A WEEKLY ILLUSTRATED JOURNAL OF SCIENCE 



" To the solid ground 

 Of Nature trusts the mind which builds for aye." — Wordsworth. 



THURSDAY, NOVEMBER 7, 1907. 



SCIENTIFIC WORTHIES. 

 XXXVI.— Sir William Crookes, F.R.S. 



SIR WILLIAM CROOKES has the rare privilege 

 of looking back upon a scientific activity ex- 

 tending already over more than fifty-five years. By 

 numerous papers and by several volumes the results 

 of his experimental researches in different departments 

 of physics and chemistry have been spread all over the 

 world. Though born in 1832, even his advanced age 

 has not diminished his scientific productiveness. 



All Sir William Crookes 's researches, with the 

 exception of the first, were made in his private 

 laboratory in Kensington Park Gardens. Although 

 the motion of the walls of this laboratory, as seen 

 under the high magnifying power of the horizontal 

 pendulum, gave rise, at first sight, to doubts as to the 

 solidity of its construction (Philosophical Transactions, 

 1876, Crookes, " On Repulsion, &c.," § 134), it has 

 stood the test of lime. The perennial stability, however, 

 of many of the stones joined by Crookes to the edifice 

 of science never was questionable. Most of those who 

 have risen to eminence in physics have done so by 

 giving their exclusive attention to that science, and 

 it is only rarely that the physicist can do pioneer work 

 also in chemistry. Rarer still is the case of Sir William 

 Crookes, whose series of physical papers is frequently 

 interrupted by communications concerning his chem- 

 ical discoveries. 



In the Philosophical Magazine of April, 1861, 

 Crookes tells us : 



" In the year 1850, Prof. Hofmann placed at my 

 disposal upwards of 10 lb. of the seleniferous deposit 

 from thf sulphuric acid manufactory at Tilkerode, in 

 the Hartz Mountains, for the purpose of extracting 

 from it the selenium, which was afterwards employed 

 in an investigation of the selenocyanides." 



In the examination, by the spectroscope, of the residue 

 left in the purification of the crude selenium, Crookes's 



NO. 1984, VOL. yy] 



attention was attracted by a bright green line, which 

 he had never met with before. In following up its 

 appearance, he succeeded in isolating a new metal, 

 which he called thallium, after the emerald green line 

 which has become now as familiar to chemists, even 

 if not brought up in a spectroscopic atmosphere, as 

 the lines of sodium and lithium ; and the physicist 

 again and again enjoys the homogeneity of thallium 

 light when observing interference for large differences 

 of path, either with his Rowland or his Michelson 

 grating, or with his Fabry and Perot apparatus, or 

 with his Lummer and Gehrcke plate. 



The year 1861 brought the first great triumph to 

 Crookes. During the next twelve years he carried 

 out minute investigations of the many properties of the 

 new element, culminating in his determination of its 

 atomic weight — 203'642, or when reduced with the 

 now accepted values for the atomic weights of oxvgen 

 and nitrogen, 204*04. Extreme care was given to the 

 necessary weighings, and the pains taken to start 

 with pure substances were enormous. The inter- 

 national committee for the atomic weights and other 

 authorities regard Crookes's determination of the 

 atomic weight of thallium as the best we possess, 

 though thirty-four years have elapsed since the date 

 of its publication. 



Crookes finished his determination not without tribu- 

 lation, having been troubled with discouraging irre- 

 gularities in his weighings. In order to improve his 

 results, the weighings were made in a partial vacuum, 

 but even under these conditions the balance behaved 

 most capriciously. Sometimes the substance appeared 

 to be heavier when cold than when in a heated con- 

 dition ; sometimes the action was opposite. Working 

 further with indefatigable ardour he came to what 

 he then called "repulsion resulting from radiation," 

 and going on he invented in 1S75 ''f apparatus in 

 illustration of the thoroughly novel and striking pheno- 

 mena he had observed, the radiometer. His researches 

 in this new field, contained in 485 paragraphs, and 

 published in the Philosophical Transactions of 1874. 

 1S75, 1876, 1S7S, 1879, represent an immense amount 



B 



