70 THE ADVANCE OF SCIENCE 



a vast number of jmenomena, such as 

 those of light, heat, electricity, magnet- 

 ism, and those of the physical and chemi- 

 cal changes, which do not involve molar 

 motion. Newton's corpuscular theory of 

 light was an attempt to deal with one 

 great series of these phenomena on me- 

 chanical principles, and it maintained its 

 ground until, at the beginning of the 

 nineteenth century, the undulatory theory 

 proved itself to be a much better working 

 hyjDothesis. Heat, up to that time, and 

 indeed much later, was regarded as an im- 

 ponderable substance, caloric ; as a thing 

 which was absorbed by bodies when they 

 were warmed, and was given out as they 

 cooled ; and which, moreover, was capable 

 of entering into a sort of chemical combi- 

 nation with them, and so becoming latent. 

 Rumford and Davy had given a great 

 blow to this view of heat by proving that 

 the quantity of heat which two portions 

 of the same body could be made to give 



