receiver (Experiment 25). When, therefore, we wish to see how much a quantity of gas expands through heat, we must take care that the air which surrounds the gas does not change its pressure: in other words, we must take a bladder with some air in it, and find how much it expands when heated in the open air— that is to say, under the constant pressure of the atmosphere-between the freezing and the boiling points of water.

When this is done, it is found that if a bladder not completely filled with air have a volume equal to 1,000 cubic inches at the freezing-point, its volume at the boiling-point will be 1,367 cubic inches. If therefore we have a large quantity of ice-cold water in a vessel and force this bladder containing 1,000 cubic inches beneath the water, we shall find the water rise in the vessel through a space denoting 1,000 cubic inches-this being the increase of volume due to the bladder. But if we take the same vessel, only filled with boiling water, and plunge the bladder into it, we shall find the water rise through a space denoting 1,367 cubic inches-this being the volume of the bladder at this temperature.

55. Remarks on Expansion.-Liquids and solids expand with immense force. If you were to fill an iron ball quite full of water, shut it tightly down by means of a screw, and then heat the ball; the force of the expansion would be great enough to burst the ball.

In large iron and tubular bridges allowance must be made so that the iron has room to expand; for in the middle of summer the bridge will be somewhat longer than in the middle of winter, and if it has not room

to lengthen out, it will be injured by the force tendïng to expand it. There is an arrangement for this purpose in the Menai Tubular Bridge.

We take advantage of the force of expansion and contraction in many ways-for instance, in making carriage wheels. The iron tire is first made red-hot, and in this state is fitted on loosely upon the wheel; it is then rapidly cooled, and in so doing it contracts, grasps the wheel firmly, and becomes quite tight.

56. Specific Heat.-Some bodies require a greater amount of heat than others in order to raise their temperature one degree. The quantity of heat required. to raise a pound weight of any substance one degree is called its specific heat. Water has a very great specific heat; that is to say, it requires more heat to raise a pound of water one degree than it does to raise almost any other substance. The heat required to raise a pound of water one degree will raise through one degree 9 lbs. of iron, 11 lbs. of zinc, and no less than 30 lbs. of mercury or gold.

EXPERIMENT 39.—To convince you of the great specific heat of water, let us take 2 lbs. weight of mercury and heat it to 100°, or the boiling-point of water, and let us then mix it with 1 lb. of water at an ordinary temperature. Now note the height of a thermometer placed in the water both before and after the mixture, and you will find that it has hardly risen more than 5° in consequence of the hot mercury being poured in.

57. Change of state. You have already heard about the three states of matter—the solid, the liquid, and the gaseous. I have now to tell you that substances when heated pass first from the solid to the liquid, and

then from the liquid to the gaseous state. You have already been told in the Introductory Primer that ice, water, and steam have precisely the same composition, and that ice becomes water if it be heated, while water becomes steam if we continue the heat. The very

same change will happen to other substances if we treat them in the same way. Let us, for instance, take a piece of the metal called zinc and heat it; after some time it will melt, and if we still continue to heat it, it will at last pass away in the shape of zinc vapour. Even hard, solid iron or steel may be made to melt, and even driven away in the shape of vapour; and by means of an agent called electricity (of which more hereafter) we can probably heat any substance sufficiently to drive it away in the state of vapour or gas.

We cannot, however, cool all bodies sufficiently to bring them into the solid or even into the liquid state. Thus, for instance, pure alcohol has never been cooled into a solid; but we know very well that all we have to do is to obtain greater cold in order to succeed in freezing alcohol. In like manner, we have never been able to cool the atmospheric air sufficiently to bring it into the liquid form; but we know very well that all we require in order to succeed is to obtain greater cold. You must not, however, imagine from what I have said, that cold means anything else than the absence of heat. A cold body is a body which has little heat, and a still colder body has still less heat; but even the coldest body which we can produce has a little heat left. Do not be guided in this respect by your feeling of touch. Two bodies may be of the same temperature, as shown by the thermometer;

and yet the one may feel much colder to you than the other; and if you keep one hand for some time in very cold and the other in very hot water, and then plunge them both into water of ordinary heat, this water will seem hot to the one hand and cold to the other. Do not therefore be guided by anything else than the thermometer, or imagine that cold is anything else than the absence of heat.

To return to our subject. Probably all bodies, if we could cool them enough-that is to say, take away enough of their heat-would assume the solid state; and then, when each was again heated sufficiently, it would become liquid, until at last, if still heated, it would be driven off in the shape of gas or vapour. There would, however, be a great difference between the different bodies in the ease with which they would yield. Ice soon melts if we apply heat; tin or lead require to be heated to 200 or 300 degrees before they will melt; iron is more difficult to melt than lead; and platinum is more difficult than iron. A body very difficult to melt is called refractory.

In the following table we have the temperature at which some of the most useful substances begin to melt. Ice melts at Phosphorus. Spermaceti Potassium Sodium

44°

49°

58°

97°

1,000°

Platinum is so difficult to melt that we cannot tell at what temperature it does so. And carbon is still more difficult to melt-indeed in the very hottest fire the coal or carbon is always solid; and no one ever heard of the coals melting down and trickling out through the furnace bars.

We thus see that the same sort of change takes place in all bodies through heat; that is to say, if we could reach a temperature sufficiently low, all bodies would become solid like ice, and if we could reach one sufficiently high, all would become gaseous like steam in fact, the change that takes place is always of the same kind, and we cannot do better than use water as a type of all other things in this respect, and study the behaviour of this substance under heat, beginning with its solid state when it appears in the shape of ice.

58. Latent heat of Water.-Let us take some very cold ice, pound it into small pieces, and put the bulb of our thermometer into this pounded ice. Let us suppose that the reading of our instrument shows a temperature 20 degrees below the point we call °. Now let us heat the ice, and its temperature will rise like that of any other solid under like circumstances until it comes to o°, but at this point it will stop, and rise no further as long as any ice remains. What then does the heat do if it does not raise the temperature above this point? We reply, it melts the ice. At first the heat is wholly spent in raising the temperature of the very cold ice, but when this temperature has reached o° the heat has quite a different office to perform; its power is now wholly spent in melting the ice, and when the ice is all melted