32 Electric and Magnetic Science 



after the time of Newton the doctrine of the return of the- 

 electric effluvia gradually lost credit. 



Newton died in 1727. Of the expositions of his philosophy 

 which were published in his lifetime by his followers, one at 

 least deserves to be noticed for the sake of the insight which 

 it affords into the state of opinion regarding light, heat, and 

 electricity in the first half of the eighteenth century. This was 

 the Physices elementa matlwmatica experimentis confirmata of 

 Wilhelm Jacob s'Gravesande (b. 1688, d. 1742), published at 

 Ley den in 1720. The Latin edition was afterwards reprinted 

 several times, and was, moreover, translated into French and 

 English : it seems to have exercised a considerable and, on the 

 whole, well-deserved influence on contemporary thought. 



s'Gravesande supposed light to consist in the projection of 

 corpuscles from luminous bodies to [the eye ; the motion being 

 very swift, as is shown by astronomical observations. Since 

 many bodies, e.g. the metals, become luminous when they- -are 

 heated, he inferred that every substance possesses a natural 

 store of corpuscles, which are expelled when it is heated to 

 incandescence ; conversely, corpuscles may become united to a 

 material body ; as happens, for instance, when the body is exposed 

 to the rays of a fire. Moreover, since the heat thus acquired is 

 readily conducted throughout the substance of the body, he 

 concluded that corpuscles can penetrate all substances, however 

 hard and dense they be. 



Let us here recall the ideas then current regarding the 

 nature of material bodies. From the time of Boyle (1626-1691) 

 it had been recognized generally that substances perceptible to 

 the senses may be either elements or compounds or mixtures ; 

 the compounds being chemical individuals, distinct from mere 

 mixtures of elements. But the substances at that time accepted 

 as elements were very different from those which are now known 

 by the name. Air and the calces* of the metals figured in the 

 list, while almost all the chemical elements now recognized were 



