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 mathematica experimentis confirmata of Wilhelm Jacob s'Gravesande (b. 1688, d. 1742), published at Leyden 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. tho 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 calees[1] of the metals figured in the list, while almost all the chemical elements now recognized were
- ↑ i.e, oxides.
