prior to the Introduction of the Potentials. 35 



rapid progress. It was frankly advocated by another member 

 of the Dutch school, Hermann Boerhaave* (6. 1668, d. 1738), 

 Professor in the University of Leyden, whose treatise on 

 chemistry was translated into English in 1727. 



Somewhat later it was found that the heating effects of the 

 rays from incandescent bodies may be separated from their 

 luminous effects by passing the rays through a plate of glass, 

 which transmits the light, but absorbs the heat. After this 

 discovery it was no longer possible to identify the matter of heat 

 with the corpuscles of light ; and the former was consequently 

 accepted as a distinct element, under the name of caloric.^ In 

 the latter part of the eighteenth and early part of the nineteenth 

 centuries} caloric was generally conceived as occupying the 

 interstices between the particles of ponderable matter an idea 

 which fitted in well with the observation that bodies commonly 

 expand when they are absorbing heat, but which was less com- 

 petent to explain the fact that water expands when freezing. 

 The latter difficulty was overcome by supposing the union 

 between a body and the caloric absorbed in the process of 

 melting to be of a chemical nature; so that the consequent 

 changes in volume would be beyond the possibility of prediction. 



As we have already remarked, the imponderability of heat 

 did not appear to the philosophers of the eighteenth century to 

 be a sufficient reason for excluding it from the list of chemical 

 elements ; and in any case there was considerable doubt as to 

 whether caloric was ponderable or not. Some experimenters 

 believed that bodies were heavier when cold than when hot; 

 others that they were heavier when hot than when cold. The 

 century was far advanced before Lavoisier and Eumford finally 



* Boerhaave followed Homberg in supposing the matter of heat to be present ia 

 all so-called vacuous spaces. 



t Scheele in 1777 supposed caloric to be a compound of oxygen and phlogiston, 

 and light to be oxygen combined with a greater proportion of phlogiston. 



J In suite of the experiments of Benjamin Thompson, Count Eumford (b. 1753, 

 .d. 1814), in the closing years of the eighteenth century. These should have 

 -sufficed to re-establish the older conception of heat. 



This had been known since the time of Boyle. 



D 2 



