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 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 maintained 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. 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-corpuscles 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