NA TURE 



[November 7, 1907 



of experimental work of the greatest interest and 

 ingenuity. 



Under the influence of the dynamical theory of 

 gases the general nature of the perplexing phenomena 

 was recognised and referred to the intervention of 

 the residual gas. The genius of Schuster, Osborne 

 Reynolds, Tait, Dewar, and Maxwell was associated 

 with this explanation, but special mention should 

 here be made of the more personal, yet beautiful 

 and ennobling example of scientific cooperation given 

 by Sir William Crookes and Sir George Stokes, the 

 documents relating to which have just been published. 

 The new and fascinating chapter in the dynamical 

 theory of gases, relating to the stresses in rarefied 

 gases arising from inequalities in temperature, which 

 thus sprang up in connection with Crookes 's experi- 

 mental work, is, notwithstanding the no references to 

 the literature of the radiometer in a modern German 

 text-book, still unfinished. We may be sure that quan- 

 titative experiments concerning the radiometer actions 

 under entirely new conditions will again prove the 

 importance of the chapter, emblazoned on its cover by 

 Crookes 's light-mill. 



Crookes thus was brought into touch with the 

 dynamical theory of gases and with experimental work 

 in high vacua, and so came to his experiments con- 

 cerning the electric discharge in gases. In this pro- 

 vince we are indebted to him for some very striking 

 discoveries relating to the now well-known kathode 

 rays, then already associated with the names of 

 Plucker {1S59), Hittorf (i86g), and Goldstein (1876). 

 His brilliant experiments (" The Trajectory of Mole- 

 cules," " Molecular Physics in High Vacua," " Phos- 

 phorogcnic Properties of Molecular Discharge ") were 

 published in the Philosophical Transactions for 1879, 

 but became generally known to the world — not to the 

 scientific world alone — by his lecture on " Radiant 

 Matter," delivered on Friday, August 22, 1879, at Shef- 

 field, to the British Association for the Advancement 

 of Science. Even now the reading of this lecture, 

 though the facts in it have become familiar, brings one 

 under its irresistible charm, and Lenard and Tesla, 

 describing in eloquent terms the impression made by 

 it on their young minds, certainly give utterance 

 to a prevalent opinion. In the beautiful volumes on 

 " Ions, Electrons, Corpuscules," for which physi- 

 cists are indebted to the Soci^t^ frangaise de Physique, 

 only one lecture has been inserted, that of Sir William. 

 There exists perhaps only one lecture given on 

 a similar occasion which has become as popular and 

 made on the hearers as deep an impression, both by 

 its contents and its accomplished form; I mean the 

 lecture delivered before the Association of German 

 Naturalists at Stuttgart in 1889 by Hertz, in which 

 his great discoveries were expounded. 



All the wonderful and important properties of the 

 constituents of the kathode rays or of radiant matter : 

 its darling in a straight line from the negative pole, 

 the position of the positive electrode being unim- 

 portant; its casting of a shadow when intercepted by 

 NO. 1984, VOL. 77] 



solid matter ; the strong mechanical action radiant 

 matter seems to exert where it strikes ; the change of 

 direction by a neighbouring magnet ; the heat pro- 

 duced when its motion is arrested ; the remarkable 

 power which the molecular rays possess of causing 

 phosphorescence in preparations of calcium sulphide 

 shining with blue-violet, yellow, orange or green 

 light, in diamonds shining with nearly all colours of 

 the rainbow, in rubies glowing with a rich full red ; 

 all these results Crookes tried to explain by the hypo- 

 thesis that the kathode raj-s, or streams of radiant 

 matter, or of matter in an ultra-gaseous state are 

 particles or molecules negatively charged and pro- 

 jected with great velocity from the negative electrode. 

 The inherent truth of Sir William Crookes 's hypo- 

 thesis concerning the nature of the kathode rays is, 

 after much controversy for a space of nearly twenty 

 years, now established, and the original hypothesis, 

 with finer contents, is now accepted by all physicists. 



In Crookes's experiments for the first time the 

 majestic simplicity of the kathode rays became clearly 

 apparent. In the irritating complexity of the other 

 phenomena of the vacuum tube, appearances of great 

 purity had been isolated, so that Crookes could risk 

 the opinion " that we are here brought face to face 

 with Matter in a Fourth state or condition," neither 

 solid, liquid, nor gaseous. 



Crookes alone among his contemporaries recog- 

 nised the essential importance of the kathode rays, and 

 with almost prophetic insight foresaw the part 

 radiant matter would have to play in the development 

 of physical science. In the splendid evolution of 

 electronic theory we are now witnessing, we see how- 

 true Crookes's foreshadowing of the r6\c of radiant 

 matter was. 



" In studying this Fourth stale of Matter, we seem 

 at length to have within our grasp and obedient to 

 our co'ntrol the little indivisible particles which, with 

 good warrant, are supposed to constitute the physical 

 basis of the universe. We have seen that in some of 

 its properties Radiant Matter is as material as this 

 table, whilst in other properties it almost assumes the 

 character of Radiant Energy. We have actually touched 

 the border land where Matter and Force seem to 

 merge into one another, the shadowy realm between 

 Known and Unknown, which for me has always had 

 peculiar temptations. I venture to think that the 

 greatest scientific problems of the future will find their 

 solution in this Border Land, and even beyond; here, it 

 seems to me, lie Ultimate realities, subtle, far-reaching, 

 wonderful. 



" Yet all these were, when no Man did them know, 

 Yet have from wisest .Ages hidden beene ; 



And later Times thinges more unknowne shall show. 

 Whv then should witlesse Man so much miswecne. 



That nothing is, but that which he hath scene? " 



All the experiments in this lecture now have' become 

 classical, and several of them are repeated every 

 year in every university of the world. The most 

 familiar and representative of the group is perhaps 

 that one with the Maltese cross in the pear-shaped 

 Crookes's tube, in which the black shadow of the cross 

 is projected on the hemispherical phosphorescent end 



