320. =Capacity for Heat.=--The more heat a substance can take in and render latent the greater is its capacity for heat, as it is expressed. Thus water has a much greater capacity for heat than mercury. This can be proved by various experiments. Thus, if we take two vessels just alike, and having, the one a certain quantity of water in it, and the other the same quantity of mercury, and expose them to the same degree of heat, it will take much longer to raise the water to any specified temperature than the mercury. Why is this, when they are both receiving the same amount of heat? It is because the water renders a much larger portion of the heat latent than the mercury does. We can reverse this experiment. Take these same vessels with their contents raised to the same temperature, as indicated by the thermometer, and allow them to cool in the air side by side. The mercury will cool much faster than the water, because it has much less of latent heat to part with. The difference in capacity for heat between water, oil, and mercury may be shown by the experiment represented in Fig. 219. A pound of water is put into one Florence flask, a pound of oil into another, and a pound of mercury into a third. They are all heated to 212°, and are then placed in funnels filled with pounded ice, the funnels resting in glass jars of the same size. Now in cooling these fluids down to a certain point, say 32°, different amounts of the ice will be melted, in the proportions of 100 and 50 and 3. This shows the proportions of latent heat in them which become sensible or free as their temperatures are lowered. [Illustration: Fig. 220.] 321. =Relation of Latent Heat to Density.=--The more dense a substance becomes the less is its capacity for heat. The heat produced by hammering iron is the latent heat rendered free by condensation, this lessening the capacity of the iron for heat. The same thing can be better illustrated in the condensation of a very compressible substance, as air. In Fig. 220 you have represented a glass syringe with a closed end. If there be placed in this end a little bit of cotton wool moistened with ether, and the piston be forced downward very quickly, the ether will be set on fire. This is because the compression of the air lessens so much its capacity for heat that a great deal of its latent heat is made sensible or free. The heat which is concealed in it in its ordinary state is, as we may say, fairly squeezed out, as you would squeeze out the water that is concealed in the interstices of a sponge. 322. =Coldness of Air at Great Heights.=--You learned in § 152 that the atmosphere is thinner the farther you go from the earth. It is very thin, therefore, on the summits of high mountains. This is the chief reason why it is so cold there, for the rarer the air is the greater is its capacity for heat, and the more of sensible or free heat therefore can it render latent.