92 THE AIM AND ACHIEVEMENTS OF SCIENTIFIC METHOD. 



if a definite " amount of heat " is " given " repeatedly to a mass 

 of air whose volume in one experiment is kept constant and 

 in other experiments is made to vary in an arbitrary manner, it 

 will be impossible to find a manipulation of the mass and 

 temperature changes of the air which will yield a constant 

 number that can be equated to the " quantity of heat given." 

 In other words, the change of temperature of the air is, in 

 this case, not a complete " notice " of the change in the body 

 which acted as the " source " of the heat. Some other partial 

 expression or expressions of that notice must be sought, and 

 in this case will be found, in the u work done " by or upon 

 the gas during the experiment. As is well known, it was 

 the perception of the significance in this and other cases of 

 the " work done " that led Mayer, Joule, and (later) Helmholtz 

 to the enunciation of the essentially modern conception of 

 " energy." 



36. 



The result of such a process as has been described in the 

 foregoing section is always to bring the bodies between which 

 the new relations have been established into some sort of 

 unity.' This unity will take various forms and display various 

 degrees of permanence, to attempt a classification of which 

 would be scarcely profitable. But it is interesting to note 

 how frequently the " notice " taken by one body of another's 

 behaviour is thought of in the form of a transference of 

 substance. Besides heat and energy, momentum, electricity, 

 magnetism, and other cases will suggest themselves. This 

 transference sometimes is thought of in a shape which implies 

 that one part of the system is gaining what another part is 

 losing. Such is the case of the transference of heat which we 

 have just considered. In other cases the transference takes a 

 "circulatory" form as in the case of the "current" of electricity. 

 In this case the various bodies concerned wire, battery, 

 galvanometer needle, &c. are thought of as forming one 



