36 Electric and Magnetic Science 



proved that the temperature of a body is without sensible 

 influence on its weight. 



Perhaps nothing in the history of natural philosophy is more 

 amazing than the vicissitudes of the theory of heat. The true 

 hypothesis, after having met with general acceptance throughout 

 a century, and having been approved by a succession of illus- 

 trious men, was deliberately abandoned by their successors 

 in favour of a conception utterly false, and, in some of its 

 developments, grotesque and absurd. 



We must now return to s'Gravesande's book. The pheno- 

 mena of combustion he explained by assuming that when a body 

 is sufficiently heated the light-corpuscles interact with the 

 material particles, some constituents being in consequence sepa- 

 rated and carried away with the corpuscles as flame and smoke. 

 This view harmonizes with the theory of calcination which had 

 been developed by Becher and his pupil Stahl at the end of the- 

 seventeenth century, according to which the metals were sup- 

 posed to be composed of their calces and an element phlogiston. 

 The process of combustion, by which a metal is changed into its- 

 calx, was interpreted as a decomposition, in which the phlogiston 

 separated from the metal and escaped into the atmosphere ; 

 while the conversion of the calx into the metal was regarded as 

 a union with phlogiston.* 



s'Gravesande attributed electric effects to vibrations induced 

 in effluvia, which he supposed to be permanently attached to 

 such bodies as amber. " Glass," he asserted, " contains in it, and 

 has about its surface, a certain atmosphere, which is excited by 

 Friction and put into a vibratory motion ; for it attracts and 



* The correct idea of combustion had been advanced by Hooke. "The disso- 

 lution of inflammable bodies," he asserts in the Micrographia, " is performed by a 

 substance inherent in and mixed with the air, that is like, if not the very same 

 with, that which is fixed in saltpetre." But this statement met with little favour 

 at the time, and the doctrine of the compound nature of metals survived in full 

 vigour until the discovery of oxygen by Priestley and Scheele in 1771-5. In 1775 

 Lavoisier reaffirmed Hooke's principle that a metallic calx is not the metal minus 

 phlogiston, but the metal plus oxygen; and this idea, which carried with it the 

 recognition of the elementary nature of metals, was generally accepted by the end' 

 of the eighteenth century. 



