338 



THE POPULAR EDUCATOR. 



expands a little by its influence, and then, as the particles get 

 farther separated, it assumes the liquid state ; and finally, in 

 the case of many substances, the heat altogether overcomes the 

 cohesion, and the particles fly apart in the form of vapour. 

 When the source of heat is removed, and that already acquired 

 by the substance has been imparted to surrounding objects, 

 cohesion again comes into play, and the substance resumes the 

 liquid or solid state. 



Advantage is frequently taken of this property which the 

 metals possess of expanding and again contracting. Some years 

 ago the walls of a large building in Paris had bulged outwards 

 considerably so as to endanger the structure. A number of 

 iron rods were accordingly taken and passed through the 

 building from side to side, the ends passing outside through 

 large face-plates, and being secured by nuts screwed on to 

 them. When these were screwed up as far as possible, the 

 alternate rods were expanded by being heated, and then the 

 nuts could be screwed up further on them. As they cooled 

 the walls were drawn together to a slight extent, and the same 

 process was then repeated with the other rods ; and in this 

 way the walls were gradually brought to the perpendicular. 



For a similar reason the tire is always made hot before 

 being put on a wheel, and then as it cools it forces the dif- 

 ferent pieces more closely together, and renders the wheel much 

 stronger. So, too, in the manufacture of Armstrong guns, the 

 different coils are shrunk on ; and in making boilers the plates are 

 riveted together with hot rivets. The contraction of the metal 

 while cooling renders the joint in each case much more close 

 and tight than it would otherwise be. 



In large iron bridges, like that over the Menai Straits, or 

 some of those across the Thames, the heat of the sun's rays is 

 sufficient to curve and raise the bridge in the middle, producing 

 often a greater deflection than the heaviest load does. 



By reference to the table of expansions given above, it will be 

 seen that some metals expand more than others for a similar 

 increase of temperature. Hence, if thin bars of two different 

 metals as, for example, copper and iron be taken, and riveted 

 firmly together, and then exposed to an elevated temperature, 

 the copper will expand more than the iron, and the bar will 

 become curved, the iron being on the inner side. If, on the 

 other hand, it bo exposed to a lower temperature, the copper bar 

 will become the shorter, and thus that will be the inner one in 

 the curve. This fact is sometimes turned to account in the 

 manufacture of compensating pendulums. As has been ex- 

 plained, any increase in the length of a pendulum makes it 

 vibrate more slowly ; hence in hot weather a chronometer would 

 lose a little. To guard against this, different forms of com- 

 pensating pendulum have been tried. The most usual plan is 

 that known as the gridiron pendulum, which was explained in 

 our Lessons in Mechanics. Another plan is represented in Fig. 

 8, a, I, c. A compound bar of copper and iron, with balls at each 

 end, is fixed to the pendulum rod, the copper side of the bar 

 being underneath, as that metal is the more expansible. When 

 the temperature falls the pendulum rod contracts and raises the 

 bob ; the strip, however, curves downwards, as shown in the 

 middle figure, and thus the centre of gravity remains stationary. 

 If the temperature rises, the strip curves upwards, and thus the 

 balls at the end of it rise and compensate for the increase in 

 the length of the rod. A similar plan is adopted in the balance- 

 wheels of the best watches. 



Another application of the same principle is made in Bre- 

 guet's metallic thermometer (Fig. 9). A compound ribbon is 

 here twisted into a spiral, which is fixed to the stand at its upper 

 end, and carries a needle below. This spiral coils or uncoils as 

 the temperature changes, and the needle shows the readings on 

 the graduated disc. 



There is one more experiment which must be described here, 

 as it is a good illustration of expansion, and at the same time 

 illustrates the conversion of heat into motion. The apparatus 

 employed is known as the "rocker," or Trevelyan instrument, 

 from the name of the gentleman who first constructed it. He had 

 one day laid a hot soldering iron on a block of lead to cool, and 

 was surprised soon after by hearing a distinct sound given off 

 by the iron. On investigation he found that it was thrown into 

 rapid vibration, which caused the sound. The best form of 

 rocker for trying the experiment is represented in Fig. 10. A 

 piece of brass, A, is taken, about five inches long and an inch 

 and a half wide. Its section is almost triangular, but a small 



groove is made along the apex, c, and a piece of wire terminating- 

 in a knob, B, is fixed in one end. Let the rocker now bo raised 

 to a high temperature, and then placed so that the knob, B, may 

 rest upon a table, while the grooved edge of brass lies upon a 

 block of lead. A succession of quick taps will be heard, and 

 the rocker will be found to be in rapid vibration. By increasing- 

 the width of the groove the vibi-ations may be rendered more- 

 and more rapid until a distinct musical note is obtained. 



The explanation of this is easily given. When the rocker is. 

 laid on the block a portion of one edge of it comes in actual con- 

 tact with the lead. This metal, being very expansible, imme- 

 diately throws out a small protuberance, and thus tilts the rocker, 

 which therefore rests upon a fresh portion. This immediately 

 expands in like manner, and in this way it is kept in rapid vibra- 

 tion, and produces the sound which is heard. The heat which, 

 the rocker possessed becomes slowly lost, being employed in im- 

 parting motion to the brass, and this motion becomes in turn 

 communicated to the air, manifesting itself in the form of sound. 



Thus far we have been concerned with the expansion of solids. 

 We have now to see how liquids expand under the influence of 

 heat, and in their case it is evidently the cubical and not the 

 linear expansion with which we have to deal. As, however, the 

 liquid must be contained in some vessel, and that vessel expands 

 as well as the liquid, we must distinguish between the apparent 

 and the real expansion of the liquid, the latter being the larger 

 of the two by just the amount that the vessel is increased in 

 capacity. Thus, let the liquid in the flask (Fig. 6) stand at the 

 level A, and when it is immersed in a jar of hot water let it rise 

 to the level B ; the apparent expansion is the quantity contained 

 in the tube between A and B. If, however, the flask had re- 

 tained exactly its original capacity, the liquid would have risen 

 higher in the stem, showing that the real expansion is greater. 



Liquids generally do not expand uniformly : the amount of 

 expansion between 50 and 60, for example, would not be 

 the same as that between 190 and 200. Mercury, however, 

 is an exception to this rule, as between 32 and 212 it 

 expands uniformly, and hence it is specially fitted for use in the 

 construction of thermometers. The following table shows the- 

 apparent expansion in glass of several liquids when raised from 

 32 to 212 : 



Mercury 

 Water 



Sulphuric Acid 

 Ether 



Olive Oil 

 Alcohol . 



The way in which the real expansion of mercury is ascer- 

 tained is by filling two vertical tubes with it, and making them_ 

 communicate by a small tube opening into their lower ends 

 (Fig. 11). One tube is now surrounded by a jacket containing: 

 boiling water, while the other is surrounded by melting ice. 

 The mercury in the hot one will stand at a higher level than 

 that in the other. This difference is measured by a telescope 

 properly adjusted, and shows the real expansion. 



There is an interesting experiment in connection with the 

 expansion of water which shows a departure from the usual 

 rule. Let a tall glass vessel be filled with water, with a small 

 thermometer at the bottom of it, and a second near the; 

 top. Now put the whole in a place where the temperature is. 

 below the freezing point ; both thermometers will fall, the lower 

 one, however, more rapidly than the other till it reaches about. 

 40, when it will become stationary. The upper one will con- 

 tinue to fall down to 32, and then the water will begin to> 

 freeze, and the vessel will probably be cracked. 



The explanation of this is found in the fact that at first the 

 cooler water from the top and sides, being more dense, sinks to 

 the bottom. When, however, water attains the temperature of 

 39'4, it has attained its maximum density, and then, instead of 

 continuing to contract, it expands slightly till it reaches the 

 freezing point, when it suddenly expands still further. Thus, in 

 the above experiment, the water at 39'4 was at its greatest den- 

 sity, and hence remained at the bottom. This provision is of 

 great importance to us, as, were it not for it, the coldest water 

 would sink to the bottoms of our seas and rivers till all attained 

 a temperature of 32, and they would then be slowly converted 

 into masses of solid ice, whereas now the colder water and ice 

 on the top protects that below. 



The great expansion of water on becoming converted into ice 

 is often so painfully manifested in the bursting of our water- 

 pipes and plugs during a frost, that it need not be illustrated 

 further. It is well, however, to guard against the common error 



