317. Dew-Point.—What is called the dew-point of the air is that degree of temperature to which any substance must be brought down in order to have dew deposited upon it. This depends upon the amount of water there is in the atmosphere. The more there is the higher is the dew-point. When water condenses on a cold tumbler in a hot day there is much more water in the air, and the dew-point is higher, than when no moisture is condensed upon the tumbler. So after a very hot clear day the earth needs not to be much cooled to produce a deposit of dew, because the air has become so highly charged with moisture through the evaporation of the earth under the hot sun. We can very readily at any time ascertain the dew-point. Take a glass of water, and, having a thermometer in it, drop into it some pieces of ice, and watch the outside of the glass. As soon as it begins to be dimmed with moisture look at the thermometer, and you have the dew-point.
318. Freezing Mercury.—Mercury can be frozen by radiation when the cold is excessively severe, although the thermometer may indicate a temperature considerably above -39°, the degree at which mercury freezes. Suppose that in a clear, still night the temperature of the air is at -20°. In order to freeze the mercury it must be cooled down 19 degrees below this. Now this can be done by surrounding the mercury with some good non-conductor, as charcoal. This cuts off the supply of heat to the mercury, while it is all the while giving off heat into space by radiation. In like manner can ice be formed in an atmosphere that is above the freezing point, and this is often done in warm climates.
319. Latent Heat.—You have seen, § 270, that our sensations do not inform us accurately of the amount of heat in any substance. The same is also true of the thermometer. This only indicates the sensible or free heat. There may be a great deal of heat locked up, as we may say, in the substance, which can be brought out or made free by some change in the substance. This heat thus locked up is called latent heat.
Fig. 219.
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.
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.