A lump of ice brought out of the open air in very cold weather may have a temperature of 30°, or 20°, or lower. If such a lump is brought into a warm room it gradually becomes warmer, but remains unchanged otherwise, until it has risen to 32°. Then it begins to melt, and remains at 32° as long as it is melting; and the water which proceeds from it is at first also at 32°.
If you were to throw a lump of ice into the middle of a hot fire, so long as a particle of ice remained as such, it would have a temperature of 32° and no more. This is a fact exactly parallel to that which is observed when water is raised to the boiling-point. So long as any of the water remains unconverted into steam it becomes no hotter. Moreover the steam itself is at first at 212°.
44. Ice the solid, Water the liquid, and Steam the gas, are three states of one natural object; the Condition of each State being a certain Amount of Heat.
Ice, liquid water, and steam, are three things as unlike as any three things can well be. What do we mean then by saying that they are states of one substance, water?
What we really mean is that if we take a given quantity of water, say a cubic inch, and change it first into ice and then into steam, there is something which remains identically the same through all these changes. This something is, in the first place, the weight of the material substance. The water weighs 252½ grains, the ice into which it is converted weighs 252½ grains, and the steam produced from it weighs 252½ grains. In the second place, the same force would cause the ice, the water, and the steam, to move with the same rapidity; and, when set in motion, they would produce the same effect upon anything movable against which they struck.
In the third place, when you study chemistry, you will learn that the ice, the steam, and the liquid water, would yield the same weight of the same two gases, oxygen and hydrogen, and nothing else. Every one cubic inch of water, 1,700 cubic inches of steam, and 1
111 cubic inch of ice, yield 281
18 grains of hydrogen, with 2248
18 grains of oxygen, and nothing else. (See § 50.)
As there is not the slightest difference in weight between a given quantity of water and the ice, or the steam, into which it may be converted, it is clear that the heat which is added to or taken from the water to give rise to these several states, can possess no weight. If then heat is a material body, it must be devoid of weight—and hence, in former times, heat was called an imponderable substance. It was thought to be a kind of fluid, called caloric, which had no weight, and which drove the particles of bodies asunder, when it entered them as they were heated, and let them come together as it left and they grew cool.
45. The Phenomena of Heat are the Effects of a rapid Motion of the Particles of Matter.
This much, however, is certain: that heat can be caused by motion. Every boy knows that a metal button may be made quite hot by rubbing it. A skilful smith will hammer a piece of iron red hot. The axles of wheels become red hot by rubbing against their bearings, if they are not properly lubricated; and even two pieces of ice may be melted by the heat evolved when they are rubbed together. And there are abundant other reasons, as you will find when you study physics, for the belief that the sensation we call heat, and all the phenomena which we ascribe to heat, are the effects of the rapid motion of matter.
However, a quiescent body may be made hot without exhibiting the least appearance of motion. The surface of the water in a tumbler at 100° is just as unruffled as that of the same water at 32°. What, then, is meant by saying that heat is a kind of motion, and that the greater the heat in any body the greater the amount of motion in that body?