HEAT, COLD, AND LIGHT. 315 



of producing Heat. There are several ways by which 

 Heat may be generated. 1. By means of the sun's 

 rays. 2. By exciting vibrations in solids. 3. By the 

 taking place of certain chemical attractions. 4. By 

 conversion of vapours into fluids, and of fluids into 

 solids. 5. By animal powers. 6. In volcanos. 



And first, if a cold body be exposed to the rays 

 of the sun, it will be heated. 



It has been frequently conjectured that the sun 

 is fire, burning and heating ocher bodies in like 

 manner as a culinary fire: but the celebrated Dr. 

 Fordyce has asserted, that the sun is probably not 

 at all hot in itself; neither are the solar rays hot, 

 but have only a power of producing Heat on being 

 applied to other bodies. An opinion very little 

 different from this is also entertained by Dr. tVil- 

 liam Herschel, and supported with the authority 

 of demonstration. As a farther consideration of 

 this subject may tend to correct our notions con- 

 cerning Light, it is here attempted. 



It is a constant rule, with regard to hot bodies 

 that they heat ail colder bodies which are brought 

 near them. Though it must be allowed (as will b 

 hereafter seen) that some bodies will receive Heat 

 with more readiness than others: however, thi* 

 will not easily form an objection to what imme- 

 diately follows. 



Now, if we take a 'large burning glass, and hold 

 a piece of iron in its focus, such Heat will be pro- 

 duced as to melt the iron. But the glass through 

 which all the rays passed is scarcely heated at all : 

 and when they fell on the iron they were n.Q hotter 



pg than 



