592 



NA TURE 



[October i6, 1890 



the disruptive discharge which he expressed in his paper pub- 

 lished in the Philosophical Magazine for June 1883. That is 

 partly a matter of opinion, but Prof Thomson has certainly led 

 his readers to think that he considered the presence of free 

 atoms before discharge as essential. 



One of the principal difficulties of any theory of the disruptive 

 discharge is the explanation of the relation between pressure 

 and so called dielectric strength. This relation is one of the 

 crucial points by means of which every theory must be judged. 

 The two following passages will show how Prof. Thomson 

 meets the difficulty : — 



" Let us now apply these considerations to the case of the 

 electric discharge. The disturbance to which the gas in an 

 electric field is subjected makes the molecules break up sooner 

 into atoms than they otherwise would do, and thus diminishes 

 the ratio of the paired to the free times of the atoms of the gas ; 

 as the intensity of the electric field increases, the disturbance in 

 some places may become so violent that in these regions the 

 ratio of the paired to the free times approaches the value it has 

 when the gas is about to be dissociated." 



"Let us now consider what effect rarefying the gas would 

 have upon its electrical strength. In a rare gas the mean 

 distance between the molecules is greater than in a dense one ; 

 and if the temperature be the same in both cases, and con- 

 sequently the mean velocity of the molecules the same, the ratio of 

 the free to the paired time will be greater for the rare than for the 

 dense gas, for the free atoms will, on an average, be longer in 

 meeting with fresh partners. Thus the rare gas will be nearer the 

 state in which it begins to suffer dissociation than the dense gas, and 

 thus it will not require to be disturbed so violently as the dense 

 gas in order to increase the ratio of the free to the paired time to 

 its dissociation value ; and thus the intensity of the field neces- 

 sary to produce discharge would be less for the rare gas than 

 for the denser one : in other words, the electric strength would 

 diminish with the density, and this we know is the case." 



It will be seen that the explanation entirely depends on the 

 idea that free atoms exist already before discharge. In my 

 Bakerian Lecture, I have pointed out that the existence of free 

 ions seems inconsistent with experimental fact, and I add : 

 " This seems to me to be fatal to J. J. Thomson's view of the 

 disruptive discharge. " This is the passage which Prof. Thomson 

 says implies a misconception, but surely, assuming that I am 

 right in my argument that no dissociation takes place before 

 discharge, I am also right in saying that this is fatal to any 

 theory which makes the relation between pressure and spark po- 

 tential depend on such dissociation. It may be said, on the other 

 side, that the idea of decomposition of molecules by the discharge, 

 which forms so important a feature in the theory which I have 

 explained in my Bakerian Lecture of 1884, occurs already in 

 Prof. J. J. Thomson's paper of 1883 ; and that it is unfair, there- 

 fore, to condemn his views because they do not account for a 

 feature of the discharge which has never been satisfactorily 

 explained. A i^w remarks are therefore necessary to explain 

 the relationship between Prof. Thomson's paper of 1883 and 

 my own of the succeeding year. 



The hypothesis that the discharge of electricity in gases is 

 similar to that in electrolytes, and that each atom of a gas such 

 as nitrogen or hydrogen carries a permanent charge, seems so 

 obvious that it must have occurred to many who have thought 

 about the matter ; but no attempt has, until recently, been made 

 to develop the hypothesis so as to account for the complicated 

 phenomena of the discharge. 



The credit of being the first to have done so undoubtedly belongs 

 to Giese, who has explained by means of it a number of observed 

 facts concerning the behaviour of flames, and of the gases rising 

 from flames, but he has not until quite lately considered any 

 other part of the subject. J. J. Thomson, in 1883, published 

 his paper "On the Theory of the Electric Discharge in Gases." 

 The hypothesis of atomic charges is not mentioned therein, and 

 the author does not discuss the question whether or not a 

 current of electricity in a gas consists of a diffiision of charged 

 atoms or not. So carefully are these matters excluded, that it 

 is difficult to avoid the conclusion that it was done intentionally, 

 in order that the investigation may be more general, and in- 

 dependent of any particular theory which might in future be 

 established. 



"As the most general assumption," the electric field is sup- 

 posed to consist ot " a distribution of velocity in the medium 

 whose vortex motion constitutes the atoms of the gas ; the dis- 

 turbance due to this distribution of velocity will cause the 



NO. 1094, VOL. 42] 



molecules to break up sooner than they otherwise would do.'' 

 The way in which the decomposition of molecules is connected 

 with the spark must be judged from the passages quoted above, 

 and from those quoted by Prof. Thomson in his letter to you. 

 The general terms in which the whole paper is expressed may 

 increase its scientific value, but it affords no help to those who 

 wish to form any more definite notion what an electric current 

 through a gas consists of. If in future it should be shown that a 

 current of electricity does not consist of a diffiasion of charged 

 atoms. Prof. Thomson's reasoning may still apply, I do not 

 know whether such general considerations may be fitly described 

 as "a theory." 



After having for a number of years attempted to trace out 

 the consequences of the electrolytic hypothesis, and discovered 

 some method by means of which it could be definitely tested, 

 I presented to the Royal Society, in 1884, a paper which forms 

 the subject of the Bakerian Lecture for that year. In that paper 

 I referred to Prof. J. J. Thomson's of the previous year, but 

 unfortunately I was not then aware of Giese's work, and that 

 gentleman has undoubtedly some right to complain of the way in 

 which his researches have, till recently, been neglected by 

 myself and others. I have never claimed that my hypothesis 

 of atomic charges was in any way original ; but I have always 

 maintained that that hypothesis, by itself alone, does not explain 

 much. My recent Bakerian Lecture shows sufficiently clearly 

 that we must form much more definite notions regarding the 

 phenomena of dissociation and the interaction between chemical 

 and electrical forces before we can say that we have a complete 

 theory of the electric discharge, and those who will overcome suc- 

 cessfully all the remaining difficulties will have done much more 

 than those who started the idea. The work of Hittorf, E. Wiede- 

 mann, Hallwachs, Warburg, Elster and Geitel, and others, has 

 thrown so much light on many points, that the final decision as to 

 the truth or otherwise of the theory under discussion cannot be long 

 deferred. I have ventured to call it the "theory of electrolytic 

 convection," which, I should say, was a sufficiently neutral and 

 distinctive name. Arthur Schuster. 



A Suggestion respecting the Syllabus of the Science 

 and Art Department. 



Will you permit me to call attention to the following con- 

 siderations which have occurred to myself and several of my 

 colleagues as the result of some years' teaching experience ? 



(i) The syllabus of Subject VIII. (Sound, Light, and Heat), 

 as it at present stands, is very extensive — too much so, indeed, 

 for the majority of students to grasp in one session. This fact is 

 tacitly acknowledged by the Department, as a student is per- 

 mitted to obtain a first class in either stage by taking two only 

 out of the three subjects. 



(2) Sound and light are pretty closely related, both being 

 forms of wave motion, and the general ideas involved in their 

 study very similar. But between these two subjects on the one 

 hand, and heat on the other, this connection is small, existing, 

 indeed (so far as the Department's syllabus extends), only in 

 the comparatively unimportant section of radiant heat. The 

 considerations involved in dealing with specific and latent heat 

 and with heat as a form of energy are of an utterly different 

 character from those presented in sound or light. 



(3) The syllabus of Subject VI. (Theoretical Mechanics) is 

 also too extensive for most students to grasp in one session, in- 

 cluding, as it does, four subjects, viz. Statics, Dynamics, Hydro- 

 statics, and Pneumatics. And although the Department does 

 not officially state that a student can obtain a first class with 

 two only of these subjects, yet the papers are always arranged to 

 admit of his doing so by taking statics and dynamics only. 

 Thus, in the elementary stage, a candidate may answer only 

 seven questions out of twelve, and in the advanced, eight out of 

 twelve, and yet in each case nine of the twelve are confined to 

 statics and dynamics. 



The result is that teachers and students pay but scant attention 

 to hydrostatics and pneumatics. 



(4) A large number of students take physics and mechanics 

 simply as accessories to engineering and the applied sciences. 

 Now, to such students a knowledge of heat is most essential, 

 while sound and light are quite useless. Again, though heat has 

 but slight connection with sound or light, it has a very strong 

 connection with hydrostatics and pneumatics. By far the most 

 important thermal phenomena are those presented by liquids 

 and gases, and moreover it is precisely these that an engineer! nt; 



