322 THE MECHANICAL EQUIVALENT OF HEAT. 



are burned ; and here the knowledge we have already ao 

 quired of the weight of gaseous bodies comes to our aid. 

 We then find that, in every case of combustion, substances 

 which previously existed in a separate state enter into an inti 

 mate union with each other, and that the total weight of the 

 substances remains the same both before and after the combi 

 nation. We thus come to know the different bodies in their 

 separate and in their combined states, and learn how to trans 

 form them from one of these states into the other ; ,we learn, 

 for instance, that water is composed of two kinds of air which 

 combine with each other in the proportion of 1: 8. An en 

 trance into chemical science is thus opened to us, and the nu 

 merical laws which regulate the combinations of matter (die 

 Stochiometrie) hang like ripe fruit before us. 



As we proceed further in our investigations, we find that 

 in all chemical operations combinations as well as decompo 

 sitions changes of temperature occur, which, according to 

 the varying circumstances of different cases, are of all de 

 grees of intensity, from the most violent heat downwards. 

 We have measured quantitatively the heat developed, or 

 counted the number of heat-units, and have so come into pos 

 session of the law of the evolution of heat in chemical pro 

 cesses. 



We have long known, however, that in innumerable cases 

 heat makes its appearance where no chemical action is going 

 on ; for instance, whenever there is friction, when unelastic 

 bodies strike one another, and when aeriform bodies are com 

 pressed. 



What then takes place when heat is evolved in such ways 

 as these f 



We are taught by history that in this case also the most 

 sagacious hypotheses concerning the state and nature of a 

 peculiar &quot;matter&quot; of heat, concerning a &quot;thermal aBther,&quot; 

 whether at rest or in a state of vibration, concerning &quot; ther 

 mal atoms,&quot; supposed to exercise their functions in the inter- 



