34 Electric and Magnetic Science 



all fixed Bodies, when heated beyond a certain Degree, emit 

 light and shine ; and is not this Emission performed by the 

 vibrating Motion of their Parts ? " and, moreover, suggested the 

 converse of this, namely, that when light is absorbed by a 

 material body, vibrations are set up which are perceived by the 

 senses as heat. 



The doctrine that heat is a material substance was main- 

 tained in Newton's lifetime by a certain school of chemists. The 

 most conspicuous member of the school was Wilhelm Homberg 

 (b. 1652, d. 1715) of Paris, who* identified heat and light with the 

 sulphureous principle, which he supposed to be one of the primary 

 ingredients of all bodies, and to be present even in the inter- 

 planetary spaces. Between this view and that of Newton it 

 might at first seem as if nothing but sharp opposition was to be 

 expected, j- But a few years later the professed exponents of the 

 Principia and the Opticks began to develop their system under 

 the evident influence of Homberg's writings. This evolution 

 may easily be traced in s'Gravesande, whose starting-point is 

 the admittedly Newtonian idea that heat bears to light a 

 relation similar to that which a state of turmoil bears to regular 

 rectilinear motion ; whence, conceiving light as a projection of 

 corpuscles, he infers that in a hot body the material particles 

 and the light-corpusclesj are in a state of agitation, which 

 becomes more violent as the body is more intensely heated. 



s'Gravesande thus holds a position between the two opposite 

 camps. On the one hand he interprets heat as a mode of 

 motion ; but on the other he associates it with the presence of 

 a particular kind of matter, which he further identifies with the 

 matter of light. After this the materialistic hypothesis made 



* Mem. del'Acad., 1705, p. 88. 



t Though it reminds us of a curious conjecture ofNewtoa'i: "Is not the 

 strength and vigour of the action between light and sulphureous bodies one reason 

 M-liy sulphureous bodies take fire more readily and burn more vehemently than 

 other bodies do? " 



J I have thought it best to translate s'Gravesande's ignis by " light-corpuscles." 

 This is, I think, fully justified by such of his statements as Quando ignis per 

 lineas rectas oculos nostros intrat, ex motu gttein fibris in fundo oculi cont/tninicai 

 ideam luminis excitat. 



