n THE PROGRESS OF SCIENCE 85 



mechanical theory, there remained a vast number 

 of phenomena, such as those of light, heat, elec- 

 tricity, magnetism, and those of the physical and 

 chemical 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 mechanical 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 hypo- 

 thesis. Heat, up to that time, and indeed much 

 later, was regarded as an imponderable 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 combination 

 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 out, by rubbing them together, was practically 

 illimitable. This result brought philosophers face 

 to face with the contradiction of supposing that a 

 finite body could contain an infinite quantity of 

 another body ; but it was not until 1843, that 

 clear and unquestionable experimental proof was 

 given of the fact that there is a definite relation 

 between mechanical work and heat ; that so much 

 work always gives rise, under the same conditions, 

 to so much heat, and so much heat to so much 



