SPECIFIC HEAT. If 1 lb. of water and 1 lb. of oil be transferred from a temperature of 40° to one of 70°, the oil will indicate 70° much quicker than the water, requiring in fact less heat to reach the same and contained in the other, so as to leave a space c C. From this space proceeds a discharge-pipe with a atop-cock E. From the bottom of the inner vessel also proceeds a pipe D; it passes air-tight through the outer vessel, and is furnished with a stop-cock. There is a third vessel, e, contained within the second, and the space B B is also filled with pounded ice. The vessel a is furnished with a cover, and there is a large tray-like cover over the whole apparatus. This is also filled with pounded ice. Now supposing such an apparatus to be introduced into a room at a temperature of 40°, it is evident that the ice in C will slowly melt, and retain B at the constant temperature of 32°. Now if we introduce the body whOse specific heat is to be determined into a at any given temperature above 32°, it would cool down to that tem perature, and in doing so will melt a quantity of ice, the water from which will be collected in the bottle below D. If this quantity of water be divided by the number of degrees through which the body in A has fallen, the quantity of ice dissolved by the heat corresponding to 1° will be found. This being divided by the weight of the body in A in pounds, the weight of ice dissolved by the heat which would raise 1 lb. of the body 1° will be determined. In this way it is found that the heat necessary to raise 1 lb. of water 1°, is that which would dis solve the 142.65th part of 1 lb. of ice. For other bodies we get the following rule :—Multiply the weight of ice dissolved by 142.65, and multiply the weight of the body which dissolves the ice, by the num ber of degrees of temperature which it loses, and divide the former product by the latter, when the quotient will be the specific heat of the body.
By the method of mixtures a mean temperature is equal weights of the same body are mingled together ; but when different fluids are mixed the result is different. For example, 1 lb. of mercury at 40° agitated with 1 lb. of water at 156°, gives a
mixture =-152°.3. In this case the water loses 3°•7, and the mercury gains Hr.& Hence 112°.3 : 3'7 1 : I : or, in other words, water being 1000, the specific heat of mercury is only 33; 1 lb. of water absorbing 30 times more heat than 1 lb. of mercury.
By the third method equal and similar volumes of two bodies are raised to the same temperature, and allowed to cool under similar circumstances. By observing the intervals of time required for equal volumes to fall 1°, we get the ratio of the quantities of heat which they lose. The quantities for equal weights may be inferred from the specific gravities of the bodies, and in this way the specific heats can be arrived at.
The specific heat of bodies diminishes with an increase of density, so that mere mechanical compression will raise the temperature of many bodies, and even make some of the metals very hot ; iron, it is said, incandescent. The sudden compression of abriform bodies is attended with the evolution of host, sufficient under proper arrange ments to i?nite monde«, or German tinder. On rarefying air an oppo site effect in observed, as is evident from the dimness seen on the Inner surface of an air•pump receiver on first beginning to exhaust the air. The capacity for heat of the remaining air is increased, and there is not sufficient heat to retain the vapour in its elastio form. So when compressed air suddenly escapes, it absorbs heat, and moisture is deposited. Similar effects may be observed in nature. A blast of cold air descending from a lofty height, has its temperature raised by the mere compression that it undergoes.
The following are a few specific heats obtained by the process of mixture or immersion. They refer to that part of the scale between 32* and 212• :— A body in the liquid state has a higher speciflo heat than the same substance in a solid form. The specific heat of water, for example, is double that of ice. Indeed the large specific heat of water has a great effect on the temperature of the globe, in moderating the rapidity of tho transitions which would otherwise occur.