74 o THE POPULAR SCIENCE MONTHLY. 



its occurrence, distribution, and the method of extraction from the 

 ore, together with its physical characteristics and chemical proper- 

 ties. 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 separat- 

 ing gold and silver from their ores. (This process was discovered in- 

 dependently, 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 ar- 

 resting 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 ap- 

 paratus 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 re- 

 sult of his investigation. At first it was thought that the movement 

 of the vanes in the exhausted bulb was due to radiation, for no move- 

 ment 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 mo- 

 tion. That this residual gas was not a mere accidental accompani- 

 ment of the phenomena was rendered probable both by the experi- 

 ments 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 ro- 



