Oct, ii, 1888] 



NATURE 



581 



increases, and of this electric glow many instances could be 

 cited, both in Nature and in the laboratory. 



Consider, in the first place, the glow surrounding a point from 

 which an electric discharge is taking place. By means of the 

 electrical repulsion, the density of the air immediately surround- 

 ing the point will be so far diminished that a single air-particle 

 will be able to traverse a sensible distance with a very great 

 velocity, and therefore give rise to the glow. Here it is not a 

 question of particles becoming electrically excited by radiation 

 from the point, but of those which are electrified by actual 

 contact with it. As soon as they have lost some of their elec- 

 trical energy they will again become sensitive to electrical 

 radiation. There must therefore be a dark space immediately 

 surrounding the point, and outside this an electric glow, which 

 explains a well-known phenomenon always observed in the 

 rarefied atmosphere of a Geissler tube. The stratification can 

 also be explained very simply, for the glow causes a diminution 

 in velocity, for when the electrical waves from the positive 

 electrode give rise to luminous instead of electrical vibrations 

 in the particles of gas, the repulsion will be diminished, and 

 therefore the velocity will gradually become less than that of 

 light, when the particle will again become sensitive to the 

 electrical radiation. The velocity will therefore again increase 

 until the glow appears again, thus giving rise to a stratified 

 appearance. The velocity in the glowing layers will naturally 

 be greatest in the neighbourhood of the positive electrode, and 

 here, therefore, light will be given off of all the colours cor- 

 responding to the critical periods of the gas contained in the 

 tube, which is in accordance with observation. According to 

 the author's theory, the electrical excitation takes place by the 

 transference of ponderable gas molecules from the positive to 

 the negative electrode. After they have parted with their 

 electrical energy to the latter, they will return in an unelectrified 

 condition to the positive electrode to which they will be 

 attracted, and at the same time repelled from the negative elec- 

 trode. There will be no dark space surrounding the negative 

 electrode, because the particles leaving it will have little or no 

 electrification. The velocity of the returning molecules will 

 increase as they approach the positive electrode, so that there 

 can be no further transformation of electrical into luminous 

 energy. In very high vacua the velocity of the returning par- 

 ticles may become great enough for electrical energy to be 

 excited in them by the red glow of the positive pole, by which 

 their velocity will be still further increased. The velocity of the 

 returning particles will in this case ultimately become so much 

 greater than that of the luminous molecules moving away from 

 the positive electrode as to cause a sensible increase in the 

 density of the gas surrounding it. The result of this will be to 

 prevent the formation of the positive glow, and the whole tube 

 will become filled by the negative glow. The density in the 

 neighbourhood of the negative electrode will therefore be 

 diminished, and the returning molecules will leave it with still 

 greater velocity. If both electrodes are at one end of the tube, 

 the molecules returning towards the positive electrode will be 

 deflected by the layer of dense gas surrounding it, against the 

 sides of the tube, giving rise to fluorescent phenomena, as 

 explained in § 11 (September 6, p. 461). If the complicated 

 phenomena which have recently been observed in Geissler tubes 

 by Crookes and Hittorf can be thus simply explained, it will 

 afford an important confirmation of the author's theory. 



These considerations may be applied to the explanation of 

 many cosmical phenomena, such as the aurora and the light of 

 comets. It is quite possible that the particles of a comet's tail 

 moving with great velocity towards the sun may become 

 electrified by means of the sun's light. 



The formulce previously obtained are applicable to the deter- 

 mination of the motion of an electrified particle, in the case in 

 which no proper luminous vibrations are given off from the 

 origin, or where these may be neglected, for the equations 



dr 

 (29) to (33) give in this case for — = c, r — r , 9fc = 9t , and 



at 

 consequently — 



!(' - #■ = m (f " 9 + ** 



2 L\dt) ~ C " A " r r dr 



r dt 



Also — 



+ 91 - % 



And dr\dt can hence only become infinite when the positive 

 quantity 5t becomes infinite, or r — o. von Helmholtz's 

 objections, therefore, do not apply to this equation. 



§ 15. — Electrical Excitation. 



The foregoing theory easily explains the different methods of 

 electrical excitation. 



(1) The friction of two bodies sets their molecules into 

 vibration, which appears in the form of heat. The resulting 

 impacts of neighbouring molecules will most readily excite 

 internal vibrations of the critical periods, for which they 

 are specially sensitive. If the molecules are exceptionally 

 sensitive to vibrations of very short periods, they will be 

 easily electrified, the process being exactly analogous to 

 the production of luminous vibrations by heating gases, as de- 

 scribed in § 4 (August 23, p. 407). Electro-positive bodies 

 will be those which are most sensitive, and these will, according 

 to the theory, attract other less electrified bodies. In the 

 ordinary frictional electrical machine the glass will therefore be 

 more strongly excited than the rubber. The explanation of the 

 collecting action of points on the prime conductor is given by 

 the consideration that at a point the molecules are more fully 

 exposed to the electrical radiation from the glass plate, and 

 being electrically excited by this radiation communicate their 

 electrification to the prime conductor by conduction, as explained 

 in § 13. 



(2) Electrification by the action of heat takes place in the 

 same manner, and it is clear that the molecules in crystals, being 

 regularly disposed with their axes in definite directions, will be 

 electrified. Thermo-electrical currents are also explained. For 

 if one of the junctions of a circuit consisting of two dissimilar 

 metals is heated, the more sensitive metal will receive more 

 electrical energy than the other, and give rise to a positive 

 current. The potential difference at the junction will depend 

 on the internal constants of the molecules in the two metals, so 

 that we cannot expect to be able to express it by any simple 

 general law. 



(3) Electrification by simple contact of two dissimilar metals 

 is not so easily explained if the effects of heat, pressure, and 

 friction are excluded. It is, however, possible that the close 

 contact of differently vibrating molecules may disturb the internal 

 and therefore the external energy, and thus give rise to electri- 

 fication. The electrification of similar metals by contact could 

 be explained in the same way. 



(4) Electrification by chemical action is completely explained 

 by the author's theory, the production of electrical vibrations by 

 this means being exactly analogous to the similar production of 

 heat- and light-vibrations. Such chemical action must, in the 

 author's opinion, play an important part in the galvanic cell, 

 though contact electrification may also have a share in the 

 action. The contact between copper and sulphuric acid, for 



1 example, is a very intimate one. At ordinary temperatures the 

 molecules of both substances will be in motion. When two 

 different molecules collide, their internal equilibrium will be 

 destroyed, and they will therefore, according to § 8 (September 6, 

 p. 460) form a chemical compound, provided the critical vibrations 

 of the compound are, at the given temperature, less easily excited 

 than those of the separate elements, which we must assume to be 

 the case, from the strong chemical affinity which is experimentally 

 known to exist between copper and sulphuric acid. During 

 this process electrification will take place if the maximum 

 internal electrical energy is less for the compound than for the 

 constituents, exactly as hydrogen in combining with oxygen to 

 form water produces light, and chlorine in combining with 

 hydrogen to form hydric chloride produces heat. The electricity 

 set free will be carried away by the copper, the latter being a 

 good conductor. The accumulation of electricity in the copper 

 is prevented, however, by its being used up again in forming a 

 chemical compound with the zinc. 



G. W. DE TUNZELMANN. 



( To be continued. ) 



COMPRESSIBILITY OF WA TER, SALT W A TER, 

 MERCURY, AND GLASSY 



T^HE pressures employed in the experiments ranged from 150 



-*■ to 450 atmospheres, so that results given below for higher or 



lower pressures [and inclosed in square brackets] are extrapolated. 



x Extracted, with the sanction of Dr. Murray, from a Report by Prok 

 Tait, now in type for a forthcoming volume of the Challenger publications. 



