GENERAL PRINCIPLES 47 



all the chemical energy will then be transformed into heat. Such 

 is the principle of thermochemistry or the art of measuring the 

 heats of combustion of substances. For this the "bomb" 

 calorimeter of Bcrthelot is generally used, modified by Mahler 

 and subsequently, by Donkin. In the chamber of that calori- 

 meter the substance is burnt in contact with compressed oxygen 

 at a pressure of 25 atmospheres. The heat can be measured by 

 correct and proved methods for which reference must be made 

 to special treatises. Simpler calorimeters are also used, such as 

 that of Bunsen, or that of Fery.f 1 ) 



But the general case is that in which 



U = T EQ. 



Let there be an electric battery driving a motor. Work T is 

 .done and q calories of heat are evolved, but if the- unused chemical 

 reaction of the battery be neglected, the calorimeter will indicate 

 q' calories. This experiment shows, in accordance with the 

 principle of equivalence that 



T 



E + q = q ' 

 In the first case : 



T 



u = -+ ?; 



In the second case : 



T 



U = q ' ; from which q ' = + q. 



E 



This is the case with living^ beings. Animals and vegetables 

 burn food internally, and in repose they produce heat exclusively. 

 But mechanical work, of which animals only are capable, leads 

 to the general relation : 



U = T + EQ. 

 (see 103). 



Lastly, as will be examined later in detail, the work can be 

 motive, or resistant, that is to say, positive or negative ( 28). 

 If it is negative, 



U = EQ T : 



the modification of internal energy will not be as great as if the 

 work were positive, which amounts to saying that negative work, 

 far from making demands on that internal energy, on the con- 

 trary tends to economise it. 



35. Principle of Carnot. Sadi-Carnot established a principle 

 of great importance to express the relation of work and heat, 



(*) Fery, Journal de Physique, 1912, p. 550. 



