Jan. 



889] 



NATURE 



321 



■\i\ its motion near gross matter — problems which the 



periments' of Fizeau (showing that a variable part of 

 iher was bound with matter and transmitted with it, 

 \\ bile another constant portion was free and blew through 

 ii began to throw light upon ; aberration problems such 

 IS have been partially solved by the genius of Stokes ; 

 liroblems connected with the motion of ether near great 

 masses of matter, like those which Michelson is so skil- 

 fully attacking experimentally : it is among thes3 that we 

 must probably relegate the question whether absolute or 

 relative motion of electric charges is concerned in the 

 production of magnetic field, and what absolute motion 

 through the ether precisely means. It is doubtless a 

 (juestion capable of being attacked experimentally, but 

 the experiments will be very difficult. I believe that 

 Prof, Ayrton has attempted them. 



Referring back to Parts I., II., and III., we find a 

 number of questions regarding momentum left unsettled. 

 Has an electric current any true momentum mechanically 

 discoverable .^ Now, this question, before it can be 

 answered in the negative, will have to be attacked under 

 a gfreat number of subdivisions. One may classify them 

 thus. Two main heads : (i) When steady, Does a magnet 

 behave in the least like a gyrostat 1 (2) When variable. 

 Is there a slight mechanical kick on starting or stopping a 

 current .'' With four or more subsidiary heads under each, 

 viz. (rt) in metallic conductors ; {b) in electrolytes ; {c) in 

 gases ; {d) in dielectrics. 



Suppose the answer turns out negative in metals, 

 it by no means follows that it should be negative in 

 electrolytes too. In fact, as matter travels with the 

 current in the case of electrolytic conduction, it is hardly 

 possible that there is not some momentum, though it may 

 be too small to observe— either a kick of the vessel as a 

 whole at starting and stopping, or a continuous impact on 

 an electrode receiving a deposit. The present writer has | 

 looked for these things, but after gradually eliminating a 

 number of spurious effects the result has been so far 

 negative. In a light quill vessel fixed to the end of a 

 torsion arm, the main disturbance was due to variations of 

 temperature which gradually introduced a minute air- 

 bubble, and by kicking this backwards and forwards 

 simulated the effects sought. In the case of the suspended 

 electrode, convection currents in the electrolyte, caused by 

 extra concentration or the reverse, seem determined to 

 mask any possible effect. 



One obvious though very troublesome source of dis- 

 turbance in all cases is the direct effect of terrestrial 

 magnetism on the circuit. To get over this, the writer 

 not only made his circuits as nearly as possible of zero 

 area, but also inclosed them in the iron case of a 

 Thomson marine galvanometer, lent for the purpose by 

 Dr. Muirhead. 



In gases, the experiment of Mr. Crookes, where a 

 stream of particles propels a mill inside a vacuum-tube 

 — perhaps even the ancient experiment of the blast from 

 a point — shows that momentum is by no means absent 

 from an electric current through a gas. 



To see if there are any momentum effects accompany- 

 ing variation of electric displacement in dielectrics, the 

 writer has suspended a mica-disk condenser at the end 

 of a torsion arm, and arranged it so that it could be charged 

 and discharged m situ. Many spurious effects, but no 

 really trustworthy ones, were observed. 



In the writer's opinion the subject is by no means 

 thoroughly explored, and he only mentions his old 

 attempts as a possible guide to future experimenters. 



Then, again, there is the influence of light on con- 

 ductivity. Annealed selenium, and perhaps a few other 

 things, improve in conductivity enormously when illu- 

 minated. The cause of this is unknown at present, and 

 whether it is a general property of matter, possessed 

 by metals and other bodies to a slight degree, is un- 

 rertain j for the experiments of Bdrnstein with an 



affirmative result for the case of metals have been 

 seriously criticized. 



Even though metals show no effect, yet electrolytes 

 might possibly do so, but the effect, if any, is small ; and 

 it is particularly difficult in their case to distinguish any 

 direct radiation effect from the similar effect of mere 

 absorbed radiation or heat. 



The writer has found that a glass test-tube kept 

 immersed in boiling water conducts distinctly better 

 when the blinds of a room are raised than when they 

 are lowered, though nothing but diffuse daylight falls upon 

 it. But as the effect could have been produced by a rise 

 in temperature of about the tenth of a degree, and as the 

 absorption of diffiise daylight is competent to produce a 

 rise of temperature as great as this in the glass of a 

 thermometer-bulb even though immersed in boiling water, 

 he feels constrained to regard the result, though very 

 clear and distinct, as after all a negative one, and has 

 accordingly not published it. 



A few months ago I should have put in a prominent 

 position among outstanding problems the production of 

 electric radiation of moderate wave-length, and the per- 

 formance with this radiation of all the ordinary optical 

 experiments — reflection, refraction, interference, diffrac- 

 tion, polarization, magnetic rotation, and the like. But a 

 great part of this has now been done, and so these things 

 come to be now mentioned under a different heading. 



Conclusion. 



" Conclusion " is an absurd word to write at the present 

 time, when the whole subject is astir with life, and when 

 every month seems to bring out some fresh aspect, to 

 develop more clearly some already glimpsed truth. The 

 only proper conclusion to a book dealing with electricity 

 at the present time is to herald the advent of the very 

 latest discoveries, and to prepare the minds of readers 

 for more. 



Referring back to Part IV^, vol. xxxviii. p. 418, we spoke 

 confidently of a radiation being excited by electric oscilla- 

 tions, a radiation which travelled at the same rate as 

 light, which is reflected and refracted according to the 

 same laws, and which, in fact, is identical with the radia- 

 tion able to affect our retina, except in the one matter of 

 wave-length. Such a radiation has now been definitely 

 obtained and examined by Dr. Hertz, of Karlsruhe, and 

 in the last month of last year, Prof, von Helmholtz com- 

 municated to the Physical Society of Berlin an account 

 of Dr. Hertz's latest researches. The step in advance 

 which has enabled Dr. Hertz to do easily that which 

 others have long wished to do, has been the invention of 

 a suitable receiver. Light when it falls on a conductor 

 excites first electric currents and then heat. The 

 secondary minute effect was what we had thought of 

 looking for, but Dr. Hertz has boldly taken the bull by 

 the horns, looked for the direct electric effect, and found 

 it manifesting itself in the beautifully simple form of 

 microscopic sparks. 



He takes a brass cylinder, some inch or two in 

 diameter, and a foot or so long, divided into two halves 

 with a small sparking interval between, and by connecting 

 the halves to the terminals of a small coil, every spark of 

 the coil causes the charge in the cylinder to surge to and 

 fro about five hundred million times a second, and 

 disturb the ether in a manner precisely equivalent to a 

 diverging beam of plane-polarized light with waves about 

 twice the length of the cylinder. 



The radiation, so emitted, can be reflected by plane 

 conducting surfaces, and it can be concentrated by 

 metallic parabolic mirrors ; the mirror ordinarily used 

 being a large parabolic cylinder of sheet zinc, with the 

 electric oscillator situate along its focal line. By this 

 means the effect of the wave could be felt at a fair 

 distance, the receiver consisting of a synchronized con- 

 ductor with a microscopic spark-gap, across which the 



