DISCOVERY 



217 



Sir William Grookes, O.M. 



The Discoverer of Thallium 



Sir William Crookls's death in the spring of last 

 vear at the advanced age of eighty-seven seemed to 

 break the last link of the chain of great chemists of the 

 past generation. He was the son of Joseph Crookes, 

 who came to London a poor lad and worked his way to 

 a man of considerable means by his own ability, so 

 that he was able to indulge his son's whim to study 

 chemistry, a profession which, it must be confessed, is 

 still little more than a hobby. 



At the early age of sixteen he entered the Royal 

 College of Chemistry under Hofmann and at once 

 showed signs of his great ability in research, publishing 

 his first paper in the Quarterly Jourjial of the Chemical 

 Society at the age of only nineteen. This was on the 

 Selenocyanides, and the seleniferous material he used 

 in this research was given him by his chief and came 

 from the sulphuric acid works at Tilkerode in the 

 Hartz ; the material is of interest apart from the paper 

 above referred to, for it was while examining it with 

 the spectroscope some years later that Crookes made 

 the disco\-ery which will always be associated with his 

 name. The spectroscope was first used as an instru- 

 ment of research by Bunsen and Kirchhoff in i860, 

 and it was in March of 1861 that Crookes was impelled 

 to look at his seleniferous material with the instrument, 

 with the result that he noticed a pecuhar green line, 

 which was found to be caused by the new element 

 Thalhum. He prepared and e.xhibited specimens 

 of this new metal for the International Exhibition of 

 1862 and gained a medal for his exhibit. The next 

 year he was elected a Fellow of the Royal Society. The 

 extreme care expended on the determination of the 

 atomic weight of the new element well illustrates the 

 accuracy which characterised all his work. In this 

 determination he made use of the vacuum balance 

 and encountered some quite unexpected phenomena, 

 which on investigation led to his second discovery 

 of the Radiometer in the year 1875. These little 

 instruments, which one used to see in the shop wndows 

 of the scientific instrument makers in London, give 

 one the nearest idea of perpetual motion that one 

 can get, as one does not at first realise that it is the 

 light which makes the small cup-like vanes rotate on 

 theu- pivot in the httle glass bulbs from which the air 

 has been withdrawn. 



Crookes next turned his attention to the study of the 



"phenomena exhibited by the electric discharge in 



rarefied gases, and more especially to the dark space 



around the negative pole, which has since been 



referred to as the Crookes or Cathode Dark Space. 



In i88o his discoveries in molecular phj'sics and 



{Continued on p. 2lS 



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