92 Anniversary Meeting, [Nov. 30, 



tellurium. The element had as yet been found in extremely small 

 quantities only; but yet he was able to prove chemically as well as 

 optically that it was a new substance, which, from some of its reactions, 

 he thought most probably belonged to the sulphur group of elements, 

 and to which he gave the name Thallium. 



The investigation of this new substance was eagerly followed out ; 

 and by inquiries in various directions he succeeded in obtaining it in 

 larger quantities, though his supply was still most inconveniently small, 

 rendering it necessary to work the same material over and over again, 

 instead of carrying on the investigation of two or more of its compounds 

 simultaneously. This investigation, which had led him to perceive that 

 the new body was a metal, which he isolated, was in progress when (in 

 the summer of 1862) he was induced to send in a preliminary notice of 

 his investigations to the Eoyal Society, instead of waiting, as he had 

 intended, till the research was complete, in consequence of the publication 

 by M. Lamy (who had been fortunate enough to meet with a compara- 

 tively fertile source of the new substance) of a paper in which he described 

 several of its compounds and announced its metallic nature. The com- 

 plete memoir was presented to the Eoyal Society early in 1863, and is 

 published in the * Philosophical Transactions ' for that year. It contains, 

 among other things, a provisional determination of the atomic weight, 

 giving a mean result of 203*96. 



The atomic weight of an element being a constant of fundamental 

 importance, Mr. Crookes spared no pains in an accurate determination 

 of that of thallium. This research, which extended over several years, 

 is described in a highly elaborate memoir which was presented to the 

 Eoyal Society in 1872, and is published in the ' Philosophical Transac- 

 tions ' for the following year. The various weighings which the investi- 

 gations required were performed and reduced with the most scrupulous 

 regard to minute accuracy, as the utmost care was taken in the prepara- 

 tion of the chemicals. Though only one method was employed, namely 

 that of determining the weight of nitrate of thallium obtained from a 

 known weight of the metal, the care with which the whole process was 

 conducted, and the near agreement of the different experiments, are such 

 'as to lead us to regard the final mean 203*642 as most probably very 

 near the truth. 



In the course of the weighings which were executed in this research, 

 Mr. Crookes noticed some minute anomalies depending on temperature 

 which did not seem referable to currents of convection. This led him 

 into a train of experimental research which resulted in the discovery of 

 an action of radiation which, whatever be its real nature (a point still 

 involved in mystery), and whether the action be direct or indirect, all 

 who have seen or followed the experiments must allow to be most re- 

 markable. An account of this research was published in the ' Philoso- 

 phical Transactions ' for 1874. 



