f 



326 



HEAT. 



geals its minute particles, and they descend to the earth in flakes of snow. If, 

 however, they are first formed into liquid spherules, and then solidified, hail is 

 produced. 



Thus there is a constant interchange of matter between the earth and its at- 

 mosphere the atmosphere continually drawing up water in the form of vapor, 

 and, when the heat which accomplishes this is diminished, precipitating it in 

 the form of dew, rain, snow, or hail. 



Different bodies are differently susceptible of the effects of heat. To pro- 

 duce a given change of temperature in some requires a greater supply of heat 

 than in others. Thus, to raise water from the temperature of 50 to the tem- 

 perature of 60 will require a fire of given intensity to act upon it about thirty 

 times as long as to raise the same weight of mercury through the same range 

 of temperature. In the same manner, if various other bodies be submitted to 

 a like experiment, it will be found that to produce the same change of temper- 

 ature on the same weights of each will require the action of the same fire for a 

 different length of time. 



The quantities of heat necessary to produce the same change of temperature 

 in equal weights of different bodies are therefore called the specific heats of 

 these bodies. If 1,000 express the specific heat of pure water, or the quantity 

 of heat necessary to raise a given weight of pure water through 1, then 33 

 will express the specific heat of mercury, or the quantity of heat necessary to 

 raise the same weight of mercury through 1 ; 70 will express the specific 

 heat of tin, 80 of silver, 110 of iron, and so on. The specific heat furnishes 

 another physical character by which bodies, whether simple or compound, of 

 different kinds may be distinguished. 



The specific heat of the same body is changeable with its density. In gen- 

 eral, as the density is increased, the specific heat is diminished. Now, if the 

 specific heat of a body be diminished, since a less quantity of heat will then 

 raise it through 1 of temperature, the quantity of heat which it actually con- 

 tains will make it hotter when it is rendered more dense, and colder when it 

 is rendered more rare. 



Hence we find that, when certain metals are hammered, so as to increase 

 their density, they become hotter, and sometimes become red hot. 



If air be squeezed into a small compass, it becomes so hot as to ignite tin- 

 der ; and the discharge of an air-gun is said to be accompanied by a flash of 

 light in the dark. 



On the other hand, if air expand into an enlarged space, it becomes colder. 

 Hence, in the upper regions of the atmosphere, where the air is not compressed, 

 its temperature is much reduced, and the cold becomes so great as to cause, on 

 high mountains, perpetual snow. 



The specific heats of compounds frequently differ much from those of the 

 components. If the specific heat of bodies be greatly diminished by their com- 

 bination, then the quantity of heat which they contain will render the compound 

 much hotter than the components before the combination took place. If, on the 

 other hand, the specific heat of the compound be greater than that of the com- 

 ponents, then the compound will be colder, because the heat which it contains 

 will be insufficient to sustain the same temperature. 



Hence we invariably find that chemical combination produces a change of 

 temperature. In some cases cold is produced, but in most cases a considera- 

 ble increase of temperature is the result. 



Heat is propagated through space in two ways : First by radiation, which 

 is apparently independent of the presence of matter, and, secondly, by conduc- 

 tion, a word which expresses the passage of heat from particle to particle of a 

 mass of matter. 



