96 THE AIM AND ACHIEVEMENTS OF SCIENTIFIC METHOD. 



combination that accounts for the change of state which the- 

 water undergoes : it becomes " latent heat."* 



40. 



But the concept which Black had handled with such 

 splendid audacity was not destined to a long career of 

 theoretical adequacy although for elementary didactic pur- 

 poses it is still indispensable.f Within fifty years of Black's 

 researches the experiments of Eomford and Davy had made it 

 clear that heat must be thought of as capable of being 

 generated without limit by mechanical work. 



The effect of this discovery may be represented symbolically 

 by the statement that it was tow' impossible any longer to- 

 conceive heat as a substance ; critically, it implied that the 

 changes of temperature of bodies cannot be regarded as a closed 

 series of events; but that a change of temperature may, in 

 fact, sometimes be the way in which a body " takes note " of 

 a change of the " mechanical " order in another body. At this 

 point, therefore, it becomes necessary to submit the funda- 

 mental notions of mechanics to a brief critical scrutiny. 



41. 



The development of these ideas presents, in many respects, 

 an interesting parallel with the development of the ideas of the 

 science of Heat.J We begin with a number of kinsesthetic and 

 " pressure " sensations whose object is conceived vaguely as the 

 " force" of the body to which they are due the heavy stone, the 

 stretched string, the bent rod, the moving missile. It is recog- 

 nised (as in the case of hotness and coldness) that these sensa- 

 tions, even when they are different, may often refer to the same 



* Black, op. cit., pp. 176-7. 



t See a lecture by the present writer printed in the Educational Times 

 for May, 1905. 



J I have considered it from this point of view in an article on 

 "'Mass' and 'Force' in School Mathematics " in School (December, 1905) 

 The next few paragraphs are in the main a condensation of that article. 



