On the occasion of a lecture about the formation of snow in the atmosphere, Professor Dove once told an anecdote, which is as interesting as it is instructive. A musician in St. Petersburg gave a concert in a large hall, where the fashionable world had assembled in great numbers. It was an icy cold night, such as is almost unknown with us; but in the overcrowded hall there was such excessive heat as only Russians can endure. Soon, however, it became too intense even for them. The hall was densely crowded; the throng was alarming; several ladies fainted. An effort was made to open a window, but without success--the window was frozen fast. A gallant officer devised means; he broke the window in. And what happened? _It commenced to snow in the concert room!_ How did this come? The vapor exhaled by the multitude of persons in the hall had collected above, where the air was hottest. The sudden entrance of the icy air through the broken window changed the particles of water into snow. Thus it was this time not heaven, but the upper space of an unventilated concert-hall, that sent down snow. In a similar way hail is formed in the atmosphere; this we shall consider at more length hereafter. At present we must turn our attention to the influence of these phenomena upon cold and heat; for it is a known fact, that rain and evaporation are not only engendered by cold and heat, but, _vice versa_, that rain and evaporation, in their turn, engender cold and heat in the air. CHAPTER VII. HOW HEAT IN THE AIR BECOMES LATENT, AND HOW IT GETS FREE AGAIN. In the preceding chapter it was shown how warm air produces evaporation, and how cold air causes rain and snow. In this chapter we desire to demonstrate how the reverse may take place, viz., the engendering of cold and heat by evaporation and rain. Although what we wish to prove in the following is firmly established, yet it is not easy to make it understood. For this reason many educated men, who have read much about "free and latent heat," have mistaken ideas about it. In order that what we shall explain may be in the reach of every one, we must again choose our examples from life itself, and request our readers to come to our aid with their thoughts. Every one knows how water is boiled. It is placed over the fire, the heat of which communicates itself to the water and heats it more and more. Now, where does the heat of the fire go? It is taken up by the water; thus to speak, the water absorbs the heat. This explains why a cooking-stove on which a dinner is cooked, does not get near as warm as it would, if the same quantity of fuel had been used without any cooking on the stove. For a portion of the heat being absorbed by the meat, it cannot heat the stove; hence the stove fails to receive the amount of heat that is used in cooking the meat. What will be the effect of taking boiling water from the stove and placing it in the room somewhere? Where will the heat of the water go then?