November 7, 1907 J 



NA TURE 



of the tube, in such a manner that a permanent im- 

 pression on the memory of the student is made. 



As an outcome of work recorded in Crookes's 

 various preceding papers, " On Repulsion resulting 

 from Radiation," ilc, and, therefore, with paragraphs 

 numbered in continuation of his " Phosphorogenic 

 Properties of Molecular Discharge," Crookes in 1881 

 jmblished a research on " The Viscosit)' of Gases at 

 High Exhaustion." Maxwell's great theoretical dis- 

 covery that the viscosity of a gas is independent of 

 the density, one of the most beautiful proofs for the 

 reality of molecular motion, had already been the start- 

 ing-point of experiments by Maxwell himself, Kundt 

 and Warburg, using the method of rotating discs. 



In Crookes's experiments the method of observ- 

 ation consisted in noticing the subsidence of the 

 vibrations of a delicately suspended lamina oscillating 

 within a bulb containing the gas. By these simple 

 yet adequate means, very careful measurements were 

 made, and the falling off of the viscosity of different 

 gases from atmospheric pressure to very high exhaus- 

 tions downwards observed, especial attention being 

 paid to the highest vacua and definite measurements 

 made of the degree of exhaustion employed. At these 

 high exhaustions Maxwell's law completely breaks 

 down, as Maxwell himself foresaw. His observations 

 were discussed in a splendid " note " by Sir George 

 Stokes, another example of the cooperation between 

 these physicists. 



Crookes's apparatus afforded at the same time 

 many other data and measurements. The apparent 

 attraction by heat was only found in air of 

 greater than one-thousandth part of ordinary density ; 

 while there is repulsion when the density is further 

 increased, the repulsion increasing to a maximum, and 

 thence fading away towards zero as the rarefaction is 

 continued. 



In 1881 Crookes's paper on radiant matter spectro- 

 scopy appeared. An entirely new method of spectrum 

 analysis is given, based on the well-known fact that 

 under the influence of the kathode rays a large number 

 of substances emit phosphorescent light, some faintly 

 and others with great intensity. Most bodies give a 

 faint continuous spectrum, but more rarely the spec- 

 trum of the phosphorescent light is discontinuous, 

 and to bodies manifesting it his attention has been 

 specially directed. This characteristic spectrum is 

 given by the group of elements known as the rare 

 earths, especially yttria in some of its compounds ; 

 and in the study of this group the method is of very 

 great importance, and has given, in the hands of Sir 

 William Crookes, at an immense arhount of trouble 

 and time, very valuable results. To give, however, an 

 adequate survey of these investigations would demand 

 much space, and uncommon chemical knowledge ot 

 the rare earths. We mention only that not long ago 

 Crookes isolated from yttria a new earth, character- 

 ised by an isolated strong group of lines high up in 

 the ultra-violet, ascribed by Sir William to a new 

 element named by him victorium. 

 NO. 1984, VOL. 77] 



In connection with his work on the photographed 

 spectra of the elements, of which it seems only a 

 small portion has been published, we record one of 

 his smaller papers, relating to " the slit of a spectro- 

 scope," that narrow, but extremely important, gate 

 to a large domain. Crookes makes the very ingenious 

 suggestion to use quartz jaws, cut in the same manner 

 as metal ones. The prismatic edges refracting away 

 all the light which falls on them, their transparency 

 offers no objection, while only the light passing 

 between the jaws comes into operation. As the quartz 

 jaws can be worked to a finer edge, they give better 

 definition. 



" With a pair of jaws in the spectroscope at present 

 in use, I can take excellent photographs when they 

 are only o'oooi inch apart. For eye observation the 

 width can easily be less than that." 



Another small paper of date 1879 is also character- 

 istic of Crookes's experimental skill, and illustrates 

 at the same time, if I may say so, the purity of his 

 work. The exceedingly small rate of leak of elec- 

 tricity in a high vacuum is illustrated by Crookes's 

 observation that a pair of gold leaves in a vacuum bulb 

 retains an electrical charge for months. 



Of Crookes's recent work, we mention his e.xperi- 

 mental work on radium. In 1900 Crookes first effected 

 the separation from uranium by two distinct chemical 

 methods of the one direct transformation product, called 

 uranium X. He discovered in 1903 that the alpha rays 

 from radium produce, probably by their bombardment, 

 phosphorescence on a target of crystalline zinc sul- 

 phide. This wonderful phenomenon, perhaps the 

 most direct proof of the discontinuous structure of 

 matter, was popularised in his spintharoscope. 



These examples must suffice to impart an idea of 

 Crookes's work. " The best history," it has been verily 

 said, " is but like the art of Rembrandt; it casts a vivid 

 light on certain selected causes, on those which were best 

 and greatest ; it leaves all the rest in shadow and un- 

 seen. " What is true in the science of history cannot be- 

 come untrue in the history of science. It would be desir- 

 able to follow a similar precept in trying to picture before 

 our mind the origin of the gratitude and admiration 

 phvsicists feel for a philosopher, who by his experi- 

 mental skill, his acute observation, and the con- 

 tinuity of his endeavours, combined with his daring 

 intuition, has impressed indelible marks in different 

 branches of physics and chemistry. This involves, 

 however, more than we can attempt here. 



Sir William Crookes is a member or corresponding 

 member of a number of scientific societies in his own 

 country and abroad. \t one time or another he has 

 occupied the presidential chair of many of the leading 

 learned and scientific societies of Great Britain. The 

 Royal Society awarded him a Royal Medal in 1S75, 

 the Davy Medal in 188S, the Copley Medal in 1904 ; the 

 French Academic des Sciences, a gold medal and a 

 prize in 1880 ; the Society of .\rts, the .'Mbert Medal in 

 1899 ; and he was knighted by the late Queen Victoria 

 in 1897. P. Zeeman. 



