Page:Popular Science Monthly Volume 4.djvu/377

 is that of the decomposition of carbonic acid by the leaves of plants, under the influence of sunshine. On this the whole vegetable world depends for its growth, and the whole animal world, directly or indirectly, for its food. The decomposition in question is essentially a deoxidation, and up to about 1840 it was generally supposed to be due to the violet rays of the spectrum, which, in accordance with the views held at that time, were regarded as producing deoxidizing actions, and were consequently known as deoxidizing rays. But this was altogether an assumption unsupported by experimental proof. Prof. Draper saw that there was but one method for the absolute solution of the problem, and that was by causing the decomposition to take place in the spectrum itself. In this delicate and beautiful experiment he succeeded, and found that the decomposition was brought about by the yellow rays, at a maximum by those in the vicinity of the Fraunhofer fixed line and that the violet rays might be considered as altogether inoperative. The memoir containing this result was first read before the American Philosophical Society, in Philadelphia, and immediately republished in London, Paris, and Berlin. It excited general interest among chemists. Even up to the present year it has furnished to the German experimenters the basis of a very interesting discussion in photo-chemistry.

In 1842 Dr. Draper discovered that not only might the Fraunhofer fixed lines in the spectrum be photographed, but that there exists a vast number of others beyond the violet, which up to that time had been unknown. He also found three great lines less refrangible than the red, in a region altogether invisible to the eye. Of these new lines, which more than doubled in number those of Fraunhofer, he published engravings. He also invented an instrument for measuring the chemical force of light—the chlor-hydrogen photometer. This was subsequently extensively used by Bunsen and Roscoe in their photo-chemical researches. In their paper, read before the Royal Society, in 1856, they say, "With this instrument Draper succeeded in establishing experimentally some of the most important relations of the chemical action of light."

Most of the papers he had written up to 1844 were in that year collected and published together, in a book bearing the title of a treatise on "the Forces producing the Organization of Plants." In this there are a great many experiments on capillary attraction, the flow of sap, endosmosis, the influence of yellow light on plants, etc.

His memoir "On the Production of Light by Heat," published in 1847, was an important contribution to spectrum analysis; among other things it gave the means for determining the solid or gaseous condition of the sun, the stars, and the nebulæ. In this paper he established experimentally that all solid substances, and probably liquids, become incandescent at the same temperature; that the thermometric point at which such substances are red-hot is about 977° Fahr.; that the