SOURCES OP HEAT AND COLD. 127 



APPENDIX TO CALORIC. 

 SEC. III. SOURCES OF HEAT AND COLD. 



I. SOURCES OF HEAT ; most of which are also sources of light. 



(a.) The sun. 

 b.) Combustion. 

 ) Chemical action without combustion. 



? Electricity and Galvanism. 

 Condensation of aeriform bodies by pressure. 

 ) Condensation of solids, by mechanical action, including 

 friction and percussion, 

 (g.) Vital action. 



(a.) The solar rays. The intensity of the solar heat being in pro- 

 portion to the rays that can be collected upon a given spot, there ap- 

 pears to be no other limit to our power of generating heat in this 

 manner, than what is found in the size of our instruments, and the 

 difficulty of using them, for it has been long known, that the effect is 

 much increased by lenses and mirrors.* 



This is especially true if the focus he received on a black and 

 rough surface, e. g. on charred cork lining a box, and covered by 

 glass ; thus a heat of 221, was produced while the air was only 75. 

 Saussure. In another case, the heat generated by similar meanSj 

 was from 230 to 237, while a bright fire gave, at the same time, 

 212. Black, Thomson. 



Dr. Hare remarks, that previously to the discovery of the heat ex- 

 cited by oxygen, by the compound blowpipe, or by the Voltaic series, 

 there was no known mode of rivalling the heat produced by large 

 burning glasses and mirrors. These have been already mentioned, 

 perhaps sufficiently, in the account of heat and light. 



It is not in our power to say what is the nature of the sun, and for 

 aught we know, the popular opinion that his body is a globe of ignited 

 matter, may be correct.f 



(b.) Combustion. After the solar influence, this is the most im- 

 portant source of heat ; it is very completely under our command $ 

 it can be applied when and where we please, and varies from ex- 



* Dr. Hare. 



t Dr. Herschel's ideas of the nature of the sun, were peculiar. He supposed the 

 sun's body to be opake ; that his atmosphere has two strata of clouds ; the one opake 

 and the other phosphorescent ; the latter he supposes to be the highest, and that they 

 emit the light ; that when the clouds are broken and ragged, the sun's opake body 

 is seen through the clouds. The fruitfulness of different seasons he supposed to be 

 connected with the quantity of light emitted fvom the luminous clouds of the sun. 

 Phil. Trans. 1801.. ' 



