Page:Popular Science Monthly Volume 10.djvu/760

740 its occurrence, distribution, and the method of extraction from the ore, together with its physical characteristics and chemical properties. He also discussed the position of thallium among elementary bodies, and gave a series of analytical notes on the new metal. In the Journal of the Chemical Society for April, 1864, he collated all the information then extant, both from his own researches and from those of others, introducing qualitative and quantitative descriptions of an extended series of the salts of the metal. In June, 1872, he laid before the Royal Society the details and results of experiments which had occupied much of his time during the previous eight years, and which consisted of laborious researches on the atomic weight of thallium.

In 1863 Mr. Crookes was elected a Fellow of the Royal Society. In 1865 he discovered the sodium amalgamation process for separating gold and silver from their ores. (This process was discovered independently, and at about the same time, by Prof. Henry Wurtz, of New York.) In 1866 he was appointed by the English Government to inquire into and report upon the application of disinfectants in arresting the spread of the cattle-plague then prevalent in England. In 1871 he was selected as a member of the English expedition to Oran for observing the total phase of the solar eclipse which occurred in December of that year.

Mr. Crookes commenced his research on "Repulsion resulting from Radiation" in 1872. These experiments were suggested by some observations made when weighing heavy pieces of glass apparatus in a vacuum balance during his researches on the atomic weight of thallium. His first paper on the subject was read before the Royal Society on December 11, 1873, and during the last three years Mr. Crookes has sent six other communications to the society on the same subject. The construction of the radiometer is one result of his investigation. At first it was thought that the movement of the vanes in the exhausted bulb was due to radiation, for no movement took place until the vacuum was so good as to be almost beyond the powers of an ordinary air-pump to produce, and as the vacuum got more and more absolute, so the force increased in power; but Mr. Crookes soon found that at a rarefaction so high that the residual gas was a non-conductor of an induction-current, there was enough matter present to produce motion, and therefore to offer resistance to motion. That this residual gas was not a mere accidental accompaniment of the phenomena was rendered probable both by the experiments of Dr. Schuster and by that of Mr. Crookes, on the movement of the floating glass case of a radiometer when the arms are fixed by a magnet, which was demonstrated to the Royal Society on March 30, 1876. Mr. Crookes has since constructed a special apparatus for measuring the vacuum. A vertical plate, instead of continuously