December 7, 1893] 



NATURE 



\'\^ 



of interference between ordinary and reflected waves, and posi- 

 tive electricity appearing where negative might have been ex- 

 pected, we hear first, it seems, in Herr von Bezold's "Researches 

 on the Electric Discharge" (1870), which Hertz gives as the 

 third paper of his collection, with interesting and ample recog- 

 nition of its importance in relation to his own work. 



In connection with the practical development of magnetic 

 waves, you will, I am sure, be pleased if I call your attention to 

 two papers by Prof. G. F. Fitzgerald, which I heard myself at 

 the meeting of the British Association at Southport in 18S3. 

 One of them is entitled "On a Method of Producing Electro- 

 magnetic Disturbances of comparatively Short Wave-lengths." 

 The paper itself is not long, and I shall read it to you in full, 

 from the " Report of the British Association," 18S3 : " This is 

 by utilising the alternating currents produced when an accumu- 

 lator is discharged through a small resistance. It is possible to 

 produce waves of as little as two metres wave-length, or even 

 less." This was a brilliant and useful suggestion. Hertz, not 

 knowing of it, used the method ; and, makingas little as possible 

 of the "accumulator," got waves of as little as 10 cm. wave- 

 length in many of his fundamental experiments. The title alone 

 of Fitzgerald's other paper, "On the Energy Lost by Radiation 

 from Alternating Currents," is in itself a valuable lesson in the 

 electromagnetic theory of light, or the undulatory theory of 

 magnetic disturbance. It is interesting to compare it with the 

 title of Hertz's eleventh paper, "Electric Radiation"; but I 

 cannot refer to this paper without expressing the admiration 

 and delight with which I see the words " rectilinear propaga- 

 tion," "polarisation," "reflection," "refraction," appearing 

 in it as sub-titles. 



During the fifty-six years which have passed since Faraday fir^t 

 offended physical mathematicians with his curved lines of force, 

 many workers and many thinkers have helped to build up the 

 nineteenth century school of pleniun ; one ether for light, heat, 

 electricity, magnetism ; and the German and English volumes 

 containing Hertz's electrical papers, given to the world in the 

 last decade of the century, will be a permanent monument of 

 the splendid consummation now realised. 



But, splendid as this consummation is, we must not fold our 

 hands and think or say there are no more worlds to conquer for 

 electrical science. We do know something now of magnetic 

 waves. We know that they exist in nature, and that they are 

 in perfect accord with Maxwell's beautiful theory. But this 

 theory teaches us nothing of the actual motions of matter con- 

 stituting a magnetic wave. Some definite motion of matter 

 perpendicular to the lines of alternating magnetic force in the 

 waves and to the direction of propagation of the action through 

 space, there must be ; and it seems almost satisfactory as a 

 hypothesis to suppose that it is chiefly a motion of ether with a 

 comparatively small but not inconsiderable loading by fringes ot 

 ponderable molecules carried with it. This makes Maxwell's 

 " electric displacement " simply a to-and-fro motion of ether 

 across the line of propagation, that is to say, precisely the vibra- 

 tions in the undulatory theory of light according to Fresnel. 

 But we have as yet absolutely no guidance towards any under- 

 standing or imagining of the relation between this simple and 

 definite alternating motion, or any other motion or displace- 

 ment of the ether, and the earliest known phenomena of 

 electricity and magnetism — the electrification of matter, and the 

 attractions and repulsions of electrified bodies ; the permanent 

 magnetism of lodestone or steel, and the attractions and repul- 

 sions due to it ; and certainly we are quite as far from the clue 

 to explaining, by ether or otherwise, the enormously greater 

 forces of attraction and repulsion now so well known aftec the 

 modern discovery of electromagnetism. 



Fifty years ago it became strongly impressed on my mind 

 that the difference of quality between vitreous and resinous 

 electricity, conventionally called positive and negative, essentially 

 ignored as it is in the mathematical theories of electricity and 

 magnetism with which I was then much occupied (and in the 

 whole science of magnetic waves as we have it now), must be 

 studied if we are to learn anything of the nature of electricity 

 and its place among the properties of matter. This distinction, 

 essential and fundamental as it is in frictional electricity, electro- 

 chemistry, thermo-electricity, pyro-electricity of crystals, and 

 piezo-electricity of crystals, had been long olDserved in the old 

 known beautiful appearances of electric glow and brushes and 

 sparks from points and corners on the conductors of ordinary 

 electric machines and in exhaustive receivers of air-pumps with 

 electricity passed through them. It was also known, probably 



as many as fifty years ago, in the vast difference of behaviour of 

 ' the positive and negative electrodes of the electric arc lamp. 

 : Faraday gave great attention to it ' in experiments and observa- 



■ tions regarding electric sparks, glows, and brushes, and parti- 



I cularly in his "dark discharge" and "dark space" in the 



1 neighbourhood of the negative electrode in partial vacuum. In 

 [1523] of his I2th series, he says, "The results connected with 

 the different conditions of positive and negative discharge will 

 have a far greater influence on the philosophy of electrical 

 science than we at present imagine." His " dark discharge " 

 ([1544-1554]) through space around or in front of the negative 

 electrode was a first instalment of modern knowledge in that 

 splendid field of experimental research which, fifteen years 

 later, and up to the present time, has been so fruitfully cultivated 

 by many of the able scientific experimenters of all countries. 



i The Royal Society's Transactions and Proceedings of the last 

 years contain, in the communications of Gassiot,- Andrews and 



\ Tait,'^ Cromwell Varley,'* De La Rue and Miiller,'' Spottis- 

 woode,'' Moultan," Plucker,'^ Crookes," Grove, ^"^ Robinson, ^^ 

 Schuster,^- J. J. Thomson, i'* and Fleming,^"* almost a complete 

 history of the new province of electrical science which has grown 

 up largely in virtue of the great modern improvements in 

 practical methods for exhausting air from glass vessels, by> which 



I we now have " vacuum tubes " and bulbs containing less than 

 1/190,000 of the air which would be left in them by all that could 



^ be done in the way of exhausting (supposed to be down to i 

 mm. of mercury) by the best air-pump of fifty years ago. A 

 large part of the fresh discoveries in this province have been 

 made by the authors of these communications, and their 

 references to the discoveries of other workers very nearly com- 

 plete the history of all that has been done in the way of investi- 

 gating the transmission of electricity through highly rarefied air 



I and gases since the time of Faraday. 



Varley's short paper of 1871, which, strange to say, has lain 

 almost or quite unperceived in the Proceedings during the twenty- 

 two years since its publication, contains an admirable first in- 

 stalment of discovery in a new field— the molecular torrent from 

 the "negative pole," the control of its course by a magnet, its 

 pressure against either end of a pivoted vane of mica according 

 as it is directed by a magnet to one end or the other, the shadow 



; produced by its interception by a mica screen. Quite indepen- 

 dently of Varley, and not knowing what he had done, Crookes 



! was led to the same primary discovery, not by accident, and not 

 merely by experimental skill and acuceness of observation. He 

 was led to it by carefully designed investigation, starting with an 



\ examination of the cause of irregularities which had troubled '" 



\ him in his weighing of thallium ; and, going on to trials for im- 

 proving Cavendish's gravitational measurement, in the course of 



I which he discovered that the seeming attraction by heat is only 



■ found in air of greater than i/iooo^'' of ordinary density ; and 

 that there is repulsion increasing to a maximum when the density 

 is decreased from i/iooo to 36/1,000,000, and thence diminish- 

 ing towards zero as the rarefaction is farther extended to density 

 1/20,000,000. From this discovery Crookes came to his radio- 

 meter, fir>t without and then with electrification, powerfully 

 aided by Sir George Stokes.'" As he went on he brought all his 

 work more and more into touch with the kinetic theory of gases ; 



I so much so that when he discovered the molecular torrent he 



I "Experimental Researches," Series 12 and 13, Jan. and Feb. 1838. 

 - Roy. Soc. Proc, vol. 10, i860, pp. 36, 269, 274, 432. 



•' Roy. Soc. Proc, vol. 10, i860, p. 274 ; Phil. Trans. 

 I ^ Roy. Soc. Proc, vol. ig. 1S71, p. 236 



5 Roy. Soc. Proc, vol. 23. 1875, p. 356 ; vol. 26. 1877, p. S19 I '^'ol. 27, 

 I 1878, p. 374 ; vjl. 29, 1879, p. 2S1 ; vol. 35, 1883. p. 202 ; vol. 36, 1884, pp. 

 151, 206 ; Phil. Trans., 1S78. pp. 55, 155 ; 1880. p. 65 ; 1883, 477. 



•> Roy. Soc. Proc, vol. 23, 1875, pp. 356. 455 ; vol. 25, 1875, pp. 73, 347 ; 

 vol. 26, 1877, pp. 90, 323; vol. 27, 1878, p. 60; vol. 29, 1879, p. 21 ; vol. 30, 

 1880, p. 302 ; vol. 32, 1881, pp. 385, 388; vol. 33, 1882, p. 423 ; Phil. Trans., 

 1878, pp. 163, 2IO ; 1879, 165 ; 1880, p. 561. 



7 Roy. Soc. Proc, vol. 29, 1879, p. 21 : vol. 30, 1880, p. 302; vol. 32, 1881, 

 pp. 385, 388 ; vol. 33, 1882, p. 453; Phil. Trans., 1879, p. 165, 1880, p. 561. 



8 Rcjy. Soc. Proc, vol 10, i8bo, p. 256. 



y Roy. Soc. Pioc, vol. 2i, 1879, pp. 347, 477 ; Phil. Trans., 1S70, p. 641 ; 

 1880, p. 135 : 1881, 387. 



1" Roy. Soc. Proc, vol. 28, 1878, p. 181. 



II Roy. Soc. Proc, vol. 12, 1862, p. 202. 



12 Roy. boc. Proc, v^l. 37, 1884, pp. 78, 317 ; vol. 42, 18S7, p. 371 ; vol. 

 47, 1S90 ; pp. 300, 506. 



1-J Roy. S iC Proc, vol. 42, 18S7, p. 343 ; vol. 49, 1891, p. 84. 



l-* Roy. So;. Proc, vol. 47, 1890, p. 118. 



l-J Tribulation, not undisturbed progress, gives life and soul, and leads 10 

 success when success can be reached, in the struggle for natural knowledge. 



I'' Crookes. "On the Viscosity of Gases at High Exhaustions," § 655, 

 Phil. Trans., February, i88r, p. 403. 



1" Phil. Trans., vol. 172 ti88i). PP- 387, 435- 



NO. 1258, VOL. 49] 



