Curiosities of Heat

by Lyman Beecher Tefft

“In this work of absorbing and radiating heat every object, earth, air, and sea, does its appropriate share. But water stands chief, and performs the largest service. Its high specific heat enables it to hold in store the largest calorific treasure, and causes it to change its temperature more slowly.

“The formation and condensation of vapor also operate in the same manner as in the transitions of day and night. During the summer the higher average temperature makes it possible for a much larger amount of vapor to be formed than in winter. You remember that at eighty degrees vapor equal to thirteen inches of water can sustain itself, while at thirty-six degrees the elastic force of vapor is equal to the pressure of only two inches and two-fifths of water, and at four degrees to three-fifths of an inch. If the mean summer temperature at any place were eighty degrees, it would be possible for more than one foot of water to be held in the form of vapor. In the formation of this vapor heat would be consumed sufficient to boil more than five and a half feet of ice water. If the mean winter temperature at the same place be thirty-six degrees, more than three-fourths of this vapor must be condensed and give out its latent heat to warm the air. It is not to be supposed that the full amount of vapor which can support itself does commonly exist, but the difference between the average amount of vapor in summer and in winter must be very great. I suppose this difference often amounts to four or six inches of water. If we suppose it to be four inches, an amount of heat is transferred from summer to winter sufficient to boil twenty-two inches of ice water. In estimating the effect of this we must consider that this heat is not given out gradually and regularly for three months, but whenever there is a sudden fall of temperature vapor is condensed, latent heat becomes sensible, and the suddenness and intensity of the fall are diminished. We need also to bear in mind that every open body of water is sending up its clouds of vapor constantly. The open lakes, and especially the sea, are like a seething caldron; and thus immensely more vapor is condensed during the winter months than is brought over from summer to winter. Much of the vapor formed in winter is to be set to the account of summer, for it is the summer’s heat absorbed by the water, which maintains its temperature and enables it to throw up such clouds of vapor, even in midwinter. But this comes in more properly at another place, and we will leave it for the present.

“There is another transition experienced by water by which heat treasured up in summer is made available for softening the rigors of winter. Who will suggest it?”

“It is the freezing of water,” said Mr. Hume. “In the process of crystallization one hundred and forty degrees of latent heat become sensible.”

“And this,” continued Mr. Wilton, “is no inconsiderable matter. Every pound of water frozen upon the surface of our lakes and rivers, every pound of water frozen in the wet earth, every pound of water frozen as snow or sleet in the air, gives out as much heat as would boil an equal amount of water at seventy-two degrees. Have you never heard of setting tubs of water in cellars to keep vegetables from freezing?”

“I have,” replied Peter. “I visited my grandfather two years ago, and his cellar sometimes froze. I asked him why he put tubs of water in his cellar, but he could not tell me, only he said that he knew that tubs of water in his cellar did keep his vegetables from being nipped with the frost.”

“Can you tell us, Peter, why tubs of water set in a cellar should have this effect?”

“I suppose that when the water begins to freeze it begins to give out its latent heat.”

“That is one part of the reason. The water is drawn from the well at perhaps fifty degrees; it must lose eighteen degrees of heat before it begins to freeze, and all the heat which the water loses the air of the cellar gains. And then, as you said, as soon as the water begins to freeze latent heat begins to become sensible. Every pound of water frozen sets free heat enough to raise a pound of water through one hundred and forty degrees. But why do not the vegetables begin to freeze as soon as the water?”

“I don’t know.”