SOURCES OF HEAT. 27 



their production ; but how the vital force operates we 

 do not know. But, as in physiology this ignorance 

 does not prevent us from comprehending the structure 

 of the human body and the uses of its different organs, 

 so ignorance in relation to the nature of heat does not 

 interfere with the acquisition of the most perfect knowl- 

 edge of its effects, and the laws according to which 

 they happen. 



46. THE MECHANICAL THEORY HERE 



What theory T ,, , ~ 



is adopted in ADOPTED. In the present volume the for- 



mer O f the views which have been men- 



Explain it. 



tioned is adopted, and heat, like light, is 

 assumed to be an exceedingly subtle imponderable fluid. 

 And, to return to the example of the heated bar, it grows 

 hot at the end farthest from the fire because the fluid 

 actually passes through its solid substance, and is so 

 communicated to the hand. 



47. DEFINITION OF COLD. Cold is a 



what is meant 



by the term relative term signifying the comparative 

 absence of heat. But the coldest bodies 

 which we know of, as ice, for example, contain heat, 

 and may be made colder by its withdrawal. 



48. SOURCES OF HEAT. The principal 



State the prin- 



cipal sources sources or heat are the sun and fixed stars, 

 of hmt. chemical action, electricity, and friction. 



It is by no means certain that these should be distin- 

 guished as different sources ; for the heat of the sun 

 may be due to chemical action, and electricity is, as 

 we know, excited both by chemical action, and by 

 friction. 



