82 THE HISTORY OF SCIENCE 



brightness, since it is present in boiling water and 

 absent in the moon's rays. 



The induction, however, is not complete till some- 

 thing positive is established. At this point in the 

 investigation it is permissible to venture an hypoth- 

 esis in reference to the essential character of heat. 

 From a survey of the instances, all and each, it ap- 

 pears that the nature of which heat is a particular 

 case is motion. This is suggested by flame, sim- 

 mering liquids, the excitement of heat by motion, 

 the extinction of fire by compression, etc. Motion is 

 the genus of which heat is the species. Heat itself, 

 its essence, is motion and nothing else. 



It remains to establish its specific differences. 

 This accomplished, we arrive at the definition : Heat 

 is a motion, expansive, restrained, and acting in its 

 strife upon the smaller particles of bodies. Bacon, 

 glancing toward the application of this discovery, 

 adds : " If in any natural body you can excite a 

 dilating or expanding motion, and can so repress 

 this motion and turn it back upon itself, that the 

 dilation shall not proceed equally, but have its way 

 in one part and be counteracted in another, you will 

 undoubtedly generate heat." The reader will recall 

 that Bacon looked for the invention of instruments 

 that would generate heat solely by motion. 



Descartes was a philosopher and mathematician. 

 In his Discourse on Method and his Joules for the 

 Direction of the Mind (1628) he laid emphasis on 

 deduction rather than on induction. In the subor- 

 dination of particulars to general principles he ex- 

 perienced a satisfaction akin to the sense of beauty 

 or the joy of artistic production. He speaks enthusi- 



