generally spoken of as a hard substance ; but when 
 we find that a diamond will not only scratch glass, but that, if the 
 diamond be of the proper shape, it will cut deeply into the glass, we 
 must allow that diamond is harder than glass. In this manner, then, 
 we can test the hardness of one solid, as compared with that of other 
 solids. Now it has been found, that while a diamond will scratch 
 any other solid whatever, no solid that we know of will scratch a 
 diamond. Therefore it is said that a diamond is the hardest of all 
 solids. This is a point of practical importance, for the men whose 
 business it is to cut and polish diamonds for jewellery, know that 
 the only substance they can use for polishing a diamond, is the dust 
 obtained from other diamonds. A diamond, after it has once been 
 polished, will remain smooth and bright, much longer than a piece of 
 glass or other substance made to imitate it, since it may rub against 
 many substances without being scratched at all. And diamonds 
 have of late been made use of, on account of their excessive hardness, 
 to bore through hard rocks. 
 
 The substance which stands next to diamond in hardness is emery, 
 which is much used for grinding and polishing steel, glass, and other 
 hard substances. Emery cloth, as it is called, is made by scattering 
 powdered emery over strong calico, which has previously been 
 covered with glue. It is used for cleaning rusty steel, and for other 
 similar purposes. 
 
 But the substance which is of most importance to us on account 
 of its hardness is steel, and of this nearly all our cutting instruments 
 
10 HARDNESS OF SOLIDS. 
 
 are made. Steel is a variety of iron, which has the wonderful pro- 
 perty of becoming hard or soft, according to the way in which it is 
 treated by the workman. If a piece of steel be made red hot, and 
 then allowed to cool slowly, it will become comparatively soft, so 
 soft that it is not very difficult to bend it, and far too soft to be of 
 any use for knives, or scissors, or chisels. The same piece of steel, 
 however, can be readily made hard, by again heating it, and when hot 
 plunging it into water, or oil, or other liquid, so as to cool it very 
 quickly. The degree of hardness which the steel acquires, depends 
 upon the quality of the steel, the temperature to which it is first 
 raised, and the rapidity with which it is cooled. Saws, chisels, 
 knives, &c., are made into their proper shapes while in a soft state, 
 and are then hardened in the manner just described. The workmen 
 vary the degree of hardness given to the different instruments, 
 according to the use to which they are to be put. We ought to 
 observe that steel becomes brittle, at the same time that it becomes 
 hard. We may, therefore, sometimes have to choose between having 
 our tools very hard and very brittle, or moderately hard and less 
 likely to break should they happen to fall. It seems to be a 
 general rule, that substances which are very hard are also very 
 brittle. 
 
 Other metals cannot be hardened in the same manner as steel. 
 Many can be hardened by mixing with them a small quantity of a 
 different metal, or alloying them as it is called. It has to be ascer- 
 tained by experiment, what metal is the most suitable to form the 
 alloy in each case. Gold in its pure state is a soft metal, and if it 
 were made into coins in that condition, these would very rapidly be 
 worn thin. It is found necessary to harden the gold used for coinage 
 and for jewellery, by alloying it with a little copper, for our Eng- 
 lish coinage, 1 part of copper by weight is added to 11 parts of gold. 
 For the same reason, silver is alloyed with copper before being made 
 into money, 92J parts of silver being alloyed with 1\ parts of 
 copper. 
 
 Bronze pennies, halfpennies, and farthings, are harder, and there- 
 fore stand wear better, than the old-fashioned copper coins which 
 they have replaced. They consist of an alloy of 95 parts of copper, 
 4 parts of tin, and 1 part of zinc. The very soft metal lead, which 
 is so much used for making bullets, shot, types for printing, &c., is 
 hardened by alloying it with a small proportion of another metal 
 called antimony. Brass, which is an alloy of copper and zinc, is 
 harder than either of those metals alone. 
 
 EXERCISES. 
 
 1. Arrange the following substances in the order of hardness copper, glass, 
 
 emery, iron, steel, lead, and diamond. 
 
 2. Explain clearly why it is found necessary to alloy gold with copper before 
 
 being coined. 
 
 3. Describe and explain the use of sand-paper, or glass-paper. 
 
 4. Why is it better to make a ruler of ebony than of ordinary deal? 
 
EFFECTS OF HEAT UPON SOLIDS. II 
 
 5. Why do uncivilised people use pieces of flint for cutting and scraping 
 
 rather than pieces of wood. 
 
 6. Name several cutting and other tools made of hard steel, and describe a 
 
 method by which they could be made less hard if required. 
 
 LESSON 6. 
 
 EFFECTS OF HEA T UPON SOLIDS. 
 
 We have now to study the changes in solid bodies, which are 
 brought about by the action of heat. Perhaps the change which we 
 most readily observe, is the softening which occurs in most bodies 
 when heated. In the preceding lesson, we learned that the hardness 
 of a solid body is due to its cohesion ; when the body becomes 
 softer, therefore, it must be on account of the cohesion becoming less. 
 And we may readily prove that such is the case, by observing that 
 we can easily cut through a stick of sealing-wax which has been 
 softened by heat, or by observing the ease with which a blacksmith 
 pierces or cuts a piece of red-hot iron. We shalr find that it is a 
 general rule, that heat lessens the amount of cohesion in bodies. 
 A piece of wire, which will easily support a certain weight whilst at 
 its ordinary temperature, will be broken by the force of the weight 
 if the wire be heated. Some bodies, such as sealing-wax, become so 
 soft when heated that they can be moulded into any shape ; and 
 what is more important, two pieces can be made to stick together 
 quite firmly. Wrought-iron is another body which, when sufficiently 
 heated, softens so much that it can be hammered (or wrought) into 
 any shape, and two pieces can be hammered or welded firmly 
 together. 
 
 If we go on increasing the degree of heat, the cohesion becomes at 
 last so far lessened, that the molecules are no longer held fixed in 
 their places. They are able to slide or move about among each 
 other, they spread out round the sides if not held, and in fact the 
 solid becomes converted into a liquid. In the second place, there- 
 fore, heat liquefies or melts solids, if it be sufficiently great. We 
 know from experience, that all solids do not liquefy at the same tem- 
 perature, that is, with the same degree of heat. Solid ice melts with 
 the heat of a warm room, butter melts on a warm summer's day, 
 sugar or lead may be melted in an iron spoon over the fire. Iron 
 requires a much greater heat, which is obtained in a special kind of 
 fireplace called a furnace ; the bricks of which the furnace is built 
 require a still greater heat to fuse or melt them. Of all the metals, 
 mercury, or quicksilver as it is sometimes called, is the most easily 
 melted, more easily than ice. So easily indeed is mercury melted, 
 that in England we may leave it out of doors on the coldest day in 
 winter, and it will remain a liquid. In colder climates than ours, 
 
12 EFFECTS OF HEAT UPON SOLIDS. 
 
 mercury becomes a solid, and can be cut and hammered like a piece 
 of lead. Tin, lead, and zinc, can be melted in an ordinary fire 
 before even they become red hot : silver, copper, and gold require a 
 very good fire indeed : iron requires a furnace with a good draught. 
 There is one metal, called platinum, which will not melt in any fire 
 or furnace, but requires the most intense heat which we can obtain, 
 viz., that produced by electricity. There is a substance which is 
 even more difficult than platinum to melt, for it is used to convey 
 the electricity when we wish to obtain the electric light, and the 
 heat sufficient to melt platinum ; and although this substance is 
 exposed to the full heat, it "has never yet been melted. This sub- 
 stance we call charcoal or carbon. The chemist tells us that coal, 
 graphite (or blacklead as it is usually called), and diamond, are all 
 composed of this substance carbon, although they differ so much 
 from each other in appearance and properties. 
 
 Heat produces a third change in solids, which is not so readily 
 observed as the changes we have already mentioned. It is easily 
 proved, however, that nearly every solid body when heated expands, 
 or becomes larger. And this expansion takes place in every direc- 
 tion, the body becoming at the same time longer, broader, and thicker. 
 Thus if we had an iron bar, which would when cold just reach from 
 one wall of a room to the opposite wall, and just fit also into a hole 
 of the same shape as itself, this bar when hot would be too long to 
 lie down between the two walls, and too large to go into the hole. 
 When the bar had cooled down again to the same temperature as at 
 first, it would be found to have returned exactly to its original size. 
 We can easily understand that if we put the bar in a very cold 
 place, it would grow smaller every way, or would contract. In the 
 third place, therefore, we say that heat expands solid bodies, and 
 cold contracts them. A very few bodies are known which do not 
 expand when heated, but we need not now trouble about them. 
 These changes which occur in the size of a body are so very small, 
 that people generally take no notice of them, but they are really 
 very important. The more strongly bodies are heated, the more they 
 expand. All bodies do not expand to the same amount, some expand 
 much more than others ; brass, for example, expands nearly twice as 
 much as iron, if the two are equally heated. The iron bars of which 
 a railway is formed are not laid close together, but a small space is 
 left between the adjoining ends of each pair. We might observe 
 that this space is less in summer than in winter, and occasionally, in 
 hot weather, the rails expand so much that their ends meet together, 
 and no space is left for any farther expansion in this direction. 
 During very hot summers it sometimes happens that, after the rails 
 have met in this manner, the heat tends to expand them still more. 
 In such cases the rails become bent and twisted, in order to find room 
 to expand, and that portion of railway becomes useless for a time. 
 Iron bridges expand or contract with every change in the tempera- 
 ture of the air. And to prevent the bridge from becoming broken 
 or bent out of shape, like the rails just mentioned, one end is not 
 
PROPERTIES OF SOLIDS. 13 
 
 fastened down tight, but is frequently put on rollers, so that it may 
 easily move backwards and forwards. For the same reason, one end 
 of the firebars of a grate or furnace should be left free, in order that 
 they may expand and contract. Drinking glasses are frequently 
 cracked by pouring hot water into them, and the reason is very 
 plain. The inside of the glass becomes heated and endeavours to 
 expand, but the outside remains cool and of its original size, 
 especially if the glass be thick, and consequently the struggle often 
 ends in the glass being cracked. The iron rim, or tire, placed 
 round wheels, is fitted on tightly while it is hot, then, as it cools, 
 it draws together with enormous force the separate pieces of wood 
 of which the wheel is built up, and they are held as if in a vice. 
 A great many other examples might be given of the expansion and 
 contraction of solid bodies bv the action of heat and cold. 
 
 EXERCISES. 
 
 1. Why does a lump of clay become hard when put into a fire? What other 
 
 changes will the clay undergo if the heat be sufficiently great ? 
 
 2. In fastening two plates of iron together, the iron nails (or bolts) are often 
 
 put in red hot ; is there any advantage in doing this ? 
 
 3. Describe the changes which take place in a piece of lead placed in an 
 
 iron ladle over a good fire. 
 
 4. Why are the bars of fire grates made of iron rather than of zinc or lead ? 
 
 5. When a glass stopper is very fast in the neck of a bottle, people some- 
 
 times slightly warm the bottle neck, and the stopper comes out easily. 
 Explain this. 
 
 6. Arrange the following substances in the order in which they would melt, 
 
 if exposed to heat gold, ice, platinum, beeswax, iron, lead, butter. 
 
 LESSON 7. 
 
 PROPERTIES OF SOLIDS. 
 
 Solids are composed of very minute portions, each of which is 
 called a molecule ; these molecules are held together by the force 
 of cohesion. In solids, this force is sufficiently strong to keep the 
 molecules fixed in the same position in the body, that is to say, the 
 molecules are not able to move about among each other. But the 
 amount of cohesion varies in different solids, and frequently also in 
 different directions in the same solid. Therefore, since the strength 
 of any particular solid depends mainly on the strength of this co- 
 hesion, different solids have different degrees of strength. And 
 since the molecules of a solid are so firmly fixed together, the solid 
 retains any particular shape that we may give to it, without the 
 
14 PROPERTIES OF SOLIDS. 
 
 necessity of supporting it, as we should have to do with a liquid. 
 The size of a solid also remains the same, unless we use considerable 
 force to alter it. By pressing with great force upon solids, doubtless 
 they may all be compressed a little ; but in general it is difficult to 
 compress a solid, and then the amount of compression is only small. 
 Heat causes the molecules of a solid to move further apart from each 
 other, and thus increases the size of the body as a whole, or expands 
 it. Cold has exactly the opposite effect, which is just what we 
 should expect when we remember, that to cool a body is simply to 
 take heat away from it. When we take heat away from a body, 
 the cohesion gets more and more power over the molecules, and 
 draws them closer together. Solids also become soft under the 
 action of heat, and when the heat is sufficiently great they melt, or 
 fuse, or liquefy. These three words melt, fuse, liquefy mean the 
 same thing when used as we have just used them, viz., to turn from 
 the solid into the liquid condition. The body, when in the liquid 
 state, is said to be melted, fused, or liquefied. The word liquefy is 
 sometimes used when we speak of a gas turning into a liquid, but 
 the other two words are used with reference to solids only. Solids 
 melt at very different temperatures or degrees of heat, platinum and 
 carbon being two bodies which are extremely difficult to melt. 
 When solid bodies are brought into the liquid state, either by the ac- 
 tion of heat upon them, or by dissolving them in some liquid, the mole- 
 cules are free to move about easily. And when the body is allowed 
 to return to the solid condition, the molecules frequently unite 
 together in a certain order, and form the regular solids which are 
 called crystals. And since, in doing this, molecules of any other 
 substances which may happen to be present in the liquid are more 
 or less excluded, or left out of the crystal, crystallisation is usually 
 performed for the sake of purifying any particular substance. There 
 is still another property common to solids, which is very charac- 
 teristic of them. A pressure put upon any portion of the surface of 
 a solid is transmitted through the solid in one direction, and in one 
 direction only, viz., in the direction in which the pressure is applied. 
 That is to say, when the molecules on the surface of a solid body 
 are pressed upon, they in their turn press upon their neighbours on 
 the opposite side, and these again pass it on in the same direction. 
 And in this manner the pressure is transmitted through the solid, to 
 the side opposite to that where it was first applied. This is one of 
 the most important characters of solids, and distinguishes them from 
 fluids. 
 
 We may sum up briefly the principal characters of solids, which 
 we have learned in the preceding lessons, thus :- > 
 
 (1.) A solid has a definite shape and size. 
 
 (2.) The force of cohesion in solids is usually strong. 
 
 (3.) Solids in general are difficult to compress. 
 
 (4.) Solids as a rule expand when heated, and melt if the 
 heat is great enough. 
 
 (5.) A solid transmits pressure in one direction only. 
 
*> K ' , 
 
 MEASUREMENT OF LENGTH. ' }' , 15 -'I 
 
 J 'ii , V 
 
 Any portion of matter which possesses the propertied jxist^namett) / ^ 
 
 we call a solid. -^Jfi * > / " 
 
 EXERCISES. //^ 
 
 1. Explain the following words : expand, fuse, cohesion, transmits. 
 
 2. Write four sentences referring to solids, each sentence containing one of 
 
 the terms in the last exercise. 
 
 3. Explain clearly the difference between meltinfftnd dissolving solids. How 
 
 would you proceed to melt sugar, and how would you proceed to 
 dissolve it ? 
 
 4. Give the names of six solids that will dissolve in water, six that will not 
 
 dissolve in water, six metals, six fibrous solids, three transparent 
 solids, and three solids that would melt if placed near an ordinary 
 fire. 
 
 LESSON 8. 
 
 MEASUREMENT OF LENGTH. 
 
 It is a universal property of matter that every body, or portion of 
 matter, must occupy some space. We cannot think of matter, with- 
 out having present in our minds the idea that the matter takes up 
 room. This property of matter is sometimes called extension. 
 We express the same fact familiarly by saying that a portion of 
 matter has a certain size. In the latter case, however, we compare 
 the portion of matter of which we are speaking with other portions 
 of matter, and we speak or think of its size as being large or small, 
 in comparison with these other portions. As soon as we begin to 
 compare things with each other as to their size, we commence to 
 measure. In every science, measurement of some kind or other is 
 necessary, in order that things may be exactly compared. In these 
 lessons, the measurements we shall have to consider are those of 
 space, of time, and of velocity. The present lesson will deal with 
 the measurement of space, or rather with so much of it as refers to 
 length. We have to consider the methods by which we ascertain 
 and express the distance between any two points, e.g., between the 
 two ends of a rod. 
 
 We might, of course, compare the lengths of different bodies by 
 laying them side by side. This could only be done when the bodies 
 were present in the same place and at the same time ; and at the 
 best it would be a very inconvenient method of measurement. In 
 all cases of measurement, it is found necessary to select some fixed 
 Standard, or unit. Of course there will be different standards 
 adopted for the measurement of different things, and for different 
 properties of these things ; but in every case the standard used must 
 be of the same kind as the thing to be measured. Thus the stan- 
 dard of length must be a certain fixed length, the standard of time 
 
16 MEASUREMENT OF LENGTH. 
 
 a certain portion of time, and the standard of weight a certain fixed 
 weight. 
 
 We can readily conceive what great inconvenience there would be, 
 if each person were allowed to fix his own standard of length. In 
 that case we should never be certain what length of cloth, for 
 instance, we were buying, unless we measured it for ourselves. And 
 in the same way, we could form no idea of the distance between two 
 places from what another person told us, unless we first ascertained 
 by what standard they measured this distance, and compared their 
 standard with our own. Dishonest tradesmen would have great 
 facilities for cheating their customers, and trade and commerce would 
 necessarily suffer on that account. 
 
 From some of the old names of the measures of length, we see how 
 people at one time tried to set about obtaining definite measures. 
 Thus, one of their standards of length was the cubit, or the length 
 of the arm from the elbow to the end of the middle finger. Another 
 measure was the length of the foot, and in like manner the nail, 
 palm, span, fathom, &c., were measures taken from certain parts 
 of the human body. But since the size of these parts varies very 
 much in different individuals, they evidently would not furnish 
 exact standards of length. 
 
 The British imperial standard of length has been fixed by law, as 
 the length between two marks on a certain bronze bar, which is care- 
 fully preserved in London. This standard is called the imperial 
 yard, and all our other measures of space are derived from it. Thus 
 the third part of it is called a foot, and the thirty-sixth part of 
 it an inch, whilst 1760 lengths of the yard form a standard mile. 
 According to some people the length of the yard was fixed as the 
 length of the arm of King Henry the Eighth, but this is not correct, 
 for the Saxons had a measure called the yard ; and in London there 
 is a standard bar older than the time of Henry the Eighth, which is 
 almost exactly the length of our present standard yard. There 
 seems to be no doubt that the length of our yard is, as nearly as 
 possible, the same as that of the old Saxon yard. 
 
 Several very exact copies of the present standard yard bar were 
 ordered by Parliament to be made, and these are kept in different 
 places, so that in case the real standard should ever be lost or 
 injured (as once happened), one of these bars might be adopted as 
 the standard. Other very carefully made copies are distributed to 
 various parts of the country, in order that every person may have an 
 opportunity of testing the accuracy of the measures he uses. A vast 
 amount of time and trouble was spent in making this standard bar 
 and these exact copies, since the measurement of everything else is 
 found to depend more or less on the measurement of length, and 
 therefore the standard of length should be as exact as we can pos- 
 sibly have it. The actual standard bar is made of an alloy which 
 does not rust, and which at the same time is very strong, while 
 every possible precaution is taken to preserve the bar from being 
 bent, or twisted, or injured in any way. From what we learned in a 
 
MEASUREMENT OF AREA AND VOLUME. 17 
 
 previous lesson about the effects of heat on solid bodies, we know 
 that the length of this standard yard bar will vary, to some extent, as 
 its temperature varies. This has been taken into account, and the 
 Act of Parliament, which orders that this bar shall be our standard 
 of length, expressly states that the true length of the bar shall be 
 its length, when the bar has a temperature of 62 F. (or 62 degrees 
 on a Fahrenheit thermometer). 
 
 Our rules, and tapes, and other common instruments for measuring 
 lengths, if properly made, represent accurately enough, for all ordi- 
 nary purposes, the different divisions of the standard yard. This 
 enables us to compare measurements with one another with great 
 ease and certainty. An architect, for example, about to construct a 
 building, can measure the ground and decide on the lengths of the 
 various walls, doors, windows, &c., and mark these measurements 
 down on his plan. The builders then construct these parts according 
 to their own measures, and the architect may feel satisfied that there 
 should be no difference between the lengths he has ordered and those 
 actually made. Compasses are instruments occasionally used for 
 setting off fixed distances, as for dividing up a line in geometry into 
 certain portions. Callipers, again, which somewhat resemble com- 
 passes, are used for measuring the diamejber of cylindrical bodies ; 
 while the openings in a wire gauge are used in a similar manner, to 
 determine the diameters of different wires. 
 
 EXERCISES. 
 
 1. In what respects are wooden "foot-rules" superior to "tape-measures," 
 
 and for what purposes are the latter more suitable than the former? 
 
 2. I have a long coil of wire, the weight of which is 1 cwt. I have ascer- 
 
 tained that a length ojf 4 feet pf this wire weighs 1 oz. "What is the 
 total length of the wire ? 
 
 3. A carriage-wheel has a circumference of 11 feet, how often will it revolve 
 
 in the course of a journey of 5 miles? 
 
 4. The French standard of length the metre is equal to about 39 English 
 
 inches, and there are 1000 metres in a kilometre. Express the dis- 
 tance from Liverpool to London (200 miles) in kilometres. 
 
 LESSON 9. 
 
 MEASUREMENT OF AREA AND VOLUME. 
 
 In considering the length of a line, we take no account of its 
 breadth. Every line must have some breadth, however fine it may 
 be drawn. Similarly we neglect the thickness of a yard measuring- 
 rod, or the width of a two-foot rule, and have regard, in each case, 
 to the length only. 
 
 B 
 
18 
 
 MEASUREMENT OF AREA AND VOLUME. 
 
 In the present lesson, however, we shall in the first instance treat 
 .of the measurement of bodies, when we take into consideration their 
 breadth, as well as their length. We have to consider the measure- 
 ment of what is called the area of a surface, in other words, the 
 extent of the surface. For this purpose, we suppose the surface of 
 the body to be divided into a number of equal squares, and we 
 express the area by stating the number of squares. If each side of 
 the square measures one inch, we speak of the surface enclosed by 
 those sides as a square inch ; similarly we speak of square feet, square 
 yards, and square miles. 
 
 Fig. 3 represents a surface which might be a piece of board, or 
 metal, or any other substance. The long sides measure 2 inches 
 each, and the short ones 1 inch each. We should say that such 
 a piece was 2 inches long, and 1 inch broad. It is easily seen 
 from the figure, that in this case the surface contains 2 square 
 
 inches, as shown by the 
 thick lines. The thin 
 lines in the figure divide 
 the sides into half inches ; 
 but if we suppose that 
 each of these divisions 
 measured one foot, we 
 should have 4 rows of 
 squares, each row contain- 
 ing 2 ; or altogether, the 
 surface would contain 8 
 square feet. The gene- 
 ral rule for finding the area of square and oblong surfaces is 
 usually stated thus : Multiply the length by the breadth, and the 
 product will be the area. Of course it will be necessary to have 
 the length and breadth expressed in the same units, e.g., both in 
 inches, or both in feet. It is not quite so simple to find the area 
 of surfaces where the sides are inclined to each other, as in triangles, 
 or where the sides are curved ; still there are rules by which the 
 area of such figures can be obtained. It is necessary to understand 
 what is meant by area, and to know how it is measured, in order 
 to ascertain the extent of surface of the land belonging to a farm, 
 the extent of ponds, lakes, &c. The rules concerning the measure- 
 ment of area are also applied in determining the length of carpet 
 required to cover a room, or the co.st pf painting the walls of a house, 
 at so much per square foot, &c. 
 
 Just as in estimating the length of a body we neglect its breadth, 
 so in estimating the surface or area of a body, we neglect its thick- 
 ness. Every body must have a certain length, breadth, and thick- 
 ness, though for convenience we often agree to neglect one or two of 
 these dimensions. When we take into account the three dimensions 
 of a body, we are said to estimate its volume. And to do this, we 
 imagine the body divided into regular blocks, each face of which 
 measures one square inch, square foot, or square yard. Such blocks 
 
MEASUREMENT OF AREA AND VOLUME. 
 
 \ 
 
 X 
 
 \ 
 
 are called cubes, and they are said to have a volume of one cubic inch, 
 
 one cubic foot, or one cubic yard, according to the area of their 
 
 faces. Let us suppose that the 
 
 block represented in fig. 4 has a 
 
 length of 3 inches, a breadth of 2 
 
 inches, and a height (or depth) of 
 
 2 inches. Altogether it contains 
 
 12 cubic inches, as is easily seen 
 
 from the figure ; and these 12 
 
 cubic inches are arranged in 3 
 
 sets, each set containing 4 of 
 
 them. Thus there are 4 in the 
 
 front face, the area of which is 
 
 4 square inches, (that is, 2 X 2) ; 
 
 and three similar sets are built 
 
 up one behind another, to give 
 
 us the 3 inches of length of the 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 \ 
 
 Fig. 4. 
 
 body. The total volume is, 
 therefore, 12 cubic inches, viz., 
 2X2X3. And to determine the volume of any similar body, we 
 take the numbers expressing the length, breadth, .and thickness (or 
 depth) of the body, and multiply them together. A board measur- 
 ing 20 inches long, 6 inches broad, and 2 inches thick, might be cut 
 up into 240 (that is, 20 X 6 X 2) blocks, each block measuring one 
 cubic inch. It is easy to determine the volume of a body, if the 
 sides are straight and the body is of regular shape ; but it becomes 
 very difficult when the shape of the body is very irregular. We 
 cannot enter here into the methods of ascertaining the volumes of 
 irregular bodies ; such methods will be found in books on men- 
 suration. 
 
 Standard measures of capacity, as they are termed, are used for 
 determining the volumes of liquids, and of certain dry solids. In 
 this country the standard measure of capacity is the imperial gallon, 
 which is made to hold exactly ten pounds of pure water. Larger 
 measures of capacity employed for dry solids corn, peas, &c. are 
 the peck and bushel ; while smaller ones, as the pint, half-pint, &c., 
 are used for both dry solids and liquids. 
 
 We have had a great deal to say in this lesson about surfaces. 
 There are two kinds of surfaces, termed respectively plane and 
 curved. A plane surface is commonly spoken of as being flat or 
 level, and is defined as being a surface which contains all straight 
 lines drawn between pairs of points on the surface. That is to 
 say, if we marked any two points on the surface, and drew a per- 
 fectly straight line, or stretched a thin string, from one point to the 
 other, every part of the string or line would touch the surface. In 
 mechanical operations, it is frequently of the greatest importance to 
 be able to produce a surface, which shall be, as nearly as possible, a 
 true plane. A perfectly plane surface has never yet been produced by 
 any workman, and probably never will be. All the surfaces which 
 
20 LIQUIDS. 
 
 appear so very flat and smooth to the naked eye, appear rough and 
 irregular when seen through a microscope. Within the last few- 
 years, however, great advances have been made in the production of 
 plane surfaces, by the use of improved tools. The ordinary carpen- 
 ter's tool called a " plane," is used for the production of plane sur- 
 faces. The base of it is itself flat or plane, and in most forms of the 
 tool this surface is considerably extended in length and breadth, so 
 that it shall not sink or rise with every small irregularity of surface 
 of the wood which is being planed. The edge of the cutting tool 
 projects slightly below the " bed," or base of the plane, and by 
 gradually shaving off the higher portions, reduces the surface of the 
 board to an approximately true plane. By means of a " straight 
 edge," or bar having its edge as perfectly straight as possible, laid 
 on the board, the workman can ascertain whether any parts are 
 above or below the general level of the surface. For if the edge of 
 his bar be perfectly straight, and the board a true plane, then accord- 
 ing to the definition given above, every portion of the edge should 
 touch the board, in whatever position it might be placed. The sur- 
 face of water in a vessel or a small pond might be almost considered 
 a plane surface, but in reality it is slightly curved, as we shall see in 
 a succeeding lesson, and corresponds with the rounded surface of the 
 earth. 
 
 EXERCISES. 
 
 1. Which is the larger board, one having a surface of 20 square inches, or 
 
 one which is 20 inches square ? By how much is one larger than the 
 other? 
 
 2. A room measures 7 yards long, by 6 yards broad. Find the cost of the 
 
 carpet required, (a) having a width of f yards, at 3s. per yard ; (b) 
 having a width of one yard, at 3s. 9d. per yard. 
 
 3. A cubic foot of marble weighs 178 Ibs., find the weight of a solid block 
 
 measuring 8 feet long, 3 feet broad, and 6 inches thick. 
 
 4. A vessel containing water measures 4 feet long, by 18 inches broad, and 
 
 the depth of the water is 2 feet ; a cubic foot of water weighs 62^ Ibs. 
 Find how many gallons of water the vessel contains. 
 
 LESSON 10. 
 
 LIQUIDS. 
 
 We come next to the consideration of matter in the condition of a 
 liquid. And the first important difference we notice between liquids 
 and solids is, that in the former the cohesion acting between the 
 molecules is very weak, compared with the force of cohesion in solid 
 bodies. It may be proved in many ways that liquids have some 
 cohesion. The" thin film of soapy water which forms the bag we 
 call a soap bubble, is composed of molecules held together by the 
 
LIQUIDS. 21 
 
 force of cohesion. If we are inclined to think that the effect is due 
 entirely to the soap, we have only to look at a pool of water on a 
 rainy day, or at a vessel into which a stream of water is rapidly 
 falling, to see bubbles of water in which no soap has been dissolved. 
 Again, if we dip a finger into water, alcohol, or most other liquids, 
 a drop of the liquid adheres to the finger, and when we remove the 
 finger from the liquid the drop will hang on to it for some time. 
 Now this drop is composed of molecules, and the lowermost mole- 
 cules are held to those immediately above them, by the force of co- 
 hesion. Drops of dew on the leaves of plants, drops of water on dusty 
 or greasy surfaces, and of mercury on wood or glass, all prove and 
 illustrate the action of cohesion in liquids. Treacle and other similar 
 liquids have so much cohesion, especially in cold weather, that a 
 large quantity adheres to a spoon or to a glass rod thrust into them, 
 and this falls off slowly in the form of a string. If the string of 
 treacle dropping from a spoon be observed, it will be seen to grow 
 gradually thinner, and at last to break. The upper portion of 
 the string is then drawn up to the spoon, to form a more or less 
 rounded drop on its under surface. This again is due to the action 
 of cohesion. Treacle and glycerine have more cohesion than water, 
 and are often spoken of as viscous liquids ; alcohol and ether Lave 
 less cohesion than water. 
 
 The cohesion in liquids is sufficient to prevent the molecules 
 from leaving each other of themselves, but is not strong 
 enough to retain them in fixed positions. The consequence is, 
 that the least force is able to move the molecules of a liquid among 
 themselves. When a liquid is put into a vessel it makes its way to 
 the bottom, and spreads out laterally on every side so as to occupy 
 every corner. A liquid has no definite shape of its own, but will 
 take any shape according to the shape of the vessel containing it. 
 The weight of the upper portions of the liquid pressing on the lower 
 portions, causes the latter to move out in any direction in which 
 there is a chance for them to escape ; and unless the liquid be 
 supported round the sides, it will spread out or flow. In an earlier 
 lesson we learned that, on this account, liquids are included with 
 gases under the term fluids. Liquids, like all other bodies, are 
 acted on by the force of gravity. And because they are able to 
 move about so freely, they always move to the lowest position they 
 can find, and settle down so as to take a level or horizontal surface. 
 A liquid keeps the same size, however much its shape may be 
 changed. That is to say, a certain portion of water will have the 
 same number of cubic inches when contained in a round bottle, as it 
 would if held in a square vessel, however large this latter vessel 
 might be. If liquids had no cohesion, the case would be different. 
 
 Experiments have proved that it is possible to compress liquids to 
 a slight extent, but so difficult is it to compress them even a very 
 little, that for a long time it was believed that they were incompres- 
 sible bodies. When the force which has compressed them is re- 
 moved, they come back exactly to their original size. 
 
22 
 
 LIQUIDS. 
 
 There is another character in which liquids differ greatly from 
 solids, viz., the manner in which they transmit any pressure put 
 upon them. We have already seen that a solid body transmits 
 pressure in one direction only, but that a collection of small solids, 
 such as sand or flour, transmits the pressure in several directions. In 
 a liquid, where the molecules can move about far more easily than 
 the particles of sand or flour can do, the pressure is transmitted 
 equally in all directions. We know, for instance, that if a tin be filled, 
 with water, and a hole made in the side of the tin, water will flow 
 from the hole ; and not merely trickle out, but will be forced out to 
 
 
 Fig. 5 
 
 some distance (fig. 5). All the water above the level of the hole, is 
 pressing downwards on the layer of water which lies at that level. 
 This layer not only presses downwards on the layer below, (as is 
 shown by the water rushing out at the lower opening), but also 
 presses out sideways, and makes its escape with some force at the 
 opening. The force with which the jet of water rushes out diminishes 
 as the amount of water above it becomes less, simply because the 
 pressure of the upper water decreases. To pet another instance of 
 this general transmission of pressure by a liquid, let us take a hollow 
 india-rubber ball and fill it with some liquid. Then laying the ball 
 on the table, and applying pressure to it by means of the finger or a 
 weight, we may observe that the pressure is transmitted to every 
 part of the ball. This is proved by the fact that the liquid will 
 escape in any direction, upwards, downwards, or sideways, 
 wherever there may happen to be an opening. 
 
 EXERCISES. 
 
 1. Name ten liquids, and state which of them, if any, are viscous liquids. 
 
 2. If a bottle containing water, and another containing castor-oil, be shaken 
 
 up, some bubbles of air will be entangled in the liquids, and will 
 begin to rise to the top. In which of these two liquids will the 
 bubbles rise the more slowly, and why ? 
 
SURFACE OF LIQUID& 23 
 
 3. If a bottle be filled with water to the top of the neck, and then a glass 
 
 stopper quickly forced into the bottle, in which direction will the 
 water in the neck move? What property of liquids does this il- 
 lustrate ? 
 
 4. Water is poured on the middle of a table, and runs towards one particular 
 
 side ; what inference can you draw from this ? 
 
 5. If two drops of water are brought close together in one place, and two 
 
 pieces of glass close together in another place, what difference will be 
 observable in the two cases? 
 
 6. What property of liquids enables them to be sold by measured volume 
 
 (gallon, pint, &c.) more satisfactorily than solids? 
 
 7. Explain why a piece of lead is melted when it is desired to cast it into 
 
 any particular shape. 
 
 LESSON 11. 
 
 SURFACE OF LIQUIDS. 
 
 It was stated in the last lesson, that liquids in vessels arrange them- 
 selves so that their surfaces are level or horizontal. In speaking of 
 the horizontal surface of a liquid, we mean that if we could measure 
 the lengths of a number of lines, drawn from the centre of the earth to 
 different parts of the surface of the liquid, these lines would all have 
 exactly the same length. Strictly speaking, therefore, the surface of 
 water is not flat but curved. This is very easily observed at the sea- 
 side, where we see vessels in the distance gradually sinking out of 
 our sight first the lower, then the upper, parts disappearing. But 
 the surface of a plate of water, or even of a pond, is so little curved, 
 that we may think of it for all our purposes as being plane. It is 
 easy to understand why a liquid cannot heap itself up in one part of 
 a vessel, when we remember that if on any side a liquid is unsup- 
 ported, it will flow or spread out in that direction, and will move to 
 as low a position as it possibly can. In small portions, however, 
 liquids do not take level surfaces, but break up into little rounded 
 masses, which we call drops. This may be seen when mercury is 
 spilt on a plate, or when water is dropped on to something which it 
 does not easily wet. The pressure of the upper portion of the drop 
 tends to make the liquid spread out into a flat sheet ; but this pres- 
 sure is not so great as the cohesion, which tends to draw together the 
 molecules of the liquid as closely as possible. 
 
 We all know that when water is poured into the body, or wide 
 part, of a kettle or teapot, some of it passes into the spout. And we 
 may all see, if we notice carefully, that the water rises just as high 
 in the spout as in the body, notwithstanding the great difference in 
 the size and shape of the two parts. What is true of the kettle, is 
 true of all pipes or vessels which consist of two portions. When a 
 liquid is poured into such vessels, it runs first to the lowest part of 
 the vessel, and then rises just as high on the one side as on the other, 
 
24 
 
 SURFACE OF LIQUIDS. 
 
 whatever may be the shapes and sizes of the two portions. This 
 has been known for a very long time, and people commonly express 
 the fact by saying, that "water finds its own level," though 
 we must remember that all other liquids besides water will do just 
 the same. Many applications are made of this principle, one of the 
 most interesting, perhaps, being its application to the production of 
 fountains. 
 
 Let us suppose that a tank or cistern of water is placed on the top 
 of a building, or on a hill. If a leaden pipe were carried from the 
 
 
 Fig. ti. 
 
 tank to the ground, and then bent up again as high as the cistern, 
 the water would descend the pipe on one side and rise up on the 
 other, until it reached the same height as the surface of the water 
 in the cistern. If the pipe were cut off against the ground, as in the 
 illustration (tig. 6), the water would issue from the pipe, with force 
 enough to carry it nearly as high as the surface of the water in the 
 cistern. This would constitute a fountain, and the height of the jet 
 of water might be increased, by placing the cistern on a higher tower 
 or hill. A more useful application of the same principle, however, 
 is afforded by the water supply of many towns, where a reservoir 
 placed on some neighbouring hill corresponds to the cistern in the 
 illustration, and the pipe in the illustration represents the pipes laid 
 along the streets. The water will be forced from any opening in 
 these street pipes to a certain height, by the pressure of the water in 
 the reservoir above it, and will thus be available in the case of fires, 
 &c. The water will not rise to exactly the same height as the 
 reservoir, since some of the force is lost in passing through the pipes, 
 by the rubbing of the water against their sides ; and again, when it 
 comes from the pipe, part of the force is spent in beating the air out 
 of the way of the ascending column of water. Another interesting 
 application is seen in the case of the water-gauge on a steam-engine, 
 by means of which the engineman can see how high the water stands 
 
SURFACE OF LIQUIDS. 25 
 
 in the boiler. The gauge consists of a small tube connected with the 
 boiler, somewhat like the spout of a kettle is connected with the body 
 of the kettle, only that it opens into the boiler at the top as well as 
 at the bottom. 
 
 Suppose that we wish to fasten a piece of string horizontally from 
 one point to another, we can assure ourselves that it is horizontal in 
 several ways. In the first place, we might get a large glass vessel 
 of water, and ascertain whether the level of the water coincided with 
 the level of the string. Or we might get a bent piece of glass tube 
 containing some water, arid see that the string passed exactly at the 
 level of the water in both parts of the tube. The instrument ^which 
 is most generally used to ascertain whether surfaces are horizontal 
 is that known as the 
 spirit-level. It consists 
 of a glass tube slightly 
 curved, as shown in fig. 
 
 7, (though not curved . 
 
 quite so much as there Fig 7> 
 
 shown), nearly filled 
 
 with alcohol (or spirit, as it is frequently called), leaving, however, 
 a small bubble of air in the tube. The alcohol always lies in the 
 lower parts of the tube, and the bubble of air in the highest part. 
 When the two ends rest on a horizontal surface, the bubble is seen 
 in the middle of the upper portion of the tube, but if one end 
 be raised ever so little, the bubble moves up nearer to that end. 
 Alcohol is used in preference to water, partly because it does not 
 freeze even in the coldest weather, and partly because the bubble 
 moves more freely in alcohol than in water, for a reason already 
 given. 
 
 EXERCISES. 
 
 1. If an open glass tube be pushed vertically downwards into a vessel of 
 
 water, how high will the water rise inside the tube ? 
 
 2. Explain why the water comes from the pipes in the upper rooms of a 
 
 house with less force than from those in the lower rooms. 
 
 3. Describe several methods by which you could determine a point on a rod 
 
 set up in one part of a field, which should be at the same horizontal 
 level as a point on a second rod in another part of the field not far 
 distant. 
 
 4. If the upper portion of the spout of a kettle were cut off, to what height < 
 
 could the kettle be filled with water ? How might such a kettle 
 placed so that the water could reach the lid ? ' 
 
 5. Describe and explain the working of " lock gates " on can 
 
2(> CAPILLARY PHENOMENA OF LIQUIDS. 
 
 LESSON 12. 
 
 CAPILLARY PHENOMENA OF LIQUIDS. 
 
 When a piece of loaf sugar is laid in a small quantity of water or 
 tea, say in a teaspoon, the liquid is seen to make its way upwards to 
 the top of the lump sugar. Similarly, when a piece of blotting 
 paper is dipped into a drop of ink, the ink rises up into the blot- 
 ting paper and remains there. The force which causes the liquid in 
 each of these cases to make its way into and through the solid, is 
 called capillary force. It is sometimes called capillary attraction, 
 because in most cases there seems to be some force in the solid which 
 attracts, or draws nearer, the liquid. 
 
 If we examine carefully the surface of the water in a glass vessel, 
 we shall see that at the sides of the vessel the water is raised up 
 against the glass, and stands slightly higher there than the surface 
 of the rest of the water. This can be well seen with water in an 
 ordinary flat-sided glass bottle, especially at the corners of the bottle. 
 Mercury in a tumbler or bottle behaves differently ; where it comes 
 against the glass, the edge is turned downwards, so as to stand 
 slightly lower than the surface of the rest of the mercury. And if we 
 try a number of liquids in succession, we shall find this rule to hold 
 good : Those liquids which are able to wet (that is, adhere to) 
 the glass, such as water and alcohol, rise up against the glass : 
 while those which do not wet the glass, such as mercury, sink down 
 where they come in contact with it. 
 
 Again, suppose we take two pieces of sheet glass, place them 
 nearly close together face to face, and then plunge them into water, 
 alcohol, glycerine, turpentine, or any other liquid which wets the 
 plates, we shall find that the liquid rises up between the plates 
 some distance higher than the liquid in the vessel on the outside of 
 them. And a few very simple experiments will soon show us, that 
 the narrower the space between the plates, the higher the liquid 
 rises ; and of course the wider the plates are apart, the less is the 
 rise of the liquid. The same effects may be seen in glass tubes, and 
 the same rule holds good, viz., that the smaller the diameter of the 
 tube the greater is the rise of the liquid. In tubes, and between plates, 
 where the space is no larger than the thickness of a hair, the effects 
 of capillary force are very plainly seen. Such spaces are called 
 capillary spaces, from a Latin word capillaris, meaning hair-like. 
 If the glass plates or tubes were plunged into mercury, or if they 
 were greased before being plunged into water, the liquid would be 
 depressed, or forced down, against the glass ; and the narrower the 
 space the greater would be this depression. 
 
 Now a piece of loaf sugar is simply a collection of crystals, having 
 small spaces between them. And blotting paper is really composed 
 of a large number of fibres matted together, having also very small 
 
PRESSURE OF LIQUIDS. 27 
 
 spaces between the numerous fibres. When these substances are 
 placed in water or ink, the liquid rises up the capillary spaces in 
 them. The force, then, which causes the elevation or depression of 
 liquids, where they come into contact with solid bodies, is called 
 capillary force, and it is so called because its action is most strik- 
 ing in hair-like or capillary spaces. 
 
 The wicks of candles and lamps are masses of cotton fibres, which 
 contain between the fine fibres a large number of capillary spaces. 
 The lower portion of the wick dips into a quantity of melted 
 tallow, or wax, or oil, or other combustible liquid, which rises up 
 the wick by reason of the capillary force acting on it. 
 
 The roots, stems, and leaves of plants, all contain numbers of very 
 fine tubes vessels they are called through which the sap rises up 
 the plant. It is supposed that the rise of the sap in plants is due, 
 for the most part, to the action of capillary force. 
 
 EXERCISES. 
 
 1. In a narrow glass tube the surface of water has the form of a hollow cup, 
 
 while the surface of mercury is rounded like part of a ball. Explain 
 this. 
 
 2. Why do many liquids, when kept in bottles, rise up between the neck of 
 
 the bottle and the glass stopper ? 
 
 3. People sometimes water plants in flower pots by letting the pot stand in 
 
 a saucer of water. Describe aud explain the manner in which the 
 water gets to the roots of the plant, and even to the top of the soil. 
 
 4. Explain why ink spreads when written on paper which has not been sized 
 
 or glazed, most kinds of brown paper for example, and does not so 
 spread when written on similar paper after it has been sized. 
 
 5. Explain why a piece of dry bread will soak up a considerable quantity of 
 
 water or tea. 
 
 6. When a tallow candle is put into a vessel of water, the liquid is depressed 
 
 round the sides of the candle. Explain this effect. 
 
 LESSON 13. 
 
 PRESSURE OF LIQUIDS. 
 
 A liquid contained in a vessel exerts a pressure, not only on the 
 base of the vessel, but also on its sides. And we have learned that 
 any additional pressure which may be put on the surface of the 
 liquid, will be transmitted by it to the base and sides of the vessel. 
 Suppose that we have a tumbler half full of water, the water will 
 be pressing on the base of the tumbler, and on the sides as far as it 
 reaches. If we then pour an additional quantity of water into^the 
 tumbler, there will be an increase of pressure on the parts just 
 named. The water last poured in will press on the water below it, 
 
28 PRESSURE OF LIQUIDS. 
 
 and this will therefore exert a greater pressure than before on the 
 base and sides. Those portions of the sides of the tumbler which 
 have to support the water last added, will have to bear a less pres- 
 sure than the lower portions of the sides. And if we consider the 
 pressure on any one point of the tumbler, whether in the base or 
 sides, it is easy to see that its amount depends on the quantity of 
 water above it. The higher the surface of the water above the point, 
 the greater will be its pressure. The same rule holds, not only for 
 water, but for any liquid. Hence we may make this general state- 
 ment : The pressure of a liquid on any part of the base or 
 sides of a vessel containing it, varies according to the height 
 of the surface of the liquid above that part. A very simple 
 experiment will prove that this rule is true for the sides of a vessel. 
 If a tall tin be filled with water, or other liquid, and then several 
 holes be made in the side at different distances from the top, the 
 water will be forced out at each of these holes, as explained in lesson 
 10. But it will rush out with greater force, and to a greater distance, 
 from the lowest hole than from any of the others, while, on the other 
 hand, the highest jet will issue with the least force (tig. 5). It is 
 plain that the pressure of the water on the sides of the vessel 
 increases as we go deeper down below its surface. 
 
 Vessels that are to contain large quantities of liquids ought there- 
 fore to be made specially strong in their lower portions ; and the 
 deeper they are, the stronger they should be. For this reason, the 
 walls of reservoirs, and the embankments of rivers and canals, are 
 usually built much thicker at the bottom than at the top. 
 
 If we invert a tumbler, and push it down into a large vessel of 
 water, only a small quantity of water enters the tumbler. The 
 tumbler was quite full of air to commence with ; if none has been 
 allowed to escape, it follows that the air has been slightly com- 
 pressed. This compression has been effected by the upward pressure 
 of the water. It may be noticed that the deeper the tumbler is 
 forced into the water, the greater is the quantity of liquid which 
 enters, and the more, therefore, the air is compressed. This experi- 
 ment proves in a simple manner, that the upward pressure of a 
 liquid varies according to the depth bek>w the surface of the 
 liquid. The tumbler, in the experiment just described, may be 
 taken to represent a diving-bell. In large vessels of this kind, 
 people have gone down to considerable depths below the surface of 
 water, air being forced down a pipe into the top of the bell, so as to 
 keep the water from entering it at all. 
 
 Let us next suppose that we have a bent tube containing some 
 water ; we know that the water will stand at the same height in the 
 two arms of the tube. Let us suppose that the surface of the water 
 in each arm measures just one square inch. If we press on the sur- 
 face of the water in one arm with a force of 1 lb., some of the water 
 will be forced into the other arm, and the level of it there will be 
 higher than before. If we wish to keep the water in both arms at 
 the same level, a pressure of 1 lb. must be put on the water in each 
 
PRESSURE OF LIQUIDS. 
 
 arm. But if, instead of the tube being of the same size throughout, 
 one arm be wider than the other, the case will be somewhat different. 
 Suppose that the surface of the water in one arm measures one 
 square inch, while in the other arm it measures two square inches. 
 Then a force of 1 Ib. must be applied to each square inch of surface 
 in the large tube, in order to balance the force of 1 Ib. applied to the 
 surface of the water in the narrower tube. That is to say, a force of 
 1 Ib. in the narrow tube balances a force of 2 Ibs. in the wider one. 
 And if, in the latter, the surface of the water measured 10 square 
 inches, it would require a force of 10 Ibs. there to balance a force of 
 1 Ib. in the narrow tube. This is the principle of a very important 
 machine called the hydrostatic press. Sometimes this machine is 
 spoken of as the hydraulic press, and occasionally it is called 
 Bramah's press, after the name of the man who first made it to work 
 successfully. 
 
 The principle of the hydrostatic press is shown in the illustration 
 (fig. 8). It consists of a strong iron cylinder, which is connected by 
 a side pipe with a cylinder 
 much smaller in diameter. 
 In each cylinder a piston 
 presses down on the water, 
 or oil, or other liquid, con- 
 tained in it. As the liquid 
 is forced in from the small 
 to the large cylinder, the 
 piston in the latter slowly 
 rises with any weight that 
 may be placed on it. If 
 the area of the surface of 
 liquid in the larger cylinder 
 be 250 times as great as the 
 area of the surface of the Fig. 8. 
 
 liquid in the smaller cylin- 
 der, a pressure of 40 Ibs. on the small piston would balance a weight 
 of 10,000 Ibs. on the large piston. The hydrostatic press is fre- 
 quently used for the lifting of very great weights, or for pressing 
 with enormous force upon certain bodies. 
 
 EXERCISES. 
 
 3. Empty bottles which have been carried down to great depths in the sea, 
 have sometimes been found to have the corks which closed them forced 
 in. Explain this. 
 
 2. If a hole be made near the bottom of the side of a vessel containing water, 
 
 the distance to which the jet of water is forced gradually decreases. 
 Explain why this is so ; and state what might be done to cause the 
 water to fall always in the same place without moving the vessel. 
 
 3. Suppose that a vessel were filled with water, and an opening made in the 
 
 bottom of the vessel. If the water were allowed to run out for 
 10 seconds into one tumbler, for the next 10 seconds into another 
 
30 
 
 SPECIFIC GRAVITY OF LIQUIDS. 
 
 tumbler, and so on, would the quantity of water collected in each of 
 the different tumblers be the same? Give some reason for your 
 answer. 
 
 4. If the area of the surface of liquid in the smaller cylinder of a hydrostatic 
 
 press be 2 square inches, and in the larger cylinder 240 square inches, 
 what weight on the larger piston will balance a pressure of 20 Ibs. on 
 the smaller piston ? 
 
 5. A glass bottle is provided with a well-fitting cork, and is completely filled 
 
 with liquid up to the cork. "What would be the effect of a smart 
 blow with a hammer on the top of the cork ? Give a reason for your 
 answer. 
 
 6. If a pail be half full of water, will the pressure on the base be altered by 
 
 placing your hand in the water? 
 
 LESSON 14. 
 
 SPECIFIC GRAVITY OF LIQUIDS. 
 
 Water, mercury, or any other liquid, when poured into a bent 
 tube, rises up into each arm, until the level of the surface of the 
 liquid is the same in each. We may regard the liquid in such 
 a case as a balance, or pair of scales. If we pour in liquid on one 
 side, and still want the balance to remain as at first, we must pour 
 in liquid to the same height on the other side. Let us suppose that 
 we have such a bent tube, and that we pour 
 in a quantity of mercury sufficient to fill the 
 bend and to rise up a little in each arm, as 
 shown in fig. 9 A. As we have seen, the 
 level of the mercury in the two limbs will be 
 the same ; and the surface of the mercury will 
 be rounded, as shown in the illustration, on 
 account of the action of capillary force. Let 
 us next pour some water into one arm of the 
 tube, say that on the left-hand side. The 
 pressure of the water, caused by its weight, 
 will force down the mercury on that side, 
 and therefore the mercury in the other arm 
 will be raised. Suppose that we have poured 
 in a quantity of water, just sufficient to raise 
 the surface of the mercury in the right arm 
 one inch above the surface of that in the 
 'Fig. 9. left arm. Probably the first thing that we 
 
 notice will be, that the column of water 
 
 is much greater in height than the column of mercury which 
 balances it. On measuring we shall find that while the column 
 of water is about 13 inches, the column of mercury is only 1 
 
SPECIFIC GRAVITY OF LIQUIDS.* 31 
 
 inch. We may regard the mercury below the dotted line in fig. 
 9 B as a balance, on one side of which we have a column of water 
 13^ inches high, balancing a column of mercury 1 inch high on the 
 other side. We learn from this experiment, that if we require the 
 same weight of water and mercury for any purpose, we must take a 
 much greater volume of water than of mercury, in fact, 13 times 
 as great. On the other hand, if we take two bottles of exactly the 
 same size, and fill one with water and the other with mercury, the 
 mercury will weigh 13 J times as much as the water. Most people 
 express this fact by saying that "mercury is much heavier than 
 water ; " but this is an inaccurate expression. A pailful of water 
 would weigh more than a cupful of mercury ; in that case, therefore, 
 the water is heavier than the mercury. What people mean to say 
 is, that a certain volume of mercury is heavier than an equal volume 
 of water. We shall express the fact by saying that mercury has a 
 greater specific gravity than water. v By the specific gravity of a 
 liquid, or solid, we mean the weight of a certain volume of the 
 body, compared with the weight of an equal volume of water. 
 Water is chosen, for several reasons, as the substance with which all 
 liquids and solids shall be compared, as regards their specific gravities. 
 The specific gravity of mercury is said to be 13 ; by this is meant, 
 that a certain volume of mercury, a gallon for example, weighs 13 
 times as much as an equal volume of water. The experiment with 
 the bent tube, which we have been considering above, has shown us 
 one means of finding the specific gravity of mercury ; there is another 
 method which we shall consider presently. 
 
 Leaving the water and mercury in the tube, as shown in fig. 9 B, 
 let us pour some salt water down the right arm of the tube, until the 
 mercury in the two arms is again at the same level. Then it will 
 be seen that the column of salt water is somewhat shorter than the 
 column of fresh water on the other side. Here again the mercury in 
 the tube serves as a balance, and we observe that to balance a cer- 
 tain quantity of fresh water, less salt water is required. We express 
 this by saying that salt water has a greater specific gravity than fresh 
 water. If we had made use of alcohol, or paraffin oil, instead of salt 
 water, we should have needed a column of liquid greater than the 
 column of fresh water on the other side. That is to say, alcohol and 
 paraffin oil have each a less specific gravity than water. 
 
 It is very important that we should understand clearly what is 
 meant by specific gravity. Perhaps it will help us to do so, if we 
 consider another method of determining the specific gravity of 
 various liquids, which is far more convenient and accurate than the 
 one already described. A bottle is taken and its exact weight is 
 ascertained. It is then filled in succession with different liquids, 
 and weighed carefully each time. Subtracting the weight of the 
 bottle itself from each result, we learn the weight of an equal volume 
 of each of the different liquids with which we have experimented. 
 We can then compare together the different numbers thus obtained. 
 For example, suppose we took a bottle which would contain exactly 
 
32 SPECIFIC GRAVITY OF LIQUIDS. 
 
 1 Ib. of pure water, we should find that the same bottle would contain 
 about the following weights of the different liquids mentioned : 
 
 Ibs. Ibs. 
 
 Mercury 13 J or 13*5 
 
 Milk . 1A- , 1-03 
 
 Sea- water 
 Pure water 
 Olive oil 
 Alcohol 
 
 1-28 
 
 TOO 
 
 9 
 
 8 
 
 Ether T V '7 
 
 The decimal numbers in the second column are those commonly 
 made use of to express the specific gravity of those liquids. 
 
 These numbers may also be taken to express something besides the 
 specific gravities of the different liquids ; or, rather, may be taken 
 to express what amounts to the same thing in a somewhat different 
 manner. We like to find a reason for things as far as we can, and there- 
 fore we try to explain why a bottle full of mercury, for example, should 
 be heavier than the same bottle full of water. If we take a box and 
 fill it loosely with sand, the box and the sand will have a certain 
 weight. It will be possible however to increase the weight of the box 
 full of sand, by pressing together the loose sand already in the box and 
 putting more in. If then we had two similar boxes filled with the same 
 kind of sand, and one box weighed more than the other, we should 
 conclude that in the heavier one the grains of sand were lying more 
 closely together than in the other. The word density is employed 
 to express the closeness with which the particles of a body are 
 packed together ; in other words, the more particles there are in a 
 cubic inch of the body, the denser it is said to be. Now we can 
 imagine that the same thing holds good of the mercury and water, 
 as of the two boxes of sand ; namely, that in the one case the par- 
 ticles of matter are more closely packed than in the other. Thus it 
 is commonly said that mercury is denser than water, or that the 
 density of mercury is 13 J times as great as that of water. The same 
 number which expresses the specific gravity of a body serves also to 
 express its density ; for water is taken as the standard of density, 
 as well as of specific gravity. 
 
 From the short list of numbers given above, we learn that mer- 
 cury, milk, and sea- water, have each a greater density or specific 
 gravity than water, while oil, alcohol, and ether, have each a less 
 density or specific gravity. Mercury is the densest of all liquids ; 
 or more strictly, the densest of all bodies that we usually meet with 
 in the liquid condition. We shall briefly consider the specific 
 gravities of solids in the next lesson. 
 
 EXERCISES. 
 
 1. A gallon of pure water weighs 70,000 grains; find the weight of a gallon 
 of olive oil, and of mercury, respectively. 
 
BUOYANCY OF LIQUIDS. 33 
 
 2. If the floor of a cistern were just strong enough to support safely 6800 
 
 gallons of water, how many gallons of mercury could it support ? 
 
 3. Will there be any difference between the weight of a bottle full of pure 
 
 milk, and the weight of the same bottle full of milk which has been 
 largely adulterated with water ? Give a reason for your answer. 
 
 4. Describe one or more methods by which you could ascertain whether a 
 
 certain liquid was pure alcohol, or a mixture of alcohol and water. 
 
 5. If the pressure of the air is sufficient to hold up a column of mercury 30 
 
 inches high in the tube of a barometer, find the height of a columu 
 of oil which the same pressure could support. 
 
 6. Describe a method by which you might ascertain whether ink or tea had 
 
 the greater density. 
 
 LESSON 15, 
 
 BUOYANCY OF LIQUIDS. 
 
 We are about to consider in this lesson a most important property 
 of liquids, and one which is of very extensive application. It will 
 be necessary that we should consider it carefully, and try to under- 
 stand it clearly, before we proceed to the subject of the next lesson. 
 We have learned that liquids transmit pressure in all directions. A 
 layer of liquid in the middle of a vessel is pressed on by the layers 
 above it ; and, in its turn, it presses on those below, on those above, 
 and on the sides of the vessel where it touches them. If we plunge 
 our hand and arm into a deep vessel of water, we feel a pressure try- 
 ing to force our arm upwards out of the water, If we take a heavy 
 stone fastened to a string, and let it dip into water, we may perceive 
 that it feels lighter when in the water than it does when hanging in 
 the air. These two observations would lead us to believe that when 
 a body is immersed in water it experiences an upward pressure. 
 If we repeated these experiments with other liquids, we should find 
 that they all exerted a similar upward pressure on bodies placed in 
 them, though the pressure in some cases would be greater than in 
 others. The upward pressure which a liquid exerts on a body placed 
 in it is termed its buoyancy, that is, its power to buoy up, or bear 
 up, the body. 
 
 Suppose that we ta]s:e a rather large glass marble, and, after fast- 
 ening a piece of thin string to it, hang it to one arm of a balance, 
 while in the pan on the othey arm we put weights to balance the 
 marble. This proceeding will, of course, give us the weight of the 
 marble as it hangs in the air. If we next allow the marble, while so 
 suspended, to hang in a tumbler nearly full of water, we may notice 
 two things first, that the level of the water in the tumbler rises 
 higher than before ; and secondly, that the pan containing the 
 weights goes down, showing that it is now heavier than the other 
 side. The explanation of the rise in the level of the water is, of 
 
 C 
 
34 BUOYANCY OF LIQUIDS. 
 
 course, very simple ; the glass marble displaced, or pushed aside, a 
 quantity of the water, which moved into a fresh position. It is easy 
 to see that the quantity of water displaced by the marble would be 
 just sufficient to make a ball of water equal in size to the ball of 
 glass. We can express this by saying, that the volume of liquid 
 displaced is equal to the volume of the body immersed. 
 
 We can imagine that the water which surrounds our glass marble 
 presses it on every side. The pressure on the right, however, is just 
 balanced by an equal pressure on the left, and so all round the sides. 
 But the case is different when we consider the pressures on the top 
 and bottom of the marble. We learned in lesson 13, that the pres- 
 sure which a liquid exerts increases with the depth below the sur- 
 face of the liquid. On this account, the upward pressure on the 
 lower surface of the ball will be greater than the downward pressure 
 on the upper surface. This will explain why the marble appears to 
 lose weight when it dips into the water, and therefore why the 
 opposite side of the balance on which the weights are hung goes 
 down. The same explanation, holds good in the case of the stone 
 and of the arm previously mentioned. 
 
 Our next proceeding would be to determine, as exactly as possi- 
 ble, how much the marble had lost in weight ; that is to say, the 
 difference between its weight as it hangs in the water, and its weight 
 when it is surrounded only by air. A very great number of experi- 
 ments have proved that the following general rule is true : that, in 
 every case, the loss of weight which a body undergoes when 
 immersed in a liquid, is equal to the weight of the liquid 
 displaced by the body. Thus, if we took a number of balls of 
 the same size, but of different materials gold, iron, marble, &c. 
 and immersed them in water, they would each suffer the same loss of 
 weight, namely, the weight of a ball of water of the same size. The 
 rule just given applies to all liquids, and not to water only. If we 
 let our marble hang in alcohol, for example, it would lose weight 
 equal to the weight of an equal volume of alcohol. 
 
 We may now go one step further, and suppose our glass ball im- 
 mersed in several liquids in succession alcohol, fresh- water, salt 
 water, &c. It will certainly lose weight in each case, but not the 
 same amount. From what we learned about the specific gravities 
 of liquids in the preceding lesson, we know that a ball of alcohol 
 would weigh less, and a ball of salt water more, than an equal sized 
 ball of fresh water. We should therefore expect that the upward 
 pressure on the glass ball, which is the cause of the loss of weight, 
 would be greatest in salt water, and least in alcohol. An ex- 
 periment in each of these cases would prove that our supposition 
 was correct. Speaking generally, we may say that the upward 
 pressure, or buoyancy, of any liquid depends upon its specific 
 gravity. The greater the specific gravity of any particular liquid, 
 the greater will be the buoyancy of that liquid. 
 
 We have already observed that solids, like liquids, have different 
 specific gravities. It is important that we should know a little 
 
SPECIFIC GRAVITY OF SOLIDS. 35 
 
 about the specific gravity of solid bodies, and about the method by 
 which the specitie gravity of an ordinary solid body could be ascer- 
 tained. The process would be very easy, if we could provide our- 
 selves with a cubic inch of each solid. We should simply need to 
 weigh the different blocks, and then compare the weights with the 
 weight of a cubic inch of water, which is taken as the standard of 
 specific gravity both for solids and liquids. Or suppose that we 
 ascertain the number of cubic inches of water required to weigh 
 just 1 lb., and that then we cut blocks of several different solids, 
 each containing the same number of cubic inches. The weight of 
 each block expressed in pounds, would give us its density or specific 
 gravity, the density or specific gravity of water being taken as 1. 
 The numbers we should get would be somewhat as follows : 
 
 Water 
 
 Cork 
 
 Pine (white) 
 
 Ice . 
 
 Ordinary stone 
 
 Glass 
 
 Diamond . 
 
 Zinc . . . 
 
 Iron (cast) . . 
 
 Copper . . , 
 
 Silver . . . 
 
 Lead . . . 
 
 Gold . . . 
 Platinum ... 22 
 
 The method just described will serve to illustrate once more what 
 is meant by specific gravity ; but it would be so difficult to obtain 
 blocks of the exact size, that people would never follow out the 
 method in practice. We must remember that to find the specific 
 gravity of any particular solid body, all we require to know is, 
 first, the weight of the body, and secondly, the weight of an equal 
 volume of water. Suppose that we have a piece of stone, and wish 
 to determine its specific gravity. We attach it to a balance and 
 weigh it, and then let it han^ in pure water while we weigh it 
 again. It will, as we know, weigh less in the water than in the air. 
 The loss of weight shows us precisely what would be the weight of 
 a quantity of water of the same size as the piece of stone. This gives 
 us all we require to know in order to determine the specific gravity 
 of the stone. Suppose, for example, that the stone weighs 12 oz. in 
 air and 8 oz. in water, then the weight of a quantity of water equal 
 in size to the stone is 4 oz., i.e., equal to the loss of weight. Since 
 the stone weighs 12 oz. and a portion of water the same size only 4 
 oz., the specific gravity of the stone is three times as great as that of 
 water. This is a very simple method for finding the specific 
 gravities of most solids, but it needs to be slightly altered in the 
 case of such substances as sugar which dissolve in water, or of 
 bodies like wood which float on water. 
 
 EXERCISES. 
 
 1. Why is it easier to support a person in the water, than to hold up the 
 
 same person on the land ? 
 
 2, What two properties of a diamond distinguish it from a piece of glass? 
 
36 FLOATING BODIES. 
 
 3. If a cubic foot of water weighs 1000 oz., what will be the weight of a 
 
 cubic foot of gold ? 
 
 4. If a piece of silver were gilt, and then oifered for sale as a piece of gold, 
 
 how could the fraud be ascertained without scratching or cutting the 
 piece ? 
 
 5. A piece of gold said to be pure weighs 76 grains in air, and 70 grains in 
 
 water. Is this gold pure ? 
 
 6. The specific gravity of a certain kind of stone is known to be 2 ; find the 
 
 weight of a block of that stone measuring 8 ft. long, 3 ft. broad, and 
 2 ft. deep. 
 
 LESSON 16. 
 
 FLOATING BODIES. 
 
 The subject of the present lesson is very closely connected with 
 that of our last lesson. We have to consider why it is that some 
 solids will float on water, while others sink when placed in water, 
 although they may float on other liquids. We shall see that it 
 depends on the buoyancy, or, what comes to the same thing, on the 
 specific gravity of the liquid, and on the specific gravity of the 
 solid. 
 
 When a solid is placed in a liquid so as to be entirely covered by 
 it, we have seen that there is an upward pressure on the solid tend- 
 ing to raise it. The amount of this pressure is equal to the weight 
 of the displaced liquid. If the weight of the solid, that is, its pres- 
 sure downwards, be greater than the upward pressure of the liquid, 
 the solid will force its way down; in other words, it will sink. It' 
 the weight of the solid be less than the upward pressure of the liquid, 
 the solid will be forced upwards, and partly out of the liquid ; that 
 is, it will float. If the two pressures be exactly equal, the solid will 
 remain at rest in the midst of the liquid. We can express the same 
 facts in slightly different terms, thus : If the specific gravity of 
 the solid be greater than that of the liquid, the solid sinks ; 
 if the specific gravity of the solid be less than that of the 
 liquid the solid floats ; if the specific gravity of the solid be 
 equal to the specific gravity of the liquid, the solid remains 
 at rest in any part of the liquid, so long as it is entirely 
 covered by the liquid. These three conditions may be easily 
 illustrated by means of an egg, some salt, and a tumbler of water. 
 If the egg be placed in the water it sinks to the bottom, since its 
 specific gravity is greater than that of the water. If then some salt 
 be dissolved in the water, the specific gravity of the liquid becomes 
 greater than before, and its buoyancy therefore becomes greater. 
 When the specific gravity of the liquid is equal to that of the egg, 
 the latter will remain in -any part of the liquid where it is placed, 
 If still more salt be added to the water, its specific gravity becomes 
 
FLOATING BODIES. 37 
 
 greater than that of the egg, and the egg rises to the top and floats 
 with some portion of its bulk out of the liquid. We can explain 
 these facts to ourselves by supposing that when the egg is put into 
 the liquid and made to displace a portion of it, this portion of liquid 
 endeavours to get back again into its place. There is, in conse- 
 quence, a struggle between the egg and the displaced liquid, as to 
 which shall occupy the lower position. The heavier one, that is, 
 the one with the greater specific gravity, wins the clay. 
 
 Let us consider more particularly the last case, where the egg 
 floats in the salt water. The displaced liquid flows round and 
 underneath the egg, and raises it up until part is uncovered. The 
 buoyancy of the salt water can only raise the top of the egg a small 
 distance out of the water. For the more of the egg there is un- 
 covered the smaller is the amount of liquid displaced, and therefore 
 the smaller will be the upward pressure. The egg rises so far only 
 that the upward pressure of the liquid is equal to the weight of the 
 egg. The greater the specific gravity of the liquid the greater will 
 be its buoyancy, and the higher the egg will be raised out of the 
 liquid. Wood or wax will float on water ; iron and lead will float 
 on mercury; and the same rules which apply to the egg in salt 
 water, apply in these and all other similar cases. The weight of 
 the quantity of liquid displaced "by a floating "body is equal to 
 the weight of the body. When a solid body is placed on a liquid, 
 it sinks until it has displaced a quantity of liquid whose weight is 
 equal to that of the body, and then it floats in that position. But 
 if, when the solid has sunk so low as to be just covered by the 
 liquid, the weight of liquid displaced be not equal to the weight of 
 the solid, the latter sinks to the bottom of the liquid. 
 
 There is a useful instrument, called the hydrometer, whose 
 action depends on the principles we have just been stating. It is 
 made somewhat in the form of a bottle, with a long neck or stem. 
 The bottle is loaded at the bottom with mercury or shot, so as to 
 float upright in water with about half its stem projecting above the 
 surface. If such an instrument be placed in salt water, it floats 
 with more of the stem uncovered than when floating in fresh water. 
 By observing what portion of the stem is uncovered in each case, 
 we could form some idea as to the relative quantities of salt con- 
 tained in different samples of salt water. The same instrument if 
 placed in alcohol or oil would sink deeper into those liquids, and 
 therefore less of the stem would be uncovered. It therefore fur- 
 nishes a very simple and convenient means of determining the 
 specific gravities of various liquids, and is used in several cases 
 to ascertain whether a certain liquid has been adulterated. For 
 example, suppose that some alcohol were mixed with water, the 
 specific gravity of the mixture would be less than that of water, but 
 greater than that of pure alcohol. If we had an instrument 
 like the hydrometer described, and having marked on the stem the 
 point to which it would sink in water, and the point to which 
 it would sink in alcohol, it could easily be ascertained whether 
 
38 FLOATING BODIES. 
 
 the liquid were water, or pure alcohol, or a mixture of the two. 
 And if it turned out to be a mixture of alcohol and water, we could 
 form a pretty good idea of the relative quantities of those liquids 
 present. Another similar instrument (called the lactometer) is 
 frequently used to determine whether milk has been adulterated 
 with water. 
 
 A ship floats on water for the same reason that an egg floats on 
 strong salt water, viz., that it displaces a quantity of liquid the 
 weight of which is exactly equal to its own weight. When the ship 
 is loaded, it sinks lower in the water and displaces a larger volume; 
 consequently the upward pressure of the liquid increases, and thus 
 the extra weight is supported. The ship and the hydrometer are 
 made hollow and broad, that they may displace far more water than 
 they would do if the same weight of wood or glass were made into 
 an entirely solid body. A solid piece of copper, an old penny for 
 example, sinks in water, but the same piece of copper if beaten out 
 into the form of a cup or boat will float. The copper boat displaces 
 as much water as a solid piece of the same size and shape would do 
 in the same position, but, on account of its being hollow, weighs 
 very much less. 
 
 Different liquids in the same vessel, if they are such as will not 
 mix with each other, arrange themselves according to their various 
 specific gravities or densities, the densest liquid being at the bottom, 
 and the one which has the least density at the top. Thus mercury 
 sinks to the bottom of a vessel of water, and oil floats on the top. 
 In some parts of the sea, near the mouth of a large river, the fresh 
 river- water is found to float on the top of the salt sea- water for a 
 considerable distance, on account of its smaller specific gravity. 
 
 EXERCISES. 
 
 1. An india-rubber ball filled with air floats on water ; the same ball filled 
 
 with water sinks. Explain this. 
 
 2. Why should a piece of wood, which at first floated easily, sink after it 
 
 has been for a long time in the water? 
 
 3. A ship when loaded in a river sinks to a certain depth in the water; 
 
 will there be any difference in this respect when she gets out to sea ? 
 
 4. Many fishes have in their bodies a vessel resembling a bladder, which is 
 
 filled with air. Of what use will such an "air-bladder" be to the 
 fish? 
 
 5. The density of platinum is 22 times as great as the density of water. 
 
 How can platinum be made to float on water ? 
 
 6. For what reason does a diver hang heavy weights about him before enter- 
 
 ing the water? Will it be as difficult for him to carry the weights 
 when in the water as when out of it ? 
 
EFFECTS OF HEAT UPON LIQUIDS. 39 
 
 LESSON 17. 
 
 EFFECTS OF HEAT UPON LIQUIDS. 
 
 In considering the properties of liquids, we must not forget to 
 consider what changes they will undergo when subjected to the 
 action of heat. We will consider a very simple experiment, which 
 may be easily performed, and which will teach us some very im- 
 portant facts. Suppose we take a small glass bottle having a rather 
 long and narrow neck, and, after filling it quite full of cold water, 
 set it in a deep basin. On pouring hot water into the basin, we may 
 readily notice a change taking place in the level of the water in the 
 bottle. At first, immediately the hot water is poured into the basin, 
 the surface of the water in the bottle would be seen to sink some- 
 what rapidly. Then it would begin to rise up above the neck of 
 the bottle, and would probably overflow. We have learned that 
 glass expands when it is heated, therefore the hot water would 
 cause the bottle to expand and become larger. This would take 
 place as soon as the hot water was poured in, and before the water 
 contained in the bottle became warmed. We see at once why the 
 level of the water in the bottle sank at first, viz., because the bottle 
 had become larger and allowed it more room. But what about the 
 subsequent rise of the water ] When the heat had passed through 
 the sides of the bottle and begun to warm the water in it, this water 
 expanded, and not only overtook the bottle in its expansion but 
 passed beyond it. 
 
 We might make similar experiments with bottles containing 
 alcohol, oil, and other liquids ; and we should soon come to the 
 conclusion that all liquids expand when heated. And since the 
 expansion of a liquid can be seen very plainly in a glass bottle, which 
 itself expands at the same time, it is quite evident that liquids ex- 
 pand more than glass, when both are equally heated. If we made 
 our experiments in bottles made of iron, we should find that the 
 liquids expanded more than the iron. And the following general 
 rule has been ascertained, that a liquid expands more than a 
 solid when both "are equally heated. 
 
 Two or three experiments, even with the common glass bottle 
 already mentioned, would soon convince us that all liquids do not 
 expand equally neither at the same rate, nor to the same extent. 
 Paraffin oil, for instance, expands quickly and to a much greater 
 extent than water ; alcohol also expands more than water, but not 
 so quickly as the oil. Mercury expands to a less extent than water, 
 but expands very rapidly, reaching its highest point long before the 
 other liquids mentioned have reached theirs. 
 
 We might reason that, since liquids expand when they are heated, 
 they would contract if cooled. To cool any body is simply to take 
 litat away from it, and is therefore just the reverse of heating it. 
 
40 EFFECTS OF HEAT UPON LIQUIDS. 
 
 Experiment would soon prove that our reasoning was correct : for 
 on placing the bottle full of water into a basin containing still 
 colder water, the contraction of volume would be apparent. 
 
 Let us suppose that we had a bottle with a long narrow neck, 
 filled with one of the liquids mentioned above to a point say half- 
 way up to the neck. We might make use of such a bottle to deter- 
 mine which of several liquids was the hottest. Placing it in one of 
 the liquids, we should allow the liquid in the bottle to rise or fall, 
 and then mark the point to which it reached when it had become 
 steady. On placing the bottle in another of the liquids, by noticing 
 the point of the neck to which the liquid in the bottle reached, we 
 should be able to say whether this liquid was of the same tempera- 
 ture as the first, or was colder, or hotter. By temperature is meant 
 simply the degree of heat. 
 
 There is a common instrument called the thermometer, which 
 is in reality a small bottle with a long narrow neck, and is used in 
 the manner and for the purpose just described. The thermometer 
 is an instrument for measuring temperature. 
 
 Most thermometers contain mercury, but there are some which 
 contain alcohol which is coloured red or blue so that it may be 
 distinctly seen. Mercury is chosen for ordinary thermometers for 
 several reasons : (1.) It is opaque, and therefore easily seen ; (2.) it 
 does not stick to the glass ; (3.) it expands very quickly, and 
 very evenly ; (4.) it does not readily freeze or boil. The very fine 
 passage down the stem of the thermometer that is, down the neck 
 of the bottle is completely closed at the top. On the stem, there 
 is marked the level at which the surface of the mercury will stand 
 when the thermometer is placed in freezing water. This point is 
 called the freezing point, and opposite to it, on ordinary ther- 
 mometers, the number 32 is placed. Many thermometers have also 
 marked on them the boiling point, or the level of the mercury when 
 the thermometer is surrounded by boiling water. The number 212 
 is put opposite to the boiling point, and the space between freezing 
 point and boiling point is divided into 180 equal parts called 
 degrees. 
 
 Since a liquid expands when heated, its density must at the same 
 time become less ; or, what comes to the same thing, it must have a 
 less specific gravity when hot than when cold. For if we were to 
 take a bottle full of cold water and weigh it, and then proceed to 
 heat the bottle, some of the water would overflow. On weighing 
 the bottle full of hot water, we should therefore find that it weighed 
 less than when full of cold water. The same reasoning applies to 
 all liquids and solids which expand when heated ; heat lessens the 
 specific gravity of all such substances. When a kettle full of cold 
 water is set over a fire, the lowermost layer of water first becomes 
 heated, expands, and becomes less dense. It straightway rises 
 towards the top, while colder water from above descends to take its 
 place. This latter, in its turn, becomes heated and ascends, and 
 thus upward and downward currents are started, which keep the 
 
CONVERSION OF LIQUIDS INTO SOLIDS AND GASES. 41 
 
 water in constant circulation. A similar circulation will be set up 
 in any other liquid if heated below. It will be readily understood 
 that no such circulation would be produced if the liquid were 
 heated above instead of below. It is believed that some, at any 
 rate, of the currents in the ocean are produced in this manner, by 
 differences in the temperature of the water at different parts. 
 
 There is still another change which heat produces in liquids, 
 namely, it lessens their cohesion just as it does in the case of solids. 
 This change is best seen in viscous liquids, such as treacle, glycerine, 
 and some kinds of oil. These liquids are more or less thick at 
 ordinary temperatures, and the colder they are made the thicker 
 they become, and the more slowly do they move. By applying heat 
 to them they can be made to flow quite easily. The heat appears 
 to loosen the particles, so that they are able to change their positions 
 more readily. Although this change is best seen in such liquids, 
 there can be no doubt that heat produces a similar but much 
 smaller change in all liquids. 
 
 EXERCISES. 
 
 1. Soundings have shown that there is a layer of cold water at the bottom 
 
 of the ocean in many parts, and that the temperature of the water 
 increases towards the surface. Explain these facts. 
 
 2. If an egg is carefully placed in the midst of a quantity of cold salt water 
 
 of the same specific gravity as itself, explain why the egg gradually 
 sinks when the vessel containing the water is set in a warm room. 
 t-J. "Why would water be an unsuitable liquid for a thermometer ? 
 
 4. To what cause would you ascribe the downward currents, which you may 
 
 see when a piece of ice is floated on warm water ? 
 
 5. When a thermometer is placed in a cold liquid the mercury is seen to 
 
 rise up suddenly in the tube and then gradually to fall. Explain 
 both the rise and the fall. 
 
 6. Alcohol adheres to a glass tube, but mercury does not. Explain why 
 
 mercury is better than alcohol, on this account, for use in a ther- 
 mometer. 
 
 LESSON 18. 
 
 CONVERSION OF LIQUIDS INTO SOLIDS AND GASES. 
 
 The action of heat produces several changes in the condition of a 
 liquid. It causes the liquid to expand, that is, to increase in volume, 
 and in consequence to become less dense ; while if the liquid be 
 heated from below, as liquids usually are, currents are set up in 
 the mass of it. As another consequence of this expansion, the co- 
 hesion between the molecules of the liquid becomes less. These 
 changes we have already considered, and we have now to consider a 
 change quite as important, namely, the conversion of a liquid into 
 
42 CONVERSION OF LIQUIDS INTO SOLIDS AND GASES. 
 
 a gas by the action of heat. Water passes into vapour or gas at all 
 temperatures, even in winter the water in vessels gradually " dries 
 up," or passes away in the form of vapour. There are some liquids 
 which pass into vapour, or evaporate as it is termed, more easily 
 than water. Ether and benzoline (such as is burned in small lamps), 
 are such liquids ; they evaporate very rapidly, especially when 
 poured on the warm hand. From the surface of seas, lakes, and 
 rivers, vapour of water is constantly passing into the air, where a 
 great part of it collects in the form of clouds, and ultimately falls 
 to the earth again as rain, snow, or hail. 
 
 If we place water in a vessel over a fire or any other source of 
 heat, the vapour comes off more and more rapidly as the temperature 
 of the water increases. And at last, if the heat be sufficient, there 
 comes a time when the vapour is given off from the water very 
 rapidly so rapidly in fact, that, as it escapes through the liquid, 
 it throws up bubbles, and keeps the surface of the liquid in a state 
 of constant agitation. We speak of this action as the boiling of the 
 water. The cohesion becomes less and less as the temperature in- 
 creases, until at last it is insufficient to hold the molecules 
 together, and they therefore move away from each other, and 
 assume the state of a gas. The most curious thing, however, is 
 this : a thermometer placed in boiling water shows us, that how- 
 ever great may be the heat of the fire, and however long the water 
 may have been boiling, the temperature of the liquid remains the 
 same as when it first began to boil. 
 
 The same facts are true of all other liquids besides water. They 
 are probably always giving off vapour ; though in the case of some 
 liquids, such as mercury, the quantity of vapour given off is so small 
 that it is very difficult to prove that there is any at all. When 
 heated they give off more and more vapour as the temperature rises, 
 until they commence to boil, when they evaporate very rapidly 
 but do not increase in temperature. The temperature at which 
 liquids boil is not the same for all. Under ordinary circumstances 
 water boils at the temperature marked 212 (212 degrees) on 
 ordinary thermometers, mercury at about 660, while alcohol boils 
 as soon as the temperature reaches about 173. 
 
 Under any circumstances, when a liquid passes into the state of 
 a gas or vapour, it must take heat from some other substance. When 
 we pour warm water or alcohol on to our hand, as soon as the first 
 sensation of warmth is over our hand begins to feel cold. In this 
 case, as the liquid evaporates it abstracts from our hand the heat 
 necessary to turn it into a vapour. Ether and benzoline evaporate 
 much more rapidly than water, and therefore take away the heat 
 more quickly from the hand. If we blow on to the liquid it eva- 
 porates still faster, and the surface it lies on is rendered still colder. 
 
 When a liquid is converted into a vapour or gas, the latter occu- 
 pies a much larger space than the liquid did, and this is always the 
 case. A gas occupies a very much larger space than the liquid 
 from which it is derived. From the boiler of a steam-engine, which 
 
CONVERSION OF LIQUIDS INTO SOLIDS AND/GASES. /f& 
 
 r> &* ' ' 
 
 is half full of water to begin with, enough steam escapes to/$ll the * 
 boiler a great many times, and yet the water is not all evaporated. 
 If a cubic inch of water could be placed in a vessel of the same siz^y ^ > 
 which had walls that could be easily expanded by the steam', when 
 all the water was converted into steam the vessel would be found to 
 have expanded to the size of about one cubic foot. That is to say, 
 steam occupies about 1700 times as much space as the water from 
 which it was formed. 
 
 If a liquid is surrounded by substances colder than itself, they 
 take heat away from it, and it gradually becomes colder than it 
 originally was. As the liquid cools it contracts, becomes denser, 
 has its cohesion increased, and at last, if made sufficiently cold, it 
 solidifies or turns into a solid. Thus, if we take some oil, and sur- 
 round the vessel containing it by ice, we can solidify or freeze the 
 oil. A mixture of pounded ice and salt, or snow and salt, is much 
 colder than ice alone. If we place some water in a vessel, and set 
 the vessel in such a mixture, the water will very soon freeze. Other 
 mixtures are known which will cool liquids still more than the mix- 
 ture of ice and salt. By means of these " freezing mixtures," as 
 they are termed, mercury has frequently been frozen and turned 
 into a solid lump, which could be cut and hammered like a lump 
 of lead. To prevent the solid mercury from liquefying, it has to be 
 kept so cold that it would hurt the fingers to touch it, as much as 
 to touch a piece of hot iron. Nearly all liquids have been solidified 
 at various times, but alcohol is one of the few which no one has yet 
 succeeded in converting into a solid. Hence thermometers which 
 are to be used in cold regions, or in experiments where mercury 
 would be frozen, contain alcohol instead of mercury. When a liquid 
 passes slowly into a solid condition, the molecules frequently arrange 
 themselves so as to build up crystals. 
 
 As a general rule, liquids shrink or contract as they solidify, and 
 the solid therefore takes up less room than the liquid. Melted lead 
 poured into a mould contracts in this manner, and the solid lead 
 does not exactly fill the mould. In all such cases the solid is denser 
 than the liquid, and while the process of melting is going on the 
 solid lumps lie at the bottom of the liquid, which flows over and 
 around them. This may be seen on melting wax or lead. But 
 water and one or two more substances take up more room in the 
 solid than in the liquid form, and consequently the solid is not so 
 dense as the liquid. In such substances the solid floats on the 
 liquid, as we see ice floating on water. The expansion of water on 
 freezing is the cause of the frequent bursting of water-pipes in 
 frosty weather. 
 
 EXERCISES. 
 
 1. When a person has been running quickly, so that his skin is covered with. 
 
 perspiration, why does he feel cold on standing still? 
 
 2. When wet clothes are hung out to dry, what becomes of the water in 
 
 them? 
 
44 PROPERTIES OF LIQUIDS. 
 
 3. Which liquid would produce the most severe scald, boiling water or boil- 
 
 ing oil ? Give a reason for your answer. 
 
 4. When a thaw sets in after a severe frost, people often find their water- 
 
 pipes burst. Has the thaw caused them to burst ? If not, why had 
 the people not discovered the damage before ? 
 
 5. Why does water, in a saucepan on the fire, get hot sooner when the lid is 
 
 on the saucepan than when it is not ? 
 
 6. Why does a boiler sometimes burst when a quantity of the water which 
 
 it contains is suddenly converted into steam ? 
 
 LESSON 19. 
 
 PROPERTIES OF LIQUIDS. 
 
 "We have now been engaged for several lessons in considering 
 the chief properties of liquids ; it will be well for us in this lesson 
 to bring together those various properties, and consider them in 
 connection with each other. Liquids, like the other conditions of 
 matter, consist of small particles called molecules. These molecules 
 are held together by that force of attraction which we call cohesion. 
 The molecules do not lie quite close to each other, for when a 
 liquid is cooled the force of cohesion becomes stronger, and draws 
 the molecules closer together, and we express this by saying that a 
 liquid contracts when it is cooled. On the other hand, heat appears 
 to force the molecules farther apart from each other, and causes the 
 liquid to expand. The force of cohesion in liquids generally is much 
 weaker than it is in solids ; it is just strong enough, in fact, to keep 
 the molecules together, but not sufficiently strong to fix them in one 
 position. The consequence is, that the molecules slip and roll 
 about among each other with the greatest ease ; far more easily 
 than the separate marbles in a bag full of marbles do, because there 
 is not so much friction between molecules of water, for example, as 
 between the marbles. One of the most characteristic 'properties of 
 real liquids is, that the slightest force is sufficient to move 
 the molecules amongst each other. In those liquids which we 
 have called viscous liquids the cohesion is greater, and they do not 
 yield so easily when we press on tbem ; they are, as it were, on 
 the way to become solids. By real liquids we mean liquids with 
 the least possible amount of cohesion. When we remember the 
 ease with which the molecules of a liquid move about, we can 
 easily understand why a liquid changes its shape, according to 
 the shape of the vessel which contains it. And further, when we 
 recollect that all liquids have some cohesion, we can understand 
 why it is that they do not of themselves change their size. The 
 size of a liquid is as constant as that of a solid, and, unless we alter 
 the temperature of the liquid, it is extremely difficult to alter its 
 
PROPERTIES OF LIQUIDS. 45 
 
 size. Liquids may be compressed, but only to a very small extent, 
 and with great difficulty. 
 
 We have seen that liquids transmit pressure equally in all direc- 
 tions. When we press on any portion of a liquid, it endeavours to 
 move away from us, and in so doing it presses on the portions of 
 liquid below it and around it, and on whatever may be above it. 
 So evenly is pressure distributed by a liquid, that if we press with 
 <a force of 1 Ib. on a square inch of the surface of a liquid, every 
 square inch of the surface of the vessel which the liquid touches 
 will be pressed with an extra force of 1 Ib. We say an "extra" 
 force, because when a liquid is placed in a vessel, its weight alone 
 causes it to press against all parts of the vessel with which it is in 
 contact. It presses sideways on the sides of the vessel, and down- 
 wards on its base ; and we have seen that the amount of this pres- 
 sure is greater the lower we go below the highest point of the 
 liquid. The lower portions of a quantity of liquid are always press- 
 ing out laterally or sideways, because they themselves are pressed 
 on by the portion of liquid lying above them. If we wish to keep 
 these lower layers from moving laterally, we must support them 
 round the sides, and thus we say that liquids need lateral support. 
 If not supported laterally, a liquid will spread out into a sheet, or 
 will flow, and on this account it is termed a fluid. For the same 
 reason a liquid always takes a horizontal surface ; since, if one 
 portion of the surface were raised higher than the rest, it would 
 not remain so unless supported at the sides. The raised portion 
 would settle down until the upper surface of the liquid all stood 
 at the same level, just as the little heap of treacle does, which is 
 formed when treacle is poured into a vessel. 
 
 Lastly, we have seen that liquids can be converted into solids on 
 the one hand, and gases or vapours on the other into vapours by 
 applying heat to them, into solids by taking heat away from them. 
 When a liquid turns into vapour it is said to evaporate ; at a particular 
 temperature, vapour is formed very rapidly, and the liquid is then 
 said to boil. Liquids differ very considerably in the temperatures 
 at which they boil, and in the temperatures at which they solidify. 
 
 We may sum up the chief properties of liquids which we have 
 considered as follows : 
 
 (1.) A liquid has a definite size, but an indefinite shape. 
 
 (2.) Liquids have little cohesion. 
 
 (3.) They are very difficult to compress. 
 
 (4.) As a rule, they expand when heated and are converted 
 into vapours. 
 
 (5.) Liquids transmit pressure equally in all directions. 
 (6.) They spread out (or flow) if not supported laterally. 
 
46 GASES. 
 
 EXERCISES. 
 
 1. Explain the following words: Evaporate, horizontal, capillary, thermo- 
 
 meter, density. 
 
 2. State all you can about mercury. 
 
 3. In what respects do liquids agree with solids, and in what respects do 
 
 they differ from them ? 
 
 4. Give the names of two liquids of greater and two of less specific gravity 
 
 than water ; also of two transparent and two opaque liquids. 
 
 5. I have a glass bottle containing some liquid in which a brown powder is 
 
 dissolved. I constantly find a quantity of this same brown powder 
 round the top of the neck of the bottle above the glass stopper. 
 Explain how it gets there. 
 
 6. Explain why a solid dissolves more readily when in powder than when in 
 
 a solid lump. 
 
 LESSON 20. 
 
 GASES. 
 
 We come now to consider the properties which belong to the third 
 state or condition of matter the gaseous state. We have learned 
 that liquids, under the action of heat, are converted into gases. The 
 gases thus obtained, however, only remain in the form of gases so 
 long as they are kept above a certain temperature. If we allow 
 them to cool down below that temperature, they pass back very 
 readily into the liquid condition. Steam, as we know, is very easily 
 condensed ; indeed, almost as soon as it leaves the funnel of a steam- 
 engine it is converted into a cloud consisting of numerous small drops 
 of liquid water. The term, vapour is commonly applied to those 
 gases which are derived from liquids by the action of heat, and which 
 pass back again into liquids when they are slightly cooled. All gases 
 can be converted into liquids by suitable means ; but such gases as 
 those composing the air oxygen and nitrogen which are extremely 
 difficult to liquefy, are often termed true gases. The properties we 
 are about to consider belong to all gases, that is to say, to those 
 which are called vapours, as well as to true gases. 
 
 In the first place, a gas has no cohesion whatever ; we can sepa- 
 rate one portion from another with the greatest ease. The molecules 
 are very much farther removed from each other than they are in the 
 case of either solids or liquids. This, of course, we should infer 
 from the fact previously noticed, that a liquid always expands con- 
 siderably when converted into a vapour. We have a striking proof 
 of the great distances which separate the molecules of a gas when 
 we endeavour to compress it. It is very easy to compress any gas ; 
 200 cubic inches of ordinary air could, with very little trouble, be 
 forced into the space of one cubic inch. When a gas is compressed, 
 
GASES. 47 
 
 the molecules are, of course, much closer together than at first. And 
 with many gases the compression can be carried on so far, that the 
 gas turns into a liquid, that is to say, the molecules are brought 
 so close together that cohesion again lays hold of them. But such 
 liquids only remain as liquids under great pressure ; so soon as the 
 pressure is removed they pass again into gases. Carbonic acid gas, 
 \vhich is found in small quantities in the air, and which we breathe 
 out of our bodies, can be thus liquefied by pressure. Some gases 
 are so difficult to liquefy, that it is necessary to make them intensely 
 cold, as well as to compress them enormously, in order to convert 
 them into liquids. By means of combined cold and pressure, oxygen, 
 hydrogen, and nitrogen, have recently been liquefied. Therefore we 
 may state generally that a gas has no cohesion : can be com- 
 pressed easily, and to a great extent : and by means of cold 
 alone, or pressure alone, or both combined, can be converted 
 into a liquid. 
 
 When a gas has been compressed, so soon as the pressure is re- 
 moved it expands again and returns to its former size. If a quantity 
 of gas be placed in a vessel, it expands so as to fill every portion of 
 the vessel, no matter how small the quantity of gas may be. A gas, 
 in fact, is always endeavouring to expand, its molecules are always 
 striving to get farther away from each other ; and they separate in 
 all directions until they are stopped by the walls of the vessel or by 
 some other means. Of course, as a gas expands, its density and 
 specific gravity become less. A gas has no definite size or shape ; 
 its shape depends entirely on the shape of the containing vessel, and 
 its size depends entirely on the amount of pressure put on the gas. 
 The volume of a gas decreases as the pressure on it increases. 
 Gases tend to expand in all directions, and therefore press 
 equally on all sides of the vessel by which they are enclosed ; and 
 they must be supported on every side, otherwise they will spread 
 out or " flow." As before stated, gases are comprised with liquids 
 under the term fluids. There is another property in which they 
 resemble liquids, and differ from solids, viz., that they transmit 
 pressure in all directions. If we take a thin india-rubber ball, filled 
 with air or gas of any kind, and press on any part of it with our 
 finger, we shall find that the enclosed gas presses more strongly than 
 before on every part of the ball, including the very part on which 
 we are pressing. And the more pressure we put on the gas the 
 greater is the amount of pressure which is transmitted to all parts 
 of the ball, 
 
 EXERCISES. 
 
 1. Name some true gases, and also some vapours. 
 
 2. Vapour of water is produced by the flame of a paraffin oil lamp. When 
 
 the glass chimney is first put on the lamp, it becomes covered with 
 moisture, which soon passes away, while no more appears. Explain 
 these facts as far as you can. 
 
 3. When a closed bladder half filled with gas is put under an air pump, and 
 
48 THE AIR PUMP. 
 
 the air which presses on the outside of the bladder is removed, the 
 bladder expands. What property of gases does this illustrate ? 
 
 4. In the last question, if the air be allowed to enter the air pump again, 
 
 state what will take place. 
 
 5. Explain why a liquid has a greater density than a gas. 
 
 6. When a glass containing cold water is brought into a warm room, the 
 
 outside is usually soon covered with moisture. Where does the mois- 
 ture come from, and why should it settle on the glass ? 
 
 LESSON 21. 
 
 THE AIR PUMP. 
 
 The air pump is an instrument by means of which we can with- 
 draw the air from a vessel ; or it can be slightly modified so as to 
 enable us to force air into a vessel. As it serves to illustrate several 
 of the properties of gases, we will, in this lesson, endeavour to under- 
 stand the construction and working of a simple form of air pump. 
 The common syringe, or squirt (fig. 10), contains several parts 
 
 similar to those of the air pump. 
 
 _/ pi| El i Thus, there is a barrel, or cylin- 
 
 "\ [j; der, which is of the same diameter 
 
 in all parts except at one end, 
 
 Fi - 10> where it narrows to a small aper- 
 
 ture. The other end of the cylinder is loosely closed by means of 
 a cork. Through this cork passes a rod, which is attached to a 
 kind of plate called the piston. The piston fits closely to the 
 sides of the cylinder, being generally wrapped round with cotton 
 or other substance, to make it fit as air tight as possible. The 
 rod which moves the piston is called the piston rod. When the 
 piston is moved along the cylinder towards the cork, the air in 
 front of it is swept out around the loosely-fitting cork : while, 
 on the other side of the piston, air will force its way in through 
 the end aperture, to fill up the space which the piston has left. 
 Therefore if we were to connect the opening at the end of the 
 cylinder with the neck of a bottle, by moving the piston in the 
 way just described, a cylinderful of air would be drawn out of the 
 bottle. But this arrangement would not be sufficient to constitute 
 an air pump, because, on pushing back the piston to its original 
 position, the air which we had withdrawn would be again forced into 
 the bottle. In the air pump, therefore, a kind of small trap-door is 
 placed at the opening, which opens and allows air to pass into the 
 cylinder, but closes when the air attempts to get out again. This 
 little trap-door is called a valve. 
 
THE AIR PUMP. 
 
 49 
 
 Fig. 11. 
 
 Let us now consider the air pump shown in fig. 11 A. It will 
 be seen that there is a cylinder, piston, and piston-rod, as in the 
 syringe. Great care is taken to make 
 the piston fit correctly in the cylinder, 
 and it is usually wrapped round with a 
 collar of leather, which is kept well 
 oiled. Two little trap-doors, or valves, 
 are shown, one (marked v l ) opening 
 towards the interior of the cylinder, 
 and one (marked v 2 ) opening outwards. 
 In reality the valves of the air pump 
 are not trap-doors of the form shown in 
 the figure. They are square pieces of 
 oiled silk which are fastened down on two 
 opposite sides, while the other two sides 
 are loose. The silk covers over a small 
 hole, and when the air presses through 
 the hole against the silk, the latter is 
 forced away, and the air makes its 
 escape at the two loose sides. When the air tries to pass in the 
 opposite direction it presses the silk closely over the opening, and 
 thus shuts the door in its own face. If the silk is on the outside, 
 it lets air pass out of the cylinder, but prevents it from passing in 
 (just as v 2 does in the figure) ; if on the inside it allows the air to 
 pass in (as v 1 in the figure), but prevents it from passing out. "When 
 the piston of the pump is drawn up, air passes into the cylinder 
 through v l t which opens for the purpose, while i 2 is closed by the 
 pressure of the outside air. When the piston is forced down, air is 
 pressed out of the valve v 2 , while the pressure of the air in the 
 cylinder closes the valve v 1 . And this takes place every time the 
 piston is moved up and down. 
 
 Suppose now that we have a small table (see fig. 11 B) with a pipe 
 passing from a hole in the centre, and so arranged that we can con- 
 nect this pipe with the side branch of the pump. When that is 
 done, each time the piston is drawn up air passes through the 
 opening in the table, and along the pipes, to get into the cylinder. 
 And if we place a glass vessel air-tight on the table, then, by means 
 of the pump, we can draw air out of this vessel. The vessel is called 
 the receiver, because it receives the objects with which we wish to 
 experiment. We can never remove the whole of the air from the 
 receiver, but every time we raise the piston we draw out more of 
 the air, until at last there is so little air left in the receiver, that it 
 is not strong enough to open the valve v 1 . When that is the case, 
 there is no advantage in pumping any longer. 
 
 There are many forms of air pump ; in some the cylinder is 
 horizontal, in others it is upright or vertical, while in others again 
 there are two cylinders to the same pump. In all the various 
 forms, however, the principles are the same as in the one we have 
 been describing. 
 
 D 
 
50 PRESSURE OF THE AIR 
 
 EXERCISES. 
 
 1. What property of gases causes the air to enter the cylinder of an air 
 
 pump when the piston is drawn away from the valve ? 
 
 2. Why is it necessary that the piston of an air pump should move air-tight 
 
 in the cylinder ? 
 
 3. If the cylinder of an air pump is just the same size as the receiver, how 
 
 much air will be left in the receiver after the first stroke of the pistoii ? 
 and how much after the second stroke ? 
 
 4. Why is lard, or some similar substance, usually put on the bottom edge 
 
 of the receiver ? 
 
 5. How would you use the air pump illustrated in the lesson, to force air 
 
 into a vessel ? 
 
 6. If you wished to force a quantity of coal-gas into a bladder, how could 
 
 you do it by means of the pump described ? 
 
 LESSON 22. 
 
 ^PRESSURE OF THE AIR. 
 
 The atmosphere is composed of a mixture of three gases nitrogen, 
 oxygen, and carbonic acid. The nitrogen forms about four-fifths of 
 the air, the oxygen about one-fifth, while the carbonic acid gas is 
 present in very small quantity. The atmosphere also contains, 
 mixed with these gases, more or less of the vapour of water. 
 
 We have said that one of the most characteristic properties of 
 gases is their constant tendency to expand, and that in conse- 
 quence of this tendency they exert a pressure on the surfaces of all 
 bodies with which they are in contact. The sides of an india-rubber 
 ball filled with air are being pressed in two opposite directions ; 
 they are being pressed outwards by the air inside the ball, and 
 inwards by the air outside. Under ordinary circumstances, these 
 two pressures are equal to each other, and consequently the sides of 
 the ball are not moved one way or the other. If we place the ball 
 under the receiver of an air pump, and remove some of the air 
 from around it the ball being supposed to be closed it will 
 expand, the sides bemg forced outwards by the pressure of the 
 enclosed air. The same result might be produced by forcing more 
 air into the ball. On the other hand, if air be withdrawn from 
 the inside of the ball by means of the mouth or an air pump, the 
 sides will be forced together by the pressure of the outside air. 
 
 When a receiver is placed on the plate of an air pump and some 
 of the air withdrawn, the receiver and the plate are closely pressed 
 together by the air outside, so that it is exceedingly difficult to 
 separate them. As soon as the proper quantity of air is restored 
 to the receiver it can easily be lifted, because the air on the inside 
 
PRESSURE OF THE AIR. 51 
 
 then presses the receiver and the plate as strongly in one direction 
 as the air on the outside presses them in the other. The same ex- 
 planation will apply to the sucker, in which the receiver is repre- 
 sented by a piece of soft moist leather. The leather is pressed on 
 to a flat stone, for instance, so as to force the air from between the 
 leather and the surface, then the pressure of the air on the outer sur- 
 face of the sucker holds it firmly against the stone. And since the 
 sucker can be fixed equally well on the floor or walls of a room, or on 
 the under surface of a table, it proves that the pressure of the air is 
 exerted equally in all directions. The amount of pressure of the 
 air is very considerable, being about 15 Ibs. on every square 
 inch. Therefore, if the sucker had a surface of 9 square inches, and 
 all the air could be removed from underneath it, it might be made to 
 lift a stone weighing nearly 144 Ibs. As a rule, there is a consider- 
 able quantity of air left underneath the sucker, and this, of course, 
 weakens its holding power. 
 
 If a tumbler be quite filled with water and a piece of paper laid 
 on the top, the tumbler may be carefully inverted without any of 
 the water running out. If the tumbler full of air be covered with 
 paper and inverted, the paper will fall by its own weight, since the 
 pressure of the air on the upper surface of the paper is equal to 
 the pressure on the lower surface. But when the tumbler is filled 
 with water, the upper pressure of the air on the lower surface of 
 the paper is greater than the pressure on the upper surface due to 
 the weight of the liquid. If the inverted tumbler full of water be 
 next placed with its mouth below the surface of the water in a 
 basin, the paper may be removed without the water in the tumbler 
 falling down. For if the water in the tumbler were to fall down 
 into the basin, it would have to raise the level of the water in the 
 latter vessel. In order to do that, it would have to overcome the 
 pressure of the air on the surface of the water, and the weight of the 
 water in the tumbler is not sufficient for that purpose. 
 
 The pressure of the air can not only support the water in a vessel 
 placed in the position of the tumbler just described, but it can raise 
 the water up into the vessel under certain circumstances. If a 
 syringe, for example, be placed with its narrow end in a vessel of 
 water, and the piston be then drawn up, the water follows the 
 piston into the cylinder. Similarly, if a glass tube or a straw be 
 placed with its lower end in a liquid, and the air be withdrawn 
 from its upper end by means of the mouth, the liquid rises up the 
 tube. This is usually said to be due to " suction," but the sucking 
 action of the mouth does no more than remove the air from above 
 the liquid in the tube ; the pressure of the atmosphere outside does 
 the rest. 
 
 A liquid will not run out of the vessel in which it is contained 
 unless, at the same time, air can enter the vessel. In pouring a 
 liquid quickly out of a narrow-necked bottle, the air passing into 
 the bottle interferes with the free passage of the liquid. Beer will 
 not run out of the tap in a barrel, unless a small hole is bored into 
 
52 THE BAROMETER. 
 
 the top of the barrel, or some other provision made, to enable the 
 air to enter as the liquid passes out. 
 
 Otto von Guericke, the inventor of the air pump, performed a 
 very striking experiment to illustrate the great pressure exerted by 
 the atmosphere. He took two hollow hemispheres of copper, the 
 edges of which fitted accurately to each other, and, having placed 
 them together, he exhausted the air from the hollow sphere thus 
 formed. The pressure of the external air forcing the hemispheres 
 together was so great that it required the united strength of several 
 horses attached to each hemisphere to tear them asunder. This 
 experiment was performed at Magdeburg in Germany, in 1650 ; the 
 apparatus for similar experiments is now always called the u Mag- 
 deburg hemispheres." 
 
 EXERCISES. 
 
 1. If you were drinking water from a tumbler by means of a straw, would 
 
 it make any difference if a hole were made through the side of the 
 straw ? Give a reason for your answer. 
 
 2. Why does a sucker stick better on a flat smooth surface than on a rough 
 
 one? 
 
 3. If the air, in the experiment with the Magdeburg hemispheres, has a 
 
 surface of 1 square foot to act on, what force will be needed to sepa- 
 rate the hemispheres ? 
 
 4. When using an air pump similar to that shown in fig. 11, lesson 21, to 
 
 pump air out of a receiver, why is it more difficult to raise the piston 
 after several strokes than at first ? 
 
 5. If a hollow india-rubber ball, having a small opening in its side, be 
 
 plunged into a vessel of water, the water does not enter the ball. 
 Explain this. 
 
 6. If the sides of the ball mentioned in exercise 5 be forced together, and 
 
 then released while the ball is still in the water, describe what takes 
 place, and explain it. 
 
 LESSON 23. 
 
 THE BAROMETER. 
 
 If a glass tube about 20 inches long, closed at one end and full 
 of mercury, be inverted with its open end dipping below the surface 
 of mercury in a basin, the mercury in the tube will not fall out. 
 We have already considered a very similar experiment, namely, 
 a tumbler filled with water and inverted into a vessel containing 
 water. The same explanation would serve for the two cases ; but, 
 as it is very important in this lesson that we should clearly under- 
 stand the reason why the mercury remains in the tube, let us go 
 over the explanation again. We know that the whole of the 
 mercury in the tube is pressing on the layer of mercury at the 
 bottom, and endeavouring to force it into the basin. But if the 
 
THE BAROMETER. 53 
 
 mercury were to fall out of the tube into the basin, the surface of 
 the mercury in the basin must necessarily be raised, and the air 
 pressing on that surface must also be raised. In the experiment we 
 are considering, it appears that the pressure of the mercury in the 
 tube is not sufficient to overcome the pressure of the air. 
 
 It might be thought that the air has a ^reat advantage over the 
 mercury in this experiment, because it has a much larger surface to 
 act upon. That such is not the case, however, may readily be seen, 
 if we reason about the matter in the following way. We know that 
 the air is pressing downwards on the surface of the mercury in the 
 basin, and that this pressure is transmitted equally in all directions 
 by the liquid. It is therefore transmitted to the mouth of the 
 tube, and presses on the mercury in the tube in an upward direc- 
 tion. And if the mouth of the tube has an area of one square inch, 
 the pressure transmitted to that square inch will be just equal to 
 the pressure of the air on one square inch of the surface of the 
 mercury in the basin. If the mouth of the tube had an area of 
 two square inches, the pressure on that surface would be equal to 
 the pressure of the air on two square inches, and so on. We can 
 therefore understand that the diameter of the tube makes no differ- 
 ence in the result. With a narrow tube there will be only a small 
 quantity of mercury to support, and the pressure of the air will be 
 transmitted to a small surface only. With a wider tube there will 
 be a greater weight of mercury to support, but the air will have all 
 the more surface to act on. In this experiment, and those about 
 to be described, we need only notice the height of the tube and not 
 trouble about its diameter. 
 
 When we make use of a tube 20 inches long, and find that tha 
 mercury does not fall out, it is evident that the pressure of the air 
 is, at least, as great as the weight of a column of mercury 20 inches 
 high. If we make a similar experiment with a tube 33 inches in 
 length, we shall find that all the mercury will not remain in the 
 tube : part of it falls down into the basin, leaving at the top of the 
 tube above the mercury an empty space called a vacuum. On 
 measuring the height of the surface of the mercury in the tube 
 above that in the basin, we shall find it to be about 30 inches ; and, 
 however carefully and often we might repeat the experiment, we 
 should always find this to be the case. We must try and imagine 
 the mercury in the tube as endeavouring to force its way out, and 
 the air outside endeavouring to force the mercury higher up the 
 tube into the vacuum. As the mercury in the tube stands still, we 
 conclude that the pressure of the air is not great enough to overcome 
 that of the mercury, nor the pressure of the mercury great enough 
 to overcome that of the air. That is to say, the pressure of the 
 mercury in the tube is exactly equal to the pressure of the air out- 
 side. If the pressure of the air were to become greater, it would 
 force the mercury higher up the tube, while, on the other hand, if 
 the pressure of the air were to become less, the mercury would force, 
 its way down lower. 
 
54 THE BAROMETER. 
 
 Suppose that we fitted up a tube such as that just described, and 
 allowed it to stand for several days. By frequently observing the 
 height of the column of mercury, we could ascertain whether the 
 pressure of the air changed at all from day to day, or from hour to 
 hour. Many such tubes are fitted up, and hung in houses and other 
 places ; they are called barometers. A barometer is an instru- 
 ment for measuring the pressure of the air. It is found that 
 the pressure of the air does frequently vary, that in fact the mercury 
 sometimes rises to about 31 inches, and at other times falls as low 
 as 28 inches. People are very interested in observing these varia- 
 tions in the pressure of the air, because they find from experience 
 that, as a rule, when the mercury rises up high in the baro- 
 meter, fine weather may be expected ; and when it falls down 
 low, storms of wind or rain will very likely follow. The baro- 
 meter is therefore sometimes called a " weather glass." Some baro- 
 meters are so constructed that the mercury, as it rises and falls, 
 moves round a pointer on a dial. The words " Fair," " Rainy," &c., 
 are printed on the dial, and it is arranged so that the pointer 
 shall move towards " Rain " when the mercury falls, and towards 
 "Fair" when it rises. We must remember that the barometer 
 does not actually tell us anything about the weather ; it simply 
 shows the amount of pressure of the atmosphere, and then we 
 have to judge from, experience what kind of weather is likely to 
 follow. 
 
 It is always desirable that we should, as far as possible, prove by 
 actual experiments that our ideas are correct. When it was first 
 suggested that it was the pressure of the air which supported the 
 mercury in the tube of a barometer, it was thought necessary to 
 ascertain in some way whether that idea was or was not correct. 
 A very simple experiment was tried, and has since been many 
 times repeated, as follows. The pressure of a gas, like that of a 
 liquid, must increase as we descend deeper below the surface of the 
 fluid. According to this rule, the pressure of the air decreases as 
 we climb a mountain, or ascend in a balloon. And it was argued 
 that, if it was really the pressure of the air which supported the 
 mercury in the barometer, the height of the column ought to be less 
 at the top than at the bottom of a mountain. A barometer was ac- 
 cordingly carried to the top of a mountain, and the height of the 
 column of mercury was less there than at the bottom. Since that time, 
 barometers have frequently been carried up mountains and taken up 
 in balloons, and in all cases it is found that the mercury gradually 
 falls in the barometer as the instrument is carried higher and 
 higher. Thus, if the mercury in a barometer stood at 30 inches at 
 the level of the sea, it would only be about 26^ inches in a baro- 
 meter at the top of Mount Snowdon, and about 15 inches in one on 
 the top of Mont Blanc. Thus it appears that on the top of the 
 latter mountain the pressure of the air on any surface is only half 
 as great as that at the sea-level. The barometer is frequently used 
 to ascertain the heights of mountains ; for it can be calculated how 
 
THE BAROMETER. 55 
 
 far it will be necessary to ascend, in order that the mercury may 
 fall a certain number of inches. 
 
 When we were considering the specific gravities of liquids, we 
 found that to balance a certain column of mercury we required a 
 column of water 13 J times as high. Similarly, in the barometer, we 
 have two columns of fluid balancing each other, namely, a column 
 of mercury about 30 inches high, and a column of air reaching from 
 the mercury in the basin to the top of the atmosphere. It is not 
 absolutely necessary that we should use mercury as the liquid in the 
 barometer, although it is the most convenient one. If we used 
 water instead of mercury, we should require a column 13 times as 
 long as that of mercury (i.e., about 33 feet of water) to balance the 
 column of air. 
 
 Suppose that the opening of the tube of our barometer had an area 
 of exactly one square inch. We said in the beginning of this lesson 
 that the upward pressure on this area would be equal to the pressure 
 of the air on one square inch of the surface of the mercury in the basin. 
 We see, therefore, that the pressure of the air on one square inch of 
 surface is equal to the weight of the column of mercury in such a 
 tube. This weight is known to be about 15 Ibs. ; and thus we have 
 a proof of the statement made in the last lesson, that the pressure 
 of the air is equal to about 15 Ibs. on every square inch of surface 
 at the sea-level. In other words, the pressure of the air on the 
 surface of the earth is equal to the pressure of an ocean of mercury 
 covering the earth to a depth of about 30 inches. 
 
 EXERCISES. 
 
 1. What difference would it make in a barometer if a little air got into the 
 
 vacuum above the mercury ? 
 
 2. If we place a tube which is open at both ends upright in a vessel of 
 
 mercury, and then connect the upper end of the tube with the air 
 pump, describe and explain what will take place as we work the 
 pump. 
 
 3. If there were two barometers, one containing mercury (specific gravity, 
 
 13^), and one containing nitric acid (specific gravity, 1), what would 
 be the height of the column of nitric acid, when that of the mercury 
 was 30 inches ? 
 
 4. If the pressure of the air be the same on two separate days, but the 
 
 temperature on one day be higher than on the other, will there be 
 any difference in the height of the column of mercury in the baro- 
 meter on the two occasions ? 
 
 5. What change in a barometer would you expect to find if you carried it 
 
 carefully down into a deep mine ? 
 
 6. Find the weight of a column of mercury 30 inches high, and having a 
 
 section of 1 square inch ; taking the weight of 1 cubic foot of water 
 as 1000 oz., and the specific gravity of mercury 13. 
 
 7. What advantage would there be in a barometer containing water or 
 
 glycerine instead of mercury ? 
 
56 THE SIPHON. 
 
 LESSON 24. 
 
 THE SIPHON. 
 
 The barometer is an instument whose action depends on the pres- 
 sure of the air. If the air had no pressure, a column of mercury 
 would not be supported in a tube in the manner we described in the 
 last lesson. We are now about to consider another instrument, 
 called the siphon, the action of which depends likewise on the 
 pressure of the air. Suppose we fill with water a small glass tube 
 which is closed at one end, and invert it carefully, so that the open 
 end dips below the surface of the water in a basin. We know that 
 the pressure of the air will prevent the water from falling out of the 
 tube. By proceeding carefully, we can raise the tube out of the 
 water in the basin without any of the water in the tube running 
 out. The water will remain in the tube so long as we are careful 
 to keep it exactly upright, but directly we incline it a little to one 
 side the water runs out on the lower side, while the air enters the 
 tube at the upper side. 
 
 As long as the tube is kept upright, the air presses upwards on the 
 under surface of the water but cannot get in, since the water in one 
 part will not give way more easily than in another. The reason 
 why the air gets in when the tube is inclined we shall understand 
 presently. Using a wider tube filled with water, it is more difficult 
 to raise it and at the same time to keep the water in, and with a 
 tumbler it is almost impossible to perform this experiment. The 
 reason of this is, that there is very great difficulty in keep- 
 ing the under surface of the water at the same level ; for 
 if one part of the surface be higher than another, it is 
 easy for the air to get in. If the surface of the tumbler 
 of water be covered with a piece of paper before it is 
 inverted, the water may then easily be kept in. For the 
 particles of the solid paper hold together, better than 
 the particles of water forming the surface sheet at the 
 mouth of the tumbler, and one part of the paper can- 
 not fall without the rest. If we next take a glass tube, 
 bent as in fig. 12, fill it with water, and then invert its 
 open ends into a basin of water, we may raise it carefully 
 out of the basin without the water running out of the 
 tube. So soon as we incline it, however, the water runs 
 Fi 19 out of the lower arm, and air rushes in at the other. 
 Once more, if we take a similarly bent tube with un- 
 equal arms (fig. 13), and proceed in the same manner, we shall 
 find that every time we raise it from the basin the water runs out 
 at the longer arm. The water in the shorter arm rises up the tube 
 and passes over to the other side, in order to fall down the longer 
 arm, while the air rushes up the shorter tube after the water to take 
 its place. 
 
THE SIPHON. 
 
 57 
 
 B 
 
 Fig. 13. 
 
 We have, so far, been describing facts which any one might easily 
 observe on making the experiments ; let us next try to explain 
 them. We said that the water remained in the single tube, because 
 of the upward pressure of the air on the water at the mouth of the 
 tube. In the same way, when we use a bent tube, we shall have 
 the pressure of the air acting at each opening of the tube. 
 And if the area of each opening is just one square inch, 
 then the pressure of the air on each surface of water at 
 the openings will be the same, and equal to about 15 Ibs. 
 In both the tubes we are considering (figs. 12, 13), the 
 pressure of the air on the surface of water at each open- 
 ing will be transmitted by the water to the top of the 
 tube. A portion of water at c (figs. 12, 13) will thus be 
 pressed in two opposite directions, towards the right and 
 towards the left. What will the water at c do ? The 
 experiment shows, that in the equal-armed tube (fig. 12) 
 it may stand still, and that in the unequal-armed tube 
 it will move towards the longer arm. The pressure 
 of the air transmitted by the water will not, in any case, 
 be so strong when it reaches c as at the opening of 
 the tube, for part of it is spent in holding up the water in 
 the arms. In the case of the tube with equal arms (fig. 
 12), there is the same weight of water to hold up on 
 each side ; therefore the pressure which passes up the right arm of 
 the tube to c will be just as great as the pressure which passes up 
 the left arm to that point. We see why, in this case, the water in 
 the tube does not move, simply because it is being pressed with 
 the same force in opposite directions. With the unequal-armed tube 
 (fig. 13) the case is different. The air has more water to support in 
 the longer arm than in the shorter ; and, in consequence, the pres- 
 sure transmitted up the longer arm to c will be less than that 
 transmitted up the shorter arm to that point. Therefore the liquid 
 in the tube will be forced towards the long arm, because the pres- 
 sure on that side is less than on the other. 
 
 When we incline the equal-armed tube to one side, as shown in fig. 
 14, it comes to the same thing as 
 making one arm longer than the 
 other, and the water runs out 
 on the lower side. The dotted 
 upright lines in the figure show 
 the difference between the two 
 columns of water, which the 
 air has to hold up in this case. 
 They are drawn from the level 
 of each opening to the level of 
 the highest point of the tube. 
 The explanation just given 
 applies equally to the tube 
 and the tumbler referred to in the first part of this lesson. 
 
58 PUMPS; 
 
 If, while the siphon is full, the shorter arm be dipped into a 
 vessel of water, as the liquid runs out of the siphon water from the 
 vessel will be forced up the tube by the pressure of the air. This 
 will continue as long as the opening of the shorter arm dips below 
 the surface of the water. The siphon can thus be employed for the 
 purpose of transferring water, or other liquids, from one vessel to 
 another ; provided, however, that the end of the arm out of which 
 we require the liquid to run, is lower than the surface of the liquid 
 into which the shorter arm dips. 
 
 Since it is the pressure of the air which forces the liquid up one 
 arm of the siphon, we can understand why, under certain circum- 
 stances, the siphon will not work. Thus, if the highest part of the 
 siphon were more than about 33 feet above the surface of the water in 
 the vessel, then the siphon would not act with water, although it might 
 with a liquid of less specific gravity, such as alcohol. For a siphon 
 to act with mercury, the air must not have to force up the mercury 
 more than about 29 or 30 inches. And in general, the height of the 
 siphon for use with a particular liquid, must not be greater than the 
 height of the column of that liquid which would be required to form 
 a barometer. 
 
 EXERCISES. 
 
 1. If two vessels containing water were set side by side, and a siphon having 
 
 equal arms filled with water placed with one leg in each, would any 
 movement of the water take place, if the level of the liquid in one 
 vessel were higher than in the other ? Give a reason for your answer. 
 
 2. In the last exercise, if any movement commenced, how long would it 
 
 continue ? 
 
 3. If, while a siphon were at work, a hole were made into the tube at the 
 
 top, would it make any difference in the action ? 
 
 4. Describe two methods by which the siphon may be filled with liquid in 
 
 order to start it. Explain why it is necessary to fill the siphon with 
 liquid at the commencement. 
 
 6. If a dry towel or handkerchief be placed in a basin of water, with one end 
 hanging over the side, the water will drop from this end, provided 
 it is lower than the surface of the water in the basin. Explain, as far 
 you can, (1) why the material becomes wet at all, and (2) the continual 
 dropping of the liquid from the end hanging outside. 
 
 LESSON 25. 
 
 PUMPS. 
 
 The familiar instrument called the syringe was described in the 
 lesson on the air pump. It consists of a cylinder which is usually 
 brought to a point at one end, and which contains a piston, wrapped 
 with cotton to make it air-tight. The piston is worked by means 
 
PUMPS. 59 
 
 of a piston-rod passing loosely through a cork. If the end of the 
 cylinder be placed in water, and the piston raised, the water is 
 forced into the cylinder by the pressure of the air acting on it. 
 When the piston is pushed down again, the water is forced out. 
 The syringe might be used for conveying a liquid from one vessel 
 to another ; it could be filled in one vessel, and then carried and 
 placed in the other, before the piston was pushed down. As a rule, 
 this would not be a very convenient process ; and therefore another 
 instrument, resembling the syringe in many respects, is used, and is 
 termed a pump. By means of a pump, any liquid may be removed 
 from one vessel to another, without the instrument itself being 
 moved. 
 
 A pump resembles the syringe in having a cylinder, usually nar- 
 rowed below, dipping into a well or vessel of water. There is also 
 a piston, made of wood or metal, and wrapped round with a leather 
 collar to make it fit air-tight. A piston-rod moves the piston, and 
 is itself worked by means of a lever handle. One great difference, 
 however, between a pump and a syringe is, that the former has in 
 it at least two valves, while a syringe has none. The valves are for 
 the same purpose as those in the air pump, viz., to allow of the passage 
 of a fluid in one direction only, and to prevent it from passing in the 
 opposite direction. In ordinary work- 
 ing pumps the valves are not squares of 
 oiled silk, as in the air pump, but are 
 more like trap-doors, made of wood 
 or metal, and usually covered with 
 
 leather. Fig. 15 illustrates a common form of valve. The prin- 
 ciple of the construction of the common pump is shown in fig. 16. 
 When the piston is raised, the water rises up through 
 the narrow " suction " pipe, raises the valve at the top of 
 that pipe, and enters the cylinder. When the piston is 
 lowered, the water in the cylinder presses down the 
 valve, and so shuts the door by which it could have 
 escaped. One or two valves are placed over the holes in 
 the piston, and as the piston passes downwards through 
 the water, the water rises through these openings, and 
 thus gets above the piston. When the piston is again 
 raised these valves close, and the water above them is lifted 
 up and passes out at the spout. This kind of pump is 
 called the suction and lifting pump. The distance 
 through which the pressure of the air has to raise the 
 water must not be above 33 feet ; and, unless the pump 
 is exceedingly well made, and works very air-tight, the 
 water will not be raised even to that height. There is 
 another variety of pump frequently employed, the prin- F . 
 ciple of the construction of which is shown in fig. 17. It 
 resembles the suction r.nd lifting pump in having at its lowest part 
 a suction pipe, with a valve at the top of it opening towards the 
 cylinder. It differs, however, from that pump in having no open- 
 
 T 
 
60 
 
 PUMPS. 
 
 Fig. 17. 
 
 ing through the piston. The water instead of passing through and 
 above the piston, remains constantly below it, and passes out into a 
 pipe opening into the side of the cylinder. This pipe is guarded by 
 a valve, which allows the water to pass into it from the 
 cylinder, but prevents it from passing back again. 
 When the piston is raised, the pressure of the air on the 
 water in the well forces it up the suction pipe into the 
 cylinder. When the piston descends, the water closes the 
 valve at the top of the suction pipe, and passes into the 
 side pipe. This water tries to force its way back into 
 the cylinder when the piston is again raised, but is pre- 
 vented by the valve during the working of the pump ; 
 therefore water is drawn from the well, and forced into the 
 side pipe. This kind of pump is called the suction and 
 force pump. The force pipe may be left short, and the 
 water forced out of it to a considerable distance ; or it 
 may be prolonged, and the water carried up it to a great 
 height. This height may be much more than 33 feet, 
 because it is the pressure of the piston, and not of the 
 air, which forces the water up the pipe ; and in this 
 fact consists the chief advantage of the force pump. The 
 common fire-engine contains two force pumps combined, working 
 alternately, first one and then the other forcing the water into a 
 chamber, from which it is carried by the delivery pipe. The cham- 
 ber contains air, and when the water is forced in by the pumps the 
 air is compressed. We know that the more air is compressed, the 
 greater the pressure it will exert on the walls of the chamber, and 
 on the surface of the water. It is the pressure of the air on the 
 water which forces the latter out of the delivery pipe. The great 
 advantage of the air chamber is, that the water issues from the pipe 
 in a constant steady flow, instead of in a series of jerks as it would 
 otherwise do. 
 
 EXERCISES. 
 
 1. Draw a diagram to illustrate the construction of the fire-engine. 
 
 2. Is there any necessity for a pump to be closed at the top ? Give a reason 
 
 for your answer. 
 
 3. If the leather collar round the piston gets very dry and shrinks, in what 
 
 way does that interfere with the working of the pump? 
 
 4. A piece of perforated iron is frequently placed at the bottom of tho 
 
 suction pipe of a pump, in order to keep out sand and gravel. Ex- 
 plain in what way those substances would injure the pump. 
 
 5. "Would a suction and lifting pump work as well on the top of a mountain 
 
 as at the bottom ? 
 
 6. If the suction pipe cracked near the top, so that air could enter, would it 
 
 make any difference in the action of the pump ? 
 
EFFECTS OF HEAT ON GASES. 61 
 
 LESSON 26. 
 
 EFFECTS OF HE A T ON GASES. 
 
 Gases constantly tend to expand, and, on this account, they exert 
 pressure on all sides of the vessels in which they are enclosed. To 
 maintain a quantity of gas at a constant size, there must be a 
 constant pressure on every side of it. This pressure may be 
 exerted by the sides of a vessel, or by surrounding portions of gas. 
 Thus, a particular portion of air in the middle of a room is pre- 
 vented from expanding, by reason of the pressure of the air sur- 
 rounding it. Air in contact with the floor, or walls, or ceiling of 
 a room, is prevented from expanding in one direction by those 
 structures, and in other directions by the air next to it. We 
 know that if we remove part or all of this pressure, the portion 
 of gas which we are considering will expand ; that is to say, its 
 molecules will move farther away from each other. If, on the con- 
 trary, we increase the pressure, the gas is compressed that is, its 
 molecules are forced nearer together. The size of a gas may there- 
 fore be changed, by altering the amount of pressure acting on it. 
 
 There is one means by which the size of a quantity of gas may be 
 changed without altering the pressure, namely, by altering the 
 temperature of the gas. If we heat a certain portion of air in a 
 room, it expands ; the heat forces the particles farther away from 
 each other. Similarly, if we take an open vessel and heat the air 
 inside it, that air expands, and part of it escapes from the vessel. 
 A bottle full of cold air will, therefore, weigh more than the same 
 bottle full of hot air ; the difference, however, is so small that it 
 will require a good balance, and careful weighing, to prove the 
 fact. It follows that cold air has a greater density and specific 
 gravity than hot air. 
 
 If we take a closed bottle full of air and heat it, it is evident 
 that the air in this case cannot expand as it did in the open bottle. 
 But it makes a great effort to expand, and, in consequence, exerts a 
 greater pressure on the sides of the bottle than it did at the first. 
 By heating the gas sufficiently, the pressure may become so great 
 as to force the cork out of the bottle, or even to burst the sides. 
 With hydrogen, or coal-gas, or any gas whatever in the bottle, the 
 same results would be obtained as we have described in the case of 
 air. We are, therefore, justified in making the following general 
 statement : The effect of heat upon a gas is to increase the 
 force with which the gas tends to expand. Whenever a gas 
 is heated it expands, if it can overcome the resistance of surround- 
 ing bodies. 
 
 Gases resemble liquids and solids in expanding under the action 
 of heat ; but they differ from both those forms of matter in that 
 
62 EFFECTS OF HEAT ON GASES. 
 
 all gases expand equally when heated to the same extent, whereas 
 different solids and liquids expand unequally. 
 
 When a portion of gas is cooled, the force with which it is en- 
 deavouring to expand is diminished. If we take a bottle filled 
 with air, close it with a well-fitting cork, and then cool it, the walls 
 of the bottle are not subjected to so great a pressure on the inside 
 as they were at first. The pressure of the outside air will therefore 
 tend to force in the sides of the bottle. If we leave the bottle open 
 while we cool it, air from the outside forces its way in, since the 
 air in the bottle has less power than before to press it outwards. 
 
 We said in an earlier part of this lesson that cold air has a greater 
 density and specific gravity than hot air. If, then, hot and cold 
 air are present at the same time in a room, we should expect to find 
 the hot air in the upper, and the cold air in the lower, parts of the 
 room. Suppose that the air in a room is all at the same tempera- 
 ture to commence with, and that a portion of it near the floor is 
 then heated. The heated air, as it expands, becomes less dense, 
 and is therefore pushed up by the colder and heavier air around. 
 The cooler air moves to take the place of that which has ascended ; 
 and, if the heating be continued, this air in its turn ascends and 
 is replaced by cooler air. In this manner, a circulation is set up 
 in the gas, precisely similar to that which occurs in a liquid 
 heated at the bottom. The moving masses of air are spoken of as 
 air-currents. Draughts are air-currents on a small scale, while 
 winds are nothing but large masses of air, moving sometimes with 
 very great velocity. Winds are produced by the unequal heating of 
 the air at different parts of the earth's surface. For example, the 
 air at the equator is considerably hotter than at the poles, and 
 currents of heated air ascend in the atmosphere there, and flow 
 northwards and southwards towards the poles. The colder air from 
 the regions to the north and south presses forwards towards the 
 equator, to take the place of the ascending heated air. It is, in 
 fact, the pressure of this cold air on each side, which causes the 
 heated air to rise. On the mountain known as the Peak of Tene- 
 riffe, in the Canary Islands, about 2000 miles north of the equator, 
 both of these winds can be perceived. At the bottom of the moun- 
 tain, a cool wind is steadily blowing from the north towards the 
 equator, while near the top of the mountain, there is a warm wind 
 blowing steadily from the equator towards the north. 
 
 The heated air may become so much lighter than the rest of the 
 air as to be able to carry up with it certain light solids. A paper- 
 bag or balloon filled with hot air will ascend to some distance, on 
 the same principle that wood ascends in water. The weight of the 
 cool air displaced by the balloon is greater than the weight of the 
 balloon itself and the air which it contains. The upward pressure 
 of the surrounding air is greater than the downward pressure of the 
 balloon. In some cases, balloons are filled with a light gas (hy- 
 drogen, or more commonly, coal-gas) instead of with hot air. 
 
PROPERTIES OF GASES. 63 
 
 EXERCISES. 
 
 1. "When an island is warmer than the water round it, in which direction 
 
 will the wind near the island blow ? 
 
 2. The air which we breathe out of our mouths is warm and impure, in 
 
 which part of a room will this impure air collect ? Where should an 
 opening be made to allow it to escape ? 
 
 3. If a piece of smoking paper be laid on the fender in front of a fire, in 
 
 which direction will the smoke pass, and why ? 
 
 4. Balloons sent up filled with hot air soon descend again. Explain the 
 
 cause of their descent. 
 
 5. What is the use of the holes in the brass plate just below the flame of a 
 
 lamp? 
 
 6. If a bladder partly filled with air be laid in a warm place, how will it be 
 
 affected ? 
 
 7. Why are the hot pipes which are used to warm certain buildings always 
 
 placed near the floor ? 
 
 LESSON 27. 
 
 PROPERTIES OF GASES. 
 
 Matter in the state of gas is readily distinguished from matter in tne 
 solid or liquid state by several properties. The most important of 
 these is the absence of all cohesion between the molecules ; they are 
 free to move in any direction. And, in fact, there appears to be 
 some repulsive force, which keeps the molecules in constant motion, 
 endeavouring to move as far as possible away from each other. As 
 a result of this, we find that a gas can adapt its shape to that of any 
 vessel we may choose to put it into, and that it is also ready to take 
 any size whatever. As we remove the pressure acting on a quantity 
 of gas, the gas expands ; while on the other hand, as we increase 
 the pressure the gas is compressed. These changes of volume would 
 apparently take place to almost any extent we liked, if we could but 
 sufficiently diminish or increase the pressure. In consequence of 
 the property by which a gas is constantly endeavouring to expand in 
 all directions, it exerts an equal pressure in every direction on the 
 surfaces of the vessel which encloses it. The amount of pressure 
 which a gas exerts increases as the gas is compressed ; in other words, 
 the more molecules there are in contact with a given surface, the 
 greater will be the pressure on the surface. 
 
 The air consists almost entirely of a mixture of two gases nitro- 
 gen and oxygen but contains also a small quantity of carbonic acid 
 gas, more or less vapour of water, and very small quantities of other 
 gases. Like other gases, and mixtures of gases, the air tends to ex- 
 
64 PROPERTIES OF GASES. 
 
 pand, and consequently it exerts pressure on all surfaces with which 
 it is in contact. The amount of this pressure does not vary to any 
 great extent, but is usually ahout 15 Ibs. on every square inch of sur- 
 iace. An instrument, termed the barometer, is employed to measure 
 the pressure of the air, and to show the changes which take place 
 in that pressure from day to day. Some people find it difficult to 
 understand why the air does not move away from the earth by 
 reason of its tendency to expand. But they forget that the force of 
 gravity acts on the air, and holds it as much to the earth as it does 
 a stone. Just as in the case of water, the pressure of the air increases 
 as we go deeper and deeper down into it. We live at the bottom 
 of the ocean of air, and, as we ascend in this ocean, the barometer 
 shows us that the pressure becomes less. 
 
 Not only does a gas exert a pressure in all directions, on account 
 of its constant tendency to expand, but if we apply pressure to any 
 part of the gas, that pressure is transmitted equally in. all directions. 
 In this respect a gas agrees with a liquid. 
 
 When a gas is heated it expands, if it be free to do so. In con- 
 sequence of this expansion its density becomes less, and ascending 
 currents of heated gas are produced. Winds are strong currents of 
 air produced in this manner, by the unequal heating of the air in 
 different parts of the earth. For example, it has been observed that 
 in many parts of the earth, cold currents of air are constantly pass- 
 ing towards the equator near the surface of the earth, while higher 
 up in the air hot winds are blowing from the equator towards the 
 poles. If a gas when heated cannot expand, its pressure is in- 
 creased. 
 
 Some gases when cooled readily condense, or turn into liquids ; 
 such cases are termed vapours. Other gases are more difl&cuit to 
 liquefy, and not only require to be made very cold, but at the same 
 time must be very much compressed. 
 
 We may sum up the chief properties of gases as follows : 
 
 (1.) A gas has an indefinite size, and an indefinite shape. 
 
 (2.) The molecules of a gas are not held together by co- 
 hesion. 
 
 (3.) All gases may easily be compressed. 
 
 (4.) They constantly tend to expand, and consequently exert 
 pressure in all directions on surrounding bodies. 
 
 (5.) The tendency of a gas to expand is increased by heat. 
 (6.) Gases transmit pressure equally in all directions. 
 
 (7.) All gases may be liquefied by cold alone, or by cold and 
 pressure combined. 
 
EXERCISES. 
 
 GENERAL RELATIONS BETWEEN SOLIDS. ETtt / . 65V/ J 
 
 <',. *f 
 
 : 'X 4 
 
 X/>\ Y * 
 
 1. Explain the following words : Vapour, fluid, vacuum, atmpspheM.i^ . /* _ 
 
 2. Give a short account of the atmosphere. N*V ^ J * t 
 
 3. In what respects do liquids and gases agree? In what respects do the$rl ' , 
 
 differ? / f 
 
 4. If a bladder partly full of air were taken up in a balloon, it would be seen 
 
 to expand as the ballooa ascended. What is the explanation of 
 this? 
 
 5. How would you proceed to ventilate a room ? 
 
 6. Name several instruments whose action depends on the pressure of the 
 
 air, and state for what purpose each is employed. 
 
 LESSON 28. 
 
 GENERAL RELATIONS BETWEEN SOLIDS, LIQUIDS, 
 AND GASES. 
 
 We have, in the previous lessons, considered somewhat fully the 
 chief properties which characterise matter in its three forms or states. 
 We commenced by inquiring what was meant by matter, and arrived 
 at the conclusion that whatever could be perceived by our senses 
 might be called matter. And we ascertained further that matter 
 had certain characters or properties, which we also know by means 
 of our senses. Some of these properties belong to every kind of 
 matter ; for example, the property of weight and the property of 
 divisibility. It is believed that matter consists of very small por- 
 tions called molecules, and that these molecules are the smallest 
 parts into which any body could possibly be divided by processes of 
 crushing or cutting. The chemist can, however, by certain other 
 processes, break up these molecules into still smaller portions which 
 he calls atoms. As a rule, it is best for our purposes to consider 
 bodies as being built up of molecules, and not to trouble about 
 atoms until we can find time to learn something of chemistry. 
 
 There are two forces acting on the molecules of bodies, one of 
 which tends to draw the molecules closer together, and to keep 
 them together, while the other force tends to separate them, and to 
 make them move away from each other. The former is said to be 
 an attractive force, and is called cohesion ; the latter is termed a re- 
 pulsive force, and is known as heat. The state or condition in which 
 we find a body at any particular time depends entirely upon the 
 relation between these two forces. In a solid body, for instance, the 
 force of cohesion which holds the molecules together, is so much 
 stronger than the force of heat which tends to separate them, that 
 they remain firmly fixed. As we increase the temperature of the 
 
 E 
 
66 GENERAL RELATIONS BETWEEN SOLIDS, ETC. 
 
 body, the repulsive force of heat becomes more and more nearly 
 equal to the attractive force of cohesion. At length the two are so 
 nearly equal, that the cohesion is no longer able to keep the mole- 
 cules fixed in one place, and they move about among each other when 
 acted on by the slightest force. In this condition we call the sub- 
 stance a liquid. There is still sufficient cohesion left, however, to 
 keep the molecules near each other. If we heat the liquid we in- 
 crease the repulsive force, until at length it becomes greater than the 
 attractive force, and then the molecules are no longer held near each 
 other, but move away from each other as far as they are allowed to 
 do. Matter in this latter condition is spoken of as a gas or a vapour. 
 Many persons now believe that the molecules which compose a gas 
 or a vapour are constantly moving about in all directions, striking 
 against each other and against the walls of any vessel in which they 
 may be placed. This, however, is a subject which we cannot further 
 enter into now. We may briefly describe the different states of 
 matter, by saying that a solid is a portion of matter in which 
 the attractive force between the molecules is greater than the 
 repulsive force ; a liquid is a portion of matter in which the 
 repulsive force is very nearly equal to the attractive force ; and 
 a gas is a portion of matter in which the repulsive force is 
 much greater than the attractive force. 
 
 The special properties of solids, liquids, and gases depend entirely 
 upon the relations which we have just been considering. In conse- 
 quence of the force of the cohesion in solids, for example, such bodies 
 resist a moderate force which tends to alter either their size or their 
 shape. For the same reason a solid body can withstand a pressure 
 without needing to be supported laterally. On the other hand, 
 liquids and gases yield to the slightest force, and are unable to bear 
 the least pressure, even the pressure due to their own weight, unless 
 they are supported round the sides. These two states of matter 
 liquids and gases are comprised, as we have seen, under the term 
 " fluids." They differ greatly in this respect, that while gases may 
 all be very easily compressed, it is extremely difficult to comDress 
 liquids. 
 
 EXERCISES. 
 
 1. How could you prove that the air always contains some vapour of water ? 
 
 Where does this vapour come from ? 
 
 2. The winds which come from the west, and blow over England, bring more 
 
 rain than those which come from the east. Can you give any reason 
 for this fact ? 
 
 3. Why should the tops of most high mountains be covered with snow? 
 
 4. On a cold day, the breath, as it passes from our mouths, forms a little 
 
 cloud of moisture ; we do not see any such cloud on a warm day. 
 Does our breath contain vapour in warm weather? How could you 
 prove that your answer is correct ? 
 
 5. How can you obtain salt from salt water? 
 
POROSITY AND COMPRESSIBILITY. 67 
 
 LESSON 29. 
 
 POROSITY AND COMPRESSIBILITY. 
 
 There are many reasons for believing that the molecules of bodies 
 are not in close contact with each other, but that small spaces are 
 left between them, and to these spaces the name of pores has been 
 given. On this account bodies are said to possess the property of 
 porosity, and the bodies themselves are said to be porous. In ordi- 
 nary language, however, those bodies only are called porous which 
 contain spaces or pores large enough to be seen, either with the eye 
 alone or by the aid of the microscope. Since no microscope as yet 
 made will enable us to see the individual molecules of which bodies 
 are composed, the spaces between the molecules are also invisible 
 under the best microscope. 
 
 If we take a bottle and fill it loosely with sand, a very little 
 observation will show us that the grains of sand do not lie quite 
 close to each other, but that they are separated by spaces which we 
 may compare to the pores separating the molecules of bodies. By 
 means of pressure, we may force an additional quantity of sand into 
 the bottle, or, what comes to the same thing, we can compress the 
 sand which filled the bottle in the first case, and make it take up 
 less room. This is done by forcing closer together the grains of 
 sand, and at the same time decreasing the size of the spaces between 
 them. Had the grains of sand in the first case been quite close 
 together, we could not have compressed the mass, so that the com- 
 pressibility proved the presence of spaces. By the use of a sufficient 
 amount of pressure it is possible to compress all bodies, and there- 
 fore we conclude that all bodies contain spaces or pores. And we 
 have learned in previous lessons that when heat is applied to a body, 
 in almost every case it causes the body to expand, that is to say, it 
 causes the molecules to move farther apart from each other, and 
 therefore increases the size of the spaces between the molecules. 
 Cold, on the other hand, causes the molecules to move nearer toge- 
 ther, and diminishes the size of the pores. From these facts we are 
 led to the conclusion that in all bodies there exist minute pores 
 or spaces between the molecules, and we express this by saying 
 that porosity is a general property of matter. 
 
 In addition to these invisible pores many bodies contain visible 
 pores. Bread, sponge, pumice-stone, &c., contain lai'^e visible pores 
 which can be seen without any difficulty. Our skin contains an 
 immense number of small pores through which the sweat or perspi- 
 ration comes, and these pores may be seen by means of a small mag- 
 nifying glass. The existence of pores in blotting-paper and many 
 similar substances is proved by the rise of water, ink, and other 
 liquids in them ; these pores may be readily seen by the aid of a 
 magnifying glass. Writing-paper had large pores similar to blotting- 
 
68 POROSITY AND COMPRESSIBILITY. 
 
 paper when first manufactured, but these are filled up with size to 
 prevent the ink running into them. In the same way cups and 
 plates, and ail articles made of burnt clay, are at first porous as we 
 may see in the case of a brick but in vessels intended to contain 
 liquids these pores are covered over by the " glaze v which is burnt 
 on to them. The existence of pores in metals has been proved in 
 several ways. One famous experiment which proved this was per- 
 formed more than two hundred years ago at Florence in Italy, and is 
 usually called the " Florentine experiment/' A hollow ball of silver 
 was filled with water and completely closed up, and then it was 
 attempted to compress the water. The water made its way through 
 the pores of the silver on to the outer surface. It was concluded 
 that water was incompressible ; but, as we have already seen, this 
 conclusion was incorrect, for water and all liquids are compressible 
 to a small extent. 
 
 That pores exist in liquids may also be proved by the fact that 
 they are compressible, and that they contract under the action of 
 cold. Another and very simple proof is the fact, that in a tumbler 
 full of water a large quantity of salt or sugar may be dissolved, with 
 little or no increase in the volume of the water. A still more satis- 
 factory proof is afforded by mixing together water and alcohol, or 
 water and sulphuric acid. These liquids when mixed together 
 shrink or contract in volume ; half a pint of water, for example, 
 mixed with half a pint of alcohol gives less than a pint of the mix- 
 ture. This contraction shows plainly that either one or both of the 
 liquids contained pores. 
 
 Gases contain pores, which are believed to be very much larger 
 than those of either solids or liquids, because of the great compressi- 
 bility of gases. A portion of air, for example, may easily be com- 
 pressed into i^th or yj^th part of the space it usually occupies, and 
 this shows us how very large the pores in ordinary air must be. 
 And just in the same way as we see water penetrating the pores of 
 brick or blotting-paper, so we may see one gas penetrating the pores 
 of another. The smoke which issues from a chimney spreads out 
 and diffuses itself into the pores of the air around. 
 
 We have said that one proof of the existence of pores in different 
 bodies is the fact that all bodies may be more or less compressed. 
 Some forms of matter are much more compressible than others, and 
 this seems to be due in a great measure to the fact that they contain 
 larger pores. As a general rule, solids and liquids are difficult to 
 compress, and even with very great pressures their volume is but 
 little altered. It is stated, for example, that the pressure of the 
 atmosphere, powerful as we know it to be, is only able to compress 
 22,001 gallons of water into about 22,000 gallons. That is to say, if 
 we were to take 22,000 gallons of water from the sea, and remove 
 the pressure of the air from it, the water would expand to about 
 22,001 gallons only. Gases are very compressible bodies, the least 
 increase of pressure on a gas decreases its volume. If we only 
 double the pressure on a certain quantity of gas, it is compressed 
 
ELASTICITY. 
 
 69 
 
 into half its original volume ; while if we increase the pressure a 
 hundred times, the volume of the gas will be only the one-hundredth 
 part of what it was at the commencement. 
 
 When a solid bar is bent (as in fig. 18 B), it is compressed on the 
 hollow side of the bend, and stretched or extended on the round 
 side. This can readily be proved 
 by measuring the length of the 
 two sides of the bar before it is A 
 bent, and then while it is bent. 
 It is very important for builders 
 and other workmen to remember 
 this condition of a bent bar, be- 
 cause we can readily see that the 
 molecules on the rounded side 
 are pulling at each other to get 
 
 apart, while those on the hollow 
 side are being forced together. 
 
 Fig 18. 
 
 If, therefore, a saw cut, or even a 
 
 nail hole, be made on the round side of the bar, it is very much 
 weakened, while such a cut would make scarcely any difference in 
 the strength of the bar, if made on the hollow side where the mole- 
 cules were being forced together. 
 
 EXERCISES. 
 
 1. When a piece of cork or chalk is put into water, a number of little bubbles 
 
 of air escape from those bodies. Explain this. 
 
 2. Unglazed porous earthenware bottles are frequently used to keep water 
 
 cool in warm weather. Explain how the water is cooled. 
 
 3. Rocks are sometimes split by driving wedges into cracks or grooves cut in 
 
 the rock, and then wetting the wedges with water. Explain this fact. 
 
 4. Why is a bag of feathers so much lighter than the same bag full of flour ? 
 
 5. Why does a bubble of air, when passing up the tube of a barometer 
 
 through the mercury, increase its size as it ascends ? 
 
 LESSON 30. 
 
 ELASTICITY. 
 
 Many bodies when they have been compressed return to their ori- 
 ginal size and shape, when the force which compressed them is 
 removed. A ball of solid india-rubber, or a hollow india-rubber 
 ball filled with air, are examples of such bodies. If we squeeze them 
 in our hands so as to alter their size and shape, they spring back 
 into their original size and shape immediately we open our hands 
 a^ain. The force which enables them to return to their original 
 condition is called elasticity, and bodies which possess this force 
 
70 ELASTICITY. 
 
 are called elastic bodies. Elasticity, however, does something more 
 than this ; it does not begin to act when we remove our hand or 
 whatever may be pressing on the body, but it is acting all the time 
 while the pressure is being applied, and endeavouring to prevent 
 any alteration taking place in the body. We may, therefore, say 
 that elasticity is a force which resists any change in the size 
 or shape of a body. If the body, after its shape or size has been 
 altered, returns exactly to its original shape or size, it is said to be 
 perfectly elastic. If its elasticity is not sufficient to bring it back 
 exactly to its former state, it is said to be imperfectly elastic ; 
 while if it has no elasticity at all, it is termed an inelastic body. 
 Good steel is an example of a perfectly elastic body, provided the 
 force acting on it is not very great ; wood and copper are examples 
 of imperfectly elastic bodies, while clay is a good example of an 
 inelastic body. It is very probable that no bodies are quite inelas- 
 tic, nor any bodies perfectly elastic, if we examine them very care- 
 fully. Practically, however, all bodies are perfectly elastic within 
 certain limits ; that is to say, all bodies will return exactly to their 
 original size and shape, provided the force which altered that size 
 or shape be not too great. This is a most important point to 
 remember. When an engineer builds an iron bridge, every train or 
 cart which passes over the bridge bends it a little ; but the engineer 
 makes his bridge so strong that, as soon as the train or cart has 
 passed over, the bridge springs back into its original position. That 
 is to say, the engineer plans his bridge so that the heaviest weight 
 which it will have to bear will never bend it beyond the " limits of 
 perfect elasticity." Of course if the elasticity of the bridge did not 
 restore it to its former position, it would grow weaker and weaker 
 each time a load passed over it. 
 
 We can test the elasticity of solid bodies in several ways ; for 
 example, we may either stretch them, or compress them, or bend 
 them, or twist them. Liquids and gases are very elastic bodies, but 
 we can only test their elasticity in one way, viz., by compression. 
 It is found that when a liquid or gas has been compressed, it 
 returns to its original size immediately the pressure is removed ; 
 they are, therefore, as far as we can ascertain, perfectly elastic 
 substances. 
 
 If we take several balls, say of clay, lead, india-rubber, glass, and 
 ivory, and let them fall on to a hard smooth surface, such as a piece 
 of polished marble, we may notice a difference in the behaviour of 
 the different balls. The lead and the clay would remain where they 
 fell, and on taking them up we should find that the portion which 
 struck the marble slab was permanently flattened. The india- 
 rubber, glass, and ivory balls would rebound or " bounce," and on 
 examining them we should find they were exactly round as at first. 
 Now if we smear the surface of these three balls with oil or other 
 substance before letting them fall, we shall see clearly by the mark 
 they will leave on the slab that they also flattened when they fell. 
 The difference, therefore, between the balls which rebound and 
 
TENACITY, DUCTILITY, AND MALLEABILITY. 71 
 
 those which do not is simply this, the former return to their original 
 shape after being flattened by the fall, the latter remain flattened. 
 In other words, india-rubber, glass, and ivory are elastic substances, 
 while lead and clay are inelastic. It is the rounding of the elastic 
 balls after they had been flattened which caused them to rebound. 
 We have all seen water rebound, or splash, from the pools in the 
 streets on a rainy day, and also from a wall or a window ; this, 
 amongst other things, proves that water is elastic. It is the elasticity 
 of air and other gases which causes them to expand when we remove 
 the pressure of the air from around them ; for example, when we 
 enclose gas in a partly-filled bladder and place the bladder under 
 the receiver of an air pump. It is the elasticity of the steam which, 
 does the work in the steam-engine. All watches and many clocks 
 are kept going by the elasticity of a steel spring, which is wound up 
 from time to time, and which, as it unwinds, turns round the wheels 
 of the watch or clock. The elasticity of steel is one of its most 
 valuable properties, as it is also in the case of the substance we call 
 india-rubber. 
 
 EXERCISES. 
 
 1. Explain the use of steel springs on carts. 
 
 2. If you stop up the hole at the end of a syringe, you are able to force down. 
 
 the piston to a considerable distance, but directly you remove your 
 hand the piston is forced back again* Explain this. 
 
 3. When an india-rubber ball is thrown straight at a smooth wall or floor, in 
 
 which direction does it rebound ? In which direction does it rebound 
 when thrown at the wall or floor in a slanting direction? 
 
 4. If you force a soft cork into the neck of a bottle, what causes the cork to 
 
 hold itself tightly in the neck ? 
 
 5. Piecea of india-rubber are often fastened to doors to shut them after they 
 
 have been opened ; how are they arranged? and how do they act? 
 
 6. Elastic springs are placed in the " buffers " of railway carriages; explain 
 
 the use of the " buffers," and of the springs. 
 
 LESSON 31. 
 
 TENACITY, DUCTILITY, AND MALLEABILITY, 
 
 In this lesson we are about to consider some properties which 
 belong only to solid bodies, and are found in various solids in very 
 different degrees. One of these properties called tenacity, is the 
 resistance which a body offers to any force which tends to 
 pull its particles asunder. If we take a bar or wire of any mate- 
 rial, iron for example, and hang a weight from the lower end of it, 
 we can imagine that the tendency of the weight will be to pull 
 

 TENACITY, DUCTILITY, AND MALLEABILITY. 
 
 asunder the molecules of the bar or wire. However small the weight 
 may be, there will be some effect produced, though it may be too 
 little for us to determine, but as we increase the weight the effect 
 will become more and more apparent. In the first place, the wire 
 or bar stretches more or less, and we can easily measure the amount 
 of elongation produced by different weights. Up to a certain weight 
 the wire or bar returns to its original size when the weight is re- 
 moved, that is to say, it is perfectly elastic. But as we increase the 
 weight we come at length to the " limits of elasticity," and if we 
 now put on more weights we produce a permanent effect, for on 
 removing the force we find the bar remains elongated, and has 
 become proportionally thinner, and therefore weaker. Finally, on 
 reaching a certain weight, the bar breaks, in other words the force 
 applied has been sufficient to overcome the cohesion at some point 
 in the bar. This " breaking weight," as it is called, is different of 
 course for wires or bars of different thicknesses, and is also found by 
 experiment to vary in bars or wires of the same thickness, but of 
 different materials. The name tenacity is given to the resistance 
 which has had to be overcome, in order to break the bar or wire in 
 this manner. Tenacity is therefore only another name for cohesion 
 acting in this particular direction, and whatever lessens the cohesion 
 lessens the tenacity. If the bar or wire be heated we have already 
 learned that its cohesion is lessened, and experiment proves that the 
 tenacity is also less. Of all substances familiarly employed steel 
 has the greatest tenacity, and it is stated that a wire of steel of any 
 thickness will support about 7 J miles of similar wire without break- 
 ing. Steel wire is very much used for wire ropes in mines, bridges, 
 &c., on account of its great tenacity. Next to steel, but very much 
 inferior to it, comes ordinary iron ; and with less tenacity still, cop- 
 per, gold, and lead come in the order named. A rod of ordinary 
 pine wood, one inch square, cut in the direction of the fibres (or 
 " with the grain," as a carpenter would say), will bear a weight of 
 about four tons. 
 
 Wires are made from a piece of metal, by a process of drawing the 
 metal through holes in plates. The wire is drawn through smaller 
 and smaller holes until it is reduced to the thickness required. 
 Some metals cannot be drawn into very thin wires, because they 
 readily break, while others can be drawn out into exceedingly thin 
 wires. The property which enables bodies to be drawn out 
 into wire is called ductility, and those bodies are said to be the 
 most ductile which can be drawn out into the thinnest wire without 
 breaking. Gold is by far the most ductile substance known, and 
 admits of being drawn out into threads of extreme fineness. Silver 
 is also very ductile ; and iron, copper, and lead stand in the order 
 named. The ease with which any particular metal may be drawn 
 out depends upon the softness of the metal, but the extent to which 
 it may be drawn depends altogether on the tenacity. Some sub- 
 stances, such as glass, are ductile only when heated. 
 
 Malleability is the property which enables a body to be 
 
MEASUREMENT OF TIME. 73 
 
 hammered or rolled out into a sheet without breaking. The 
 
 ease with which a body may be beaten out depends upon its soft- 
 ness, but the extent depends a great deal upon the tenacity of the 
 substance. Iron stands very low indeed in the scale of malleability, 
 although the kind of iron termed " wrought " or " malleable " iron 
 may be readily fashioned under the hammer into thin plates while 
 hot. Lead is more malleable than iron, and more easily hammered 
 out, but its tenacity is so small that the sheets begin to break before 
 they become thin. Tin is more malleable than either iron and lead, 
 and is rolled out into thin sheets called tin foil. Copper is also very 
 malleable, while an alloy of copper and zinc (that is, a kind of brass), 
 known as Dutch metal, is so malleable that it is rolled and beaten 
 out into very thin sheets somewhat resembling gold leaf. But gold 
 is by far the most malleable of all metals, and in fact of all sub- 
 stances. Gold leaf is a substance which is very well known, and is 
 used for gilding and other purposes. The thinnest is made from 
 very pure gold, and it is beaten out so thin that it is said more than 
 250,000 leaves would have to be laid on each other to make a pile 
 one inch in height. Gold leaf is so thin as to be almost transparent ; 
 when held between the eye and the light it appears green. The 
 gold is first rolled out into a thin ribbon between rollers, and is then 
 cut up into pieces, which are placed between sheets of prepared calf- 
 skin (called goldbeaters' skin), and beaten with a very heavy hammer. 
 Each sheet of gold is again cut into four and placed between fresh 
 sheets of skin to be beaten, and this process is continued until the 
 leaves are as thin as required. 
 
 EXERCISES. 
 
 1. Mention some purposes for which gold leaf is used. 
 
 2. What differences do you know between wrought iron and cast iron? 
 
 3. Explain why a piece of iron can be hammered out while hot and not 
 
 while cold. 
 
 4. Describe one method by which you could ascertain whether brass or 
 
 copper has the greater tenacity. 
 
 LESSON 32. 
 
 MEASUREMENT OF TIME. 
 
 It is extremely important that we should know something about 
 the measurement of time. This lesson and the succeeding one will 
 be devoted to the consideration of the methods and instruments 
 employed for the purpose of measuring time. But first let us try to 
 understand what is meant by time. 
 
 Two events must either happen together, or one must happen 
 before the other. In the first case we say the events take place at 
 the same time, in the latter case we say they occur at different times. 
 
74 MEASUREMENT OF TIME. 
 
 When one event takes place after another, we Bay that a certain 
 interval or space of time has elapsed between the two events. Let 
 us suppose that a long strip of paper was slowly and regularly moved 
 across the table in front of us, and that the instant we heard the 
 report of a gun we made a mark on the portion of the paper which 
 was then before us. Let us further suppose that at the report of a 
 second and third gun we similarly marked the paper. Then the 
 intervals or spaces on the paper between these three marks would 
 represent the intervals of time which elapsed between the several 
 reports. We give this illustration to show that it is possible to 
 measure time, just as it is possible to measure length and to mea- 
 sure weight. We appear to picture time to ourselves as passing on 
 constantly and regularly like the strip of paper just described, and 
 hence we make use of the expressions space of time, and length of 
 time, and say that a certain time has passed. Again, we say that 
 such a thing happened a long time ago, while it is only a short time 
 since something else occurred. Expressions like these show that we 
 compare together different intervals of time in other words, that 
 we measure time. 
 
 In a former lesson we observed that, in order to perform exact 
 measurement, it was necessary to select a standard. And from 
 what was there stated we know that in the present case the standard 
 must be some portion of time, and must be as definite and as 
 unchangeable as we can possibly get it. It seems only natural that 
 the first standard selected should be the day and the night, and that 
 people should speak of a certain event happening so many days and 
 nights ago. In such case the day would be fixed as the period of 
 light, and the night as the period of darkness. Or the day might be 
 fixed more precisely as the interval between the rising and the 
 Betting of the sun, and the night as the interval between the setting 
 and the rising of the sun. But it would very soon be found to be a 
 much simpler mode of reckoning, to call the period of time which 
 passes between one rising of the sun and the next rising a day, that 
 is, to let the day, considered as a period of time, include a period of 
 li^ht and a period of darkness. We know that the rising and setting 
 of the sun are caused by the rotation of the earth on its axis, and 
 that the interval between one sunrise and the next is the time 
 required for the earth to turn round once. We may say, therefore, 
 that our standard of time is the period of rotation of the earth. 
 
 Every one knows that in spring the sun rises a little earlier 
 each morning than it did on the previous morning, whereas in 
 autumn the time of its rising gradually becomes later. It fol- 
 lows that if we measured the length of our days from sunrise 
 to sunrise, they would be too short in spring and too long in 
 autumn. For this reason, and for several other reasons, it has 
 been found best to measure the length of the day from noon to 
 noon. Of course we must have some means of ascertaining when it 
 is exactly noon, and the astronomer tells us that noon is the instant 
 when the sun is exactly in the south. The astronomer, by means 
 
MEASUREMENT OF TIME. 75 
 
 of his telescopes and other instruments, can determine the exact 
 moment when the sun is in the south, and by means of his observa- 
 tions our time, as shown by clocks and watches, is regulated. These 
 observations are made every day when the sun can be seen at the 
 principal observatory in England, viz., the one at Greenwich. The 
 sun as seen from Liverpool would not appear in the south at the 
 same time as when seen from Greenwich, and noon at Liverpool is 
 really about twelve minutes later than noon at Greenwich. But as 
 there are not observatories in every town to determine when it is 
 noon there, Greenwich time is used all over England. 
 
 For convenience, we subdivide the day into hours, minutes, and 
 seconds, and arrange instruments to mark these intervals for us. 
 These instruments we shall consider in the next lesson. 
 
 It would obviously be as inconvenient for a person to say that he 
 had lived 10,401 days, as to say that from Liverpool to Manchester 
 was 54,560 yards. We need larger divisions of time than days so 
 that we may conveniently speak of long periods of time, just as we 
 need sovereigns and shillings so that we may conveniently carry 
 large sums of money. The great natural divisions of time are the 
 month and the year, the former being governed by the moon, the 
 latter by the sun. 
 
 Every one has observed the changes which take place in the 
 appearance of the moon, how we see it sometimes as a mere curved 
 line or crescent of light, how this gradually increases in size until 
 there is a complete disc which we call the full moon, and how from 
 this full disc the size diminishes until we have once more nothing 
 but a thin crescent of light. Ultimately this crescent disappears and 
 the moon is entirely invisible, its dark face being turned towards us. 
 Then the thin crescent appears again, and the changes occur in the 
 same order as before. When the dark side of the moon is turned 
 towards us, it is said to be the period of the new moon. From one 
 new moon to the next new moon is about 29-^ days, and as the inter- 
 vals between successive new moons are very regular, and well marked, 
 we need not be surprised to find that from very early ages time has 
 been measured by these periods. The period of time we call a lunar 
 month comprises only 28, instead of 29 J, days ; but the name 
 month itself indicates a period of time measured by the moon. The 
 division of the year into twelve unequal calendar months is in some 
 respects inconvenient, and no one appears to be able to give any 
 satisfactory reason why the year should be so divided. 
 
 Just as the month is regulated by the revolution of the moon 
 round the earth, and its consequent changes of appearance to us, so 
 the year is regulated by the revolution of the earth round the sun. 
 We cannot go very fully here into the principle of the determination 
 of the length of the year. It will be sufficient, perhaps, for us to say 
 that twice in each year on all parts of the earth's surface the day and 
 night are each 12 hours long, the sun rising at six o'clock in the 
 morning and setting at six o'clock in the evening. One of these 
 occasions is in the spring, about March 21st, the other is in autumn, 
 
76 INSTRUMENTS FOR MEASURING TIME. 
 
 about September 21st, and each period is called an equinox. Tlie 
 period of time between one spring equinox and the next spring equi- 
 nox is called a year. Strictly speaking, the equinox is not a day, 
 but a particular instant of time, which an astronomer can easily 
 ascertain. And the interval between any two successive spring 
 equinoxes has been found to be about 365 days 5| hours. Our ordi- 
 nary years contain only 365 days each, so that there remains 5J 
 hours over, and the equinox becomes later each year by that 
 amount. This change would make little difference for a few years, 
 but a little thought will show that in 100 years spring would com- 
 mence on April 14th instead of March 21st, and in the course of 
 time Christinas Day would come to be in the middle of summer. 
 We take no account of this loss of time for three years, until we are 
 altogether three-quarters of a day too fast, but we put ourselves 
 right again in the fourth year by making it consist of 366 days. 
 These long years are called leap years, and it is a general rule that 
 every year, the date of which is divisible by 4 without a 
 remainder, is a leap year. There is, however, still an error, for 
 in four years we add 24 hours, which is too much. To correct this 
 it has been decided to omit three days out of every 400 years, that is, 
 three years which would otherwise be leap years are to have only 
 365 days each. Every year, the date of which is divisible by 100 
 but not by 400, is to consist of only 365 days. Thus, by this rule, 
 the years 1700, 1800, and 1900 are not leap years, though divisible 
 by 4, but 2000 A.D. will be a leap year because divisible by 400. 
 
 EXERCISES. 
 
 1. Which of the following years will be leap years : 1890, 1896, 1910, 1960, 
 
 2100, 2400? 
 
 2. If Christmas Day fall on a Monday in one year, what day of the week will 
 
 it fall on in the succeeding year, (a) if the latter year be an ordinary 
 year, (6) if it be leap year ? 
 
 3. What names are given to the calendar months, and how many days are 
 
 there in each ? 
 
 4. What is meant by the first, second, third, and fourth quarters of the 
 
 moon respectively ? What is the interval between the first and second 
 quarter? 
 
 LESSON 33. 
 
 INSTRUMENTS FOR MEASURING TIME. 
 
 "We learned in the preceding lesson that time was naturally divided 
 into days by the rotation of the earth on its axis, and into years by 
 the revolution of the earth round the sun. A day, however, is not a 
 sufficiently small period to enable us to measure time with accuracy. 
 
INSTRUMENTS FOR MEASURING TIME. 
 
 It is necessary to agree on some mode of subdividing the day, 
 and then to obtain instruments fitted to measure these subdivisions. 
 The subdivisions employed are hours, minutes, and seconds ; a day 
 being divided into 24 equal parts called hours, an hour into 60 equal 
 minutes, and a minute into 60 equal seconds. 
 
 Suppose that in the middle of a field we set up a tall stick. 
 Whenever the sun shines on the stick a shadow will be cast on the 
 ground, and this shadow will necessarily always be on the opposite 
 side of the stick to that on which the sun is at the time. Thus, 
 when the sun is in the east the shadow will point to the west, when 
 the sun is in the south the shadow will point to the north, and so 
 on. If now we mark a circle on the ground round the stick, the 
 shadow will move round part of this circle, like the hand of a clock 
 moves round the dial. If we then mark on this circle the north 
 point, the shadow of the stick will fall on that point at noon each 
 day. Similarly the shadow will fall on the west point at six A.M., 
 and on the east point at six P.M. We have only to divide the spaces 
 between W. and N., and N. and E. respectively, into six eq s ua^ por- 
 tions, to have a sun-dial (fig. 19). 
 
 S 
 
 Fig. 19. 
 
 Every boy should make a small sun-dial for himself. This he 
 can easily do with a piece of board on which to draw his circle, and 
 an upright rod to cast a shadow. The only difficulty will be to 
 set the board with the figure XII. pointing directly to the north, but 
 this can easily be done if we have a clock to refer to, by arranging 
 the board so that the shadow falls on that figure at noon. 
 
 Sun-dials are sometimes arranged to stand on the ground, or more 
 frequently are placed on the top of a pillar, where the light of the 
 sun will be better able to reach them. Sometimes, again, they may 
 be seen attached to the walls of old buildings, and in these, as well 
 as in the others, it will be noticed that the rod is not upright, but is 
 inclined. There is a reason for this, but we cannot enter into it 
 here. It is very evident that sun-dials are only of use when the sun 
 is shining, and in positions where the sun can shine on them during 
 as great a part of the day as possible. Since clocks and watches 
 have been made to measure time so accurately, sun-dials have gra- 
 dually fallen out of use. They may be seen on many old buildings, 
 
78 INSTRUMENTS FOR MEASURING TIME. 
 
 and new ones are now often set up in parks and gardens, but only as 
 interesting ornaments. They are indeed interesting, as having been 
 the best and almost the only means people had of measuring time 
 until the invention of clocks and watches. 
 
 Most boys have read of the method of measuring time said to 
 have been used by King Alfred the Great, viz., by means of burning 
 candles. Candles would have one advantage over sun-dials, in 
 that they could be used at any time either day or night. But the 
 expense, and trouble, and uncertainty connected with them, would 
 prevent them from ever being much employed. 
 
 Another instrument for measuring time, which most people have 
 seen, is the sand-glass. This consists of two vessels connected by 
 a narrow neck, and contains a quantity of sand, which takes up a 
 certain time in falling through the neck from one vessel to the other. 
 It is little used now for any purpose, except occasionally to measure 
 the time required for boiling an egg. But formerly this instrument 
 was very largely used, and made to measure an hour, whence it is 
 often called the hour-glass. Although it may be made to measure a 
 certain interval of time very accurately, still, from the fact that one 
 instrument will measure only the interval of time for which it was 
 constructed, and from the trouble of turning it so frequently, it has 
 passed out of general use. 
 
 The clock (or watch, for both are constructed on the same prin- 
 ciple) is the most perfect instrument which has yet been invented 
 for the purpose of measuring time. Its chief advantages are : that 
 it can be made to measure exceedingly small intervals seconds, for 
 example with the greatest accuracy ; that it is independent of the 
 sun ; that it will continue to work for an indefinite period without 
 any further trouble than an occasional winding up ; and that it can 
 be made not only to measure but to record the time. To understand 
 the principle upon which the clock works, we must in the first place 
 direct our attention to the pendulum. If we tie an iron weight or a 
 stone to a piece of string and hold the other end of the string in our 
 hand, we shall have a pendulum with which we can experiment. 
 On setting the pendulum swinging, we may observe that the time 
 taken up by each swing of the pendulum is the same. But if we 
 shorten the string we shall find that the pendulum will swing 
 more quickly, that is, it will swing more times in each minute 
 than before. A pendulum that is a little more than 39J inches 
 long will swing once a second, and <*ne about 10 inches long will 
 swing twice in a second. The pendulum, if left to itself, gradu- 
 ally, but very soon, comes to rest. In order to make use of the 
 pendulum for the purpose of measuring time, we must connect with 
 it an arrangement which shall both keep up its motion, and also 
 record the number of swings. The number of swings which the 
 pendulum makes is registered in a very ingenious, though simple, 
 manner. Connected with the upper part of the pendulum is a bar 
 having two small arms, or projecting pieces, called "pallets" (fig. 20). 
 These pallets are so arranged that one or other of them is always 
 
MEASUREMENT OF VELOCITY. 79 
 
 caught between the teeth of a wheel. When the pendulum swings 
 to one side, the pallet on that side is lifted out from between the 
 teeth, while the other pallet drops between two teeth on the opposite 
 side. As this takes place one tooth of the wheel escapes, and for 
 each swing of the pendulum the wheel 
 advances one tooth. The wheel is called 
 the " escapement wheel," and if it con- 
 tained 60 teeth, it would turn round once 
 for 60 swings of the pendulum. This 
 wheel is so arranged that as it turns it 
 moves other toothed wheels, and to some 
 of these the hands of the clock are at- 
 tached. The weight or spring is connected 
 with the escapement wheel, and is always 
 trying to turn it round. Hence every time 
 a tooth of the wheel touches a pallet, it 
 gives it a little push, which keeps up 
 the motion of the pendulum. In fig. 20, 
 the arrow shows the direction in which Fig. 20. 
 
 the escapement wheel is moved by the weight. 
 
 The principle of the watch is precisely the same as that of the 
 clock, except that instead of a pendulum there is a " balance-wheel," 
 which swings backwards and forwards. 
 
 EXERCISES. 
 
 1. A tree stands in a position where the sun can shine on it the whole day. 
 
 Describe the changes in size and position which the shadow undergoes 
 in the course of the day. 
 
 2. By what means could you ascertain which was the north, without reference 
 
 to the sun ? 
 
 3. Mention the chief disadvantages of candles as instruments for measuring 
 
 time. 
 
 4. An ordinary clock is observed to lose time when the weather becomes 
 
 warmer. Explain the cause of this. 
 
 5. Why does a clock stop when the weight is taken off, or a watch stop when 
 
 the mainspring is broken ? 
 
 LESSON 34. 
 
 MEASUREMENT OF VELOCITY. 
 
 Having considered in previous lessons the measurement of space 
 and time, we are now in a position to consider the measurement of 
 velocity. The measurement of velocity involves, as we shall see, 
 the measurement of both time and space. Let us first understand 
 exactly what is meant by velocity. If a body remains constantly in 
 
80 MEASUREMENT OF VELOCITY. 
 
 one position, we speak of it as being at rest ; whereas, if it changes 
 its position, we speak of it as in motion. A body in motion, there- 
 fore, moves from one position to another, that is to say, it moves 
 over a certain space, and to do this it will necessarily require a cer- 
 tain time. The rate at which a body changes its position is 
 called its velocity, and to measure the velocity of any body we 
 must take into consideration both the distance moved over and the 
 time occupied in that motion. Practically the velocity is expressed 
 by stating the distance moved over by a body in a unit of time, and 
 thus we speak of a velocity of 10 feet per second, meaning that the 
 body moves at such a rate that in one second it would pass over a 
 distance of 10 feet. Of course we need not always express the dis- 
 tance in feet ; indeed, in many cases, it would be very inconvenient 
 to do so. Thus, in the case of light, we say it has a velocity of nearly 
 190,000 miles per second. Again, we sometimes take one minute or 
 one hour as the unit of time, and speak of an express train having a 
 velocity of one mile per minute, or 60 miles per hour. But whatever 
 may be the units we employ, whether feet or miles, seconds or hours, 
 or any others, we always include two quantities in our statement of 
 any velocity, viz., a portion of space and a portion of time. 
 
 Velocity is usually spoken of as being either uniform or variable. 
 By uniform velocity we mean that an equal space is passed over by 
 the body in each succeeding second, or any other portion of time. 
 In other words, a body is said to have a uniform velocity when in 
 10 seconds it passes over ten times the space that it does in one 
 second, in one-tenth of a second it passes over one-tenth of that 
 space, and so on in like proportion. It follows that in the case of 
 a body in uniform motion, if we know the distance traversed by it 
 in one second, we can find the distance passed over by the body in 
 any other given time by simple multiplication. 
 
 By variable velocity we mean velocity which changes from instant 
 to instant, either becoming greater or less. Examples of variable 
 velocity are seen in the case of a boy sliding : a train gradually 
 starting or stopping : a stone thrown upwards or falling. It is plain 
 that if we know the velocity of such a body for one second, we can- 
 not calculate its velocity for any other time by simple multiplication. 
 In the remainder of this lesson we shall occupy ourselves only with 
 uniform velocity. 
 
 Now let us consider a special case in which experiments were 
 made to ascertain the velocity of sound. For this purpose two hills 
 were selected several miles distant from each other, and the exact 
 distance between the hills was determined. A cannon placed on 
 one hill was fired, while persons on the opposite hill observed the 
 time which passed between their seeing the flash and hearing the 
 noise of the discharge. In this way they ascertained the time 
 required for sound to travel over the distance between the two hills. 
 And as we may consider that sound travels with a uniform velocity, 
 it was easy for these experimenters to calculate how far sound 
 travelled in one second. Thus, suppose the distance between the 
 
MEASUREMENT OF VELOCITY. 81 
 
 hills was 9 miles, or 47,520 feet, and that the time occupied by the 
 sound in its journey from one hill to the other was about 42 J 
 seconds, then a simple calculation shows us that in one second the 
 sound had travelled 1120 feet. The velocity of sound is therefore said 
 to be 1120 feet per second. Remembering this number we are able, 
 during a thunderstorm, to find out the distance at which any flash 
 of lightning occurs. The sound of the thunder and the light of the 
 flash start at the same time ; but while the light reaches us instantly, 
 the sound travels with a velocity of only 1120 feet per second. If 
 we then count the number of seconds which pass between our seeing 
 the lightning and hearing the thunder, and multiply that number 
 by 1120, the answer will give the distance from us in feet at which 
 the lightning discharge occurred. 
 
 We will now consider another example of finding the velocity of a 
 body viz., the velocity of a cannon ball, or shot, immediately after 
 leaving the cannon. Here, again, arrangements are made to measure 
 a distance, and a period of time. A wire screen is set up near the 
 mouth of the cannon, and a second similar screen is set up at a dis- 
 tance of a hundred feet from the first, both being so placed that the 
 shot must tear its way through them in succession. By the aid of 
 electricity the time at which the shot tears through each screen can 
 be accurately noted, and therefore the interval of time between its 
 breaking through the two screens is known. Suppose, for example, 
 that it is found in a particular case that this interval is one-tenth of 
 a second, the velocity of the shot would be .expressed as 1000 feet 
 per second, or nearly 700 miles per hour. 
 
 What is meant by this last statement that the velocity of the shot 
 would be about 700 miles per hour? It is very certain that the 
 cannon ball would not travel 700 m^les, neither would it be moving 
 for an hour. The statement means, however, that were the ball to 
 continue moving for an hour with the same velocity that it had 
 between the screens, it would then travel 700 miles. In the same 
 way, a man may have only half a mile to walk, and yet we may say 
 of him that he walks at the rate of four miles an hour ; meaning that 
 were he to walk at the same rate for the space of an hour he would 
 traverse a distance of four miles. 
 
 EXERCISES. 
 
 1. Light moves with a velocity of 186,000 miles per second ; how long will it 
 
 require to pass to the earth from the sun, when the latter is at a 
 distance of ninety-three millions of miles from the earth? 
 
 2. How far are you from a ship at sea, when you can count seven seconds 
 
 between the time of seeing the flash and hearing the report from a 
 gun fired on the vessel ? 
 
 3. How would you express the velocity of a carrier pigeon which could fly 
 
 from Liverpool to London, a distance of 200 miles, in 4 hours, 26 
 minutes, 40 seconds ? 
 
 F 
 
82 MEASUREMENT OF VELOCITY. 
 
 4. Describe a simple method by which you could measure the velocity of a 
 
 current of water. 
 
 5. What would be the length of a tunnel, through which a train passed in 
 
 4 minutes, when moving with a velocity of 36 miles per hour? 
 
 6. A person at Liverpool is carried round by the earth in its rotation, through 
 
 a distance of about 15,000 miles per day. Express his velocity in miles 
 per hour, and state how far the person moves each second. 
 
EXAMINATION QUESTIONS. 
 
 1. What is meant by matter ? Mention two or more properties which belong 
 
 to all matter. 
 
 2. Name the five senses, and state in what portion of the body each is 
 
 situated. 
 
 3. What is meant by saying that a body is transparent? Name several 
 
 transparent bodies. 
 
 4. Describe the properties of iron, and explain the terms you use. 
 
 5. Compare the properties of glass with those of coal. 
 
 6. In what three states or conditions is matter found? Give several 
 
 examples of each. 
 
 7. Mention a substance which is found to take at different times the con- 
 
 dition of a solid, a liquid, and a gas. What causes it to change from 
 one condition to another? 
 
 8. Describe the general properties of solid bodies. 
 
 9. Explain why it is easy to put your finger into a liquid, while it is diffi- 
 
 cult, or impossible, to force it into a solid body. 
 
 10. What is meant by the divisibility of a body? In what way might a 
 
 piece of salt be divided into exceedingly minute portions? What 
 name is given to the smallest portions of salt that could possibly be 
 obtained ? 
 
 11. What is meant by crystallisation ? Mention some crystallised substances. 
 
 For what purpose are they crystallised ? 
 
 12. What is the hardest known solid ? For what purposes is it employed on 
 
 account of its hardness ? 
 
 13. How is the gold which is used for coinage hardened ? Give any other 
 
 instance you know of other metals which are hardened by a similar 
 process. 
 
 14. Explain the difference between the melting and the dissolving of a solid. 
 
 Name some solids which are difficult to melt. 
 
 15. What changes occur in solid bodies when they are heated ? 
 
 16. Explain why it is necessary that there should be one fixed standard of 
 
 length in this country. 
 
 17. If one grain of gold can be drawn out into a wire 250 feet long, what 
 
 weight of gold will be required to produce 3 miles of such a wire ? 
 
84 EXAMINATION QUESTIONS. 
 
 18. How would you express the volume of a solid body which measures 19 
 
 feet long, 2 feet wide, and 8 inches deep ? 
 
 19. Find the weight of the air in a room which is 30 feet long, 18 feet 
 
 broad, and 12 feet high, the weight of 100 cubic inches of air being 
 31 grains. 
 
 20. Find the total cost of painting the four walls of a room which is 30 feet 
 
 long, 15 feet broad, and 15 feet high, at ld. per square foot. 
 
 21. What would be the cost of making an excavation 50 feet long, 30 feet 
 
 broad, and 15 feet deep, at 9d. per cubic yard ? 
 
 22. A sheet of iron, measuring 5 feet 6 inches long by 3 feet 4 inches broad, 
 
 weighs 4 cwt. ; find the weight of one square inch. 
 
 23. Describe the properties of a liquid. 
 
 24. What properties of liquids are not possessed by solids ? 
 
 25. Compare the properties of water with those of ice. 
 
 26. Explain clearly what is meant by the statement that a solid has a defi- 
 
 nite, and a liquid an indefinite, shape. 
 
 27. What is cohesion ? On what parts of a body does it act ? Give some 
 
 proof that cohesion acts in liquids. 
 
 28. What do you know of the transmission of pressure by liquids ? 
 
 29. Why are reservoirs of water placed in elevated positions ? 
 
 30. Write what you know of the force which causes the oil to rise in the 
 
 wick of a lamp. 
 
 31. When mercury is placed in a glass tube, what is -the form of the surface 
 
 of the mercury ? What would be the form of the surface of water in 
 the same tube? Explain as far as you can the difference in the two 
 cases. 
 
 32. When a quantity of liquid is placed in a vessel, what kind of surface 
 
 does it assume ? If a drop of the same liquid be placed on a flat sur- 
 face, which it does not readily moisten, what form will it take ? 
 
 33. Explain what is meant by the common expression that " water finds its 
 
 own level," and give some illustrations of the fact. 
 
 34. Write all you know of the pressure of liquids. 
 
 35. Why do the walls of an embankment or reservoir increase in thickness 
 
 towards the bottom ? 
 
 36. Describe a simple experiment to prove that the pressure of a liquid on 
 
 the sides of a vessel increases with the depth. 
 
 37. How would you proceed to prove that liquids exert an upward pressure 
 
 on bodies which are immersed in them ? 
 
 38. Explain the meaning of the term "specific gravity." How may the 
 
 specific gravity of a liquid be ascertained ? 
 
 39. What height would a column of water need to be, in order to balance a 
 
 column of mercury 1^ inches high ? 
 
 40. What is meant by the saying that "gold is nineteen times heavier than 
 
 water " ? 
 
EXAMINATION QUESTIONS. 85 
 
 41. Explain why some bodies float in water and others do not. What is it 
 
 which keeps the bodies afloat ? 
 
 42. Why does a bar of iron sink in water, while the same bar made into the 
 
 form of a boat floats ? 
 
 43. Explain clearly why a ship floats in water. Why does she sink lower in 
 
 the water when the cargo is taken on board ? 
 
 44. A cubic foot of water weighs 62^ Ibs., and the density of mercury 
 
 is 13| times that of water. Find the weight of one cubic inch of 
 mercury. 
 
 45. If several liquids which do not mix with one another are contained in 
 
 the same vessel, in what order will they arrange themselves? 
 
 46. Describe the changes which take place in a liquid as it is heated. 
 
 47. What is a thermometer ? Describe its construction. 
 
 48. Why is mercury preferred for ordinary thermometers? Under what 
 
 circumstances is mercury unsuitable ? 
 
 49. If you dip your hand in warm water, and then hold it a short time in 
 
 the air, your hand feels cold. Explain this. 
 
 50. What is the temperature of boiling water ] Have all liquids the same 
 
 temperature when they boil ? 
 
 51. What is a vapour ? how are vapours formed ? how do they change when 
 
 acted on by cold ? 
 
 52. In what respects do liquids and gases agree, and in what respects do 
 
 they differ? 
 
 53. Name three solids and three fluids. What are the differences between 
 
 solids and fluids ? 
 
 54. How may gases be converted into liquids? Name several gases which it 
 
 is difficult to liquefy. 
 
 55. If I take a thin india-rubber ball filled with gas, and press on one part of 
 
 it, the rest of the ball becomes more tightly stretched than before. 
 Explain this, and state what property of gases it illustrates. 
 
 56. Describe some simple experiment to prove that the atmosphere exerts 
 
 pressure on bodies. 
 
 57. What is the composition of the air ? What pressure would the air exert 
 
 on the whole surface of the water in a square vessel, the side of which 
 measured six inches ? 
 
 58. Why is a piece of paper not pressed hard against the table by the weight 
 
 of the air above it ? 
 
 59. What is the usual height of the column of mercury in a barometer ? 
 
 How does this height usually vary with the weather ? 
 
 60. What is a vacuum? If some air made its way into the vacuum of a 
 
 barometer, what effect would it have on the column of mercury ? 
 
 61. State all the differences you know between a barometer containing 
 
 mercury, and one containing water. Why is mercury usually pre- 
 ferred? 
 
86 EXAMINATION QUESTIONS. 
 
 62. If a barometer were carried up in a balloon, how would the column of 
 
 mercury be affected ? Give reasons for your answer. 
 
 63. Describe the common bellows, and explain its action. 
 
 64. Describe the " sucker," and the manner in which it is made to adhere to 
 
 a surface. Why does a sucker stick better on smooth than on rough 
 surfaces ? 
 
 65. "What conditions are necessary in order that a liquid may be drawn off 
 
 from one vessel into another by means of a siphon? 
 
 66. Describe the common water pump and explain its action. 
 
 67. "Why will not a pump work if the distance between the surface of water 
 
 in the well and the lower valve be greater than about 30 feet ? 
 
 68. What is a piston? What is a valve? Mention some instruments in 
 
 which those parts occur. 
 
 69. If an open flask be heated, and the mouth of it then plunged downwards 
 
 below the surface of some water, describe and explain what occurs as 
 the air cools. 
 
 70. What is wind ? Explain the roanner in which it is produced. 
 
 71. With what substances are balloons filled ? Explain why a balloon rises 
 
 in the air. 
 
 72. Explain why a closed bladder, which is only half full of air, swells out 
 
 when laid for some time near a fire. 
 
 73. In what manner is pressure transmitted by a solid, a liquid, and a gas 
 
 respectively ? 
 
 74. If a piece of ice were placed in a vessel over a fire, describe as fully as 
 
 you can the changes which would occur before the ice was fully con- 
 verted into steam. 
 
 75. What general effect has heat on bodies ? Give an example in the case of 
 
 a liquid, a solid, and a gas respectively. 
 
 76. Explain what occurs as a substance passes from the solid to the liquid 
 
 condition, and from the liquid to the solid condition. 
 
 77. Explain the meaning of the following terms : Porous, compressible, 
 
 ductile, malleable, opaque, and invisible. In each case mention one 
 or more bodies having that character. 
 
 78. Why is a gas more easily compressed than either a solid or a liquid? 
 
 What is meant by " compressibility "? 
 
 79. Explain clearly what occurs when a ball of solid india-rubber is dropped 
 
 on to a hard smooth floor. 
 
 80. What is elasticity ? Mention some bodies which are valuable on account 
 
 of possessing this property in a high degree. 
 
 81. Describe an experiment to prove that air is elastic. 
 
 82. What is tenacity? how is it measured? Name a substance which has 
 
 great tenacity. 
 
 83. Which is the most malleable metal ? What use is made of the metal on 
 
 account of that property ? 
 
EXAMINATION QUESTIONS. 87 
 
 84. Wrought iron when hot may be rolled out into sheets ; what property 
 
 does that prove it to possess ? By what other processes might a lump 
 of iron be made into the form of a sheet ? 
 
 85. Lead is neither very malleable nor very ductile. Explain the meaning 
 
 of this statement. 
 
 86. Explain with illustrations what is meant by cohesion, ductility, and 
 
 brittleness. 
 
 87. How is time divided into years, months, and days ? 
 
 88. Name the different contrivances that have been used for measuring time, 
 
 and describe one of them. 
 
 89. Describe the sun-dial. "Why is it not more commonly used for measuring 
 
 time? 
 
 90. Explain why an ordinary pendulum clock loses time in summer and gains 
 
 in winter. 
 
 91. Why do we speak of " Greenwich time "? 
 
 92. Explain the difference between a "lunar" month and a "calendar" 
 
 month. How many lunar months are there in a year ? 
 
 93. What is meant by a " leap year"? State the rule for finding what years 
 
 are leap years. 
 
 94. What is meant by velocity ? How is it measured ? 
 
 95. If a horse travels 9 miles in 57 minutes, what is its velocity expressed in 
 
 feet per second? 
 
 96. Explain how the velocity of a flying bird might be ascertained. 
 
 97. If a cannon ball moves 5 miles in 2 minutes, what is its velocity in miles 
 
 per hour ? 
 
 98. Two horses start from the same place in the same direction at the same 
 
 time ; one travels at the rate of 20 miles per hour, the other at the 
 rate of 12 miles per hour. What distance will separate them 10 
 minutes after starting ? 
 
 99. A train runs 24 miles in 32 minutes, how long will it take to go 40 miles ? 
 
 What is its velocity in miles per hour ? 
 
 100. In what space of time would a balloon be carried 5 miles by a wind 
 moving with a velocity of 14 miles per hour? 
 
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 LIST OF THE MAPS. 
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 THE WORLD IN HEMI- 
 SPHERES. 
 EUROPE. 
 ASIA. 
 AFRICA. 
 
 NORTH AMERICA. 
 SOUTH AMERICA. 
 
 AUSTRALIA. 
 
 NEW SOUTH WALES. 
 
 ENGLAND AND WALES. 
 
 SCOTLAND. 
 
 IRELAND. 
 
 PALESTINE. 
 
 INDIA. 
 
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 THE WORLD ON MERCA- 
 
 TOR'S PROJECTION. 
 THE BRITISH ISLANDS. 
 
 NEW ZEALAND, by Dr. 
 Hector and Thos. A. Bow- 
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 OCEANIA, on a Scale of two 
 degrees to an inch, shewing 
 the situation of New Zealand 
 and the Australian Colonies 
 relatively to the shores of 
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 with the intervening Islands. 
 
 Supplementary Maps to the Series. 
 
 SCHOOL WALL MAP OF THE WORLD, ON GALL'S 
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 LIST Oy TUB MAPS. 
 
 Eastern Hemisphere. Australia. 
 
 Western Hemisphere. 
 
 Europe, 
 
 Asia. 
 
 Africa. 
 
 North America. 
 
 South America. 
 
 New Zealand. 
 
 England and Wales. 
 
 Scotland. 
 
 Ireland. 
 
 Palestine. 
 
 Wanderings of the Israelites. 
 
 The above are reductions of the large series, constructed by William Hughea, 
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 %* Other Counties in Preparation. 
 
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 22 
 
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