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 rapid progress. It was frankly advocated by another member of the Dutch school, Hermann Boerhaave (b. 1668, d. 1738), Professor in the University of Leyden, whose treatise on chemistry was translated into English in 1727.

Somewhat later it was found that the heating effects of the rays from incandescent bodies may be separated from their luminous effects by passing the rays through a plate of glass, which transmits the light, but absorbs the heat. After this discovery it was no longer possible to identify the matter of heat with the corpuscles of light; and the former was consequently accepted as a distinct element, under the name of calorie. In the latter part of the eighteenth and early part of the nineteenth centuries caloric was generally conceived as occupying the interstices between the particles of ponderable matter—an idea which fitted in well with the observation that bodies commonly expand when they are absorbing heat, but which was less competent to explain the fact that water expands when freezing. The latter difficulty was overcome by supposing the union between a body and the calorie absorbed in the process of melting to be of a chemical nature; so that the consequent changes in volume would be beyond the possibility of prediction.

As we have already remarked, the imponderability of heat did not appear to the philosophers of the eighteenth century to be a sufficient reason for excluding it from the list of chemical elements; and in any case there was considerable doubt as to whether caloric was ponderable or not. Some experimenters believed that bodies were heavier when cold than when hot; others that they were heavier when lot than when cold. The century was far advanced before Lavoisier and Rumford finally D 2