The methods of determining the elementary bodies in organic compounds have been developed by Dumas, Liebig, Will, Varrentrap, and Kjeldahl, to the last of whom chemical analysis owes a debt of gratitude for the invention of a speedy and accurate method of determining nitrogen. Not much less is the debt due to Gooch for the invention of the perforated platinum crucible, carrying an asbestos felt for securing precipitates by filtration, in a form suitable to ignition without further preparation. [Illustration: WILLIAM CROOKES, F. R. S.] Through the classic researches of Arago (1786–1853) and Biot (1774–1862), polarized light has been made a most valuable adjunct to chemical research, serving as it does to measure the quantity of various alkaloids, essential oils, and sugars. Based on these researches of Biot and Arago, Ventzke, Soleil, Scheibler, Duboscq, Landolt, and Lippich have constructed apparatus, which have made an exact science of optical saccharimetry. Optical analysis is not without its relation to theoretical chemistry, for by it has been proved the assumption that optically active bodies contain an asymmetrical carbon atom,—that is, one which combines with four different atoms or radicles. Electricity has become also one of the most useful factors in chemical analysis. Many metals are easily deposited by electrolytic action, and their separation and determination rendered easy and certain. Chemical analysis has not only given us accurate knowledge of the constituents of matter, but by revealing the deportment of molecules and groups of molecules in inorganic and organic compounds, has opened up a path for organic and synthetic chemistry which otherwise must have remained forever closed. The discovery and development of spectrum analysis is one of the great achievements of the nineteenth century in chemical science. Wollaston, in 1802, first noticed that the spectrum of the sun’s light, when greatly magnified, was not composed of colors gradually changing from one to the other, but that the continuity of the colors was interrupted by dark bands. Fraunhofer, in 1814, had made a map of the solar spectrum, showing 576 of these dark lines. Fraunhofer was entirely ignorant of the cause of these dark lines, but when he had found them, not only in the light from the sun, but also from the moon and the fixed stars, he properly concluded that they were due to something entirely independent of the earth.