526 



NA TURE 



[April 19, 1877 



larger than our own, the fact of the existence of nearly 

 4,000 agricultural students is somewhat startling. In this 

 country we have one agricultural college supported by less 

 than 100 students. Yet we are the possessors of the most 

 extensive colonies in the world, far exceeding, in extent, 

 even the vast area of the United States. It may well be 

 difficult for English agriculturists to compete with foreign 

 rivals if the meagre number of agricultural students in 

 England compared with America may be taken as in any 

 degree a gauge by which interest in scientific progress 

 may be measured. 



GUMMING 'S THEORY OF ELECTRIGITY 



An Introduction to the Theory of Electricity. By 

 Linnaeus Gumming, M.A. (London : Macmillan and 

 Co., 1876.) 



MR. GUMMING deserves our thanks for having made 

 an effort to introduce into elementary teaching the 

 advances in the treatment of electricity made chiefly by 

 the labours of Green, Thomson, and Clerk- Maxwell. Mr. 

 Gumming possesses all the qualifications necessary for 

 such a task. He evidently has a full knowledge of the 

 subject, and seems to possess, in addition, experience as 

 a teacher. He has had, no doubt, great difficulties to 

 encounter. These difficulties are not alone due to the limi- 

 tations as to the mathematicial knowledge of his readers, 

 which Mr. Gumming has justly imposed on himself. 

 The books and papers out of which Mr. Gumming had to 

 take his material, were written from various points of 

 view, and they were chiefly addressed by scientific men to 

 scientific men. It was natural that the same words 

 should not be always used exactly in the same sense, 

 the great object being that men already possessing a 

 knowledge of the subject should understand each other. 

 It is only when the knowledge of a certain subject is com- 

 paratively advanced that the terms settle down into a 

 definite meaning. A text book, on the other hand, is 

 addressed to students who at the most have only a slight 

 acquaintance with the subject, and it should not only teach 

 that particular subject, but also scientific method, and 

 scientific reasoning. It is, therefore, of primary import- 

 ance that the precise meaning of the term should be 

 scrupulously adhered to. Even a good definition does 

 not always ensure this, for there is often a metaphysical 

 colouring which does not come out in the definition, but 

 which we soon discover in the way a term is used. We 

 take one example. The word potential is defined by Sir 

 Wm. Thomson thus :— 



" The potential at a point is the work which would be 

 done on a unit of positive electricity by the electric forces 

 if it were placed at that point without disturbing the 

 electric distribution, and carried from that point to an 

 infinite distance." 



Nothing could ba more precise than this, yet the word 

 potential will call forth different associations in different 

 minds, and this will greatly influence the way in which 

 the word is used. Some writers will attach no meaning to 

 the potential at a point, except in so far as they can 

 imagine an electrified particle to be placed at that point, 

 to others the expression will convey a perfectly definite 

 meaning, defining the state of the medium at that point, 



irrespective of any electrified particle which might be 

 placed there. It will generally be soon found out in what 

 sense the writer uses the word. 



The passage in Mr. Gumming's book which has sug- 

 gested these remarks, is contained in Prop. 8, p. 203. 

 The proposition runs thus : — 



*•' In computing the potential of any closed circuit, we 

 may substitute for it any closed circuit which is obtained 

 by projecting the given circuit by means of lines of 

 force." 



The expression potential of a circuit may perhaps have 

 sometimes been used for the induction or number of 

 lines of force through a circuit ; yet we imagine the term 

 suggests rather the idea of action at a distance, and would 

 be analogous to substituting in electro statics the expres- 

 sion "potentially a point" for that generally used — 

 "potential at a point." Mr. Gumming has so consistently 

 adopted the views and language of Faraday and Clerk- 

 Maxwell, that we have been struck rather unpleasantly by 

 the passage. This is, perhaps, only a matter of detail, 

 and we may here remark that Mr. Gumming might with 

 advantage have bestowed more care on the wording of 

 his propositions. On p. 40, for instance, an experi- 

 mental law is given, and two exceptions are added. With 

 very little trouble the law might have been worded so as 

 to admit of no exceptions. Other examples might be 

 given. 



There is one feature in the book about which we 

 should like to make a few remarks. An occasional allu- 

 sion to hypothetical matters cannot of course be alto- 

 gether avoided, yet we think that in books like Mr. Gum- 

 ming's, which are intended to give an outline of what is 

 known, and not of what is unknown, of the subject, such 

 allusions should be reduced to a minimum. 



On p. 119 five propositions are given on molecules 

 and atoms, which are not alluded to throughout the book 

 except in the following passage : — 



" From the last statement it can be easily seen that 

 when the molecules of two different solids impinge on 

 each other, as at the surface of contact, they cannot ac- 

 commodate themselves to each other's motion, but con- 

 strain each other, this constraint producing a loss of 

 energy. If, however, the two soUds are of the same kind 

 and at the same temperature, the molecules on each side 

 of the surface of contact are swmging in exactly the same 

 manner, and can easily accommodate themselves to each 

 other's motion without more constraint than exists in the 

 solid part of either body. It is this loss of energy owing 

 to the unsymmetrical swinging of the molecules at the 

 surface of contact which reappears as difference of poten- 

 tial between the two solids, or as the energy of electrical 

 separation. 



" The opposed electricities so separated will, for the 

 most part, be heaped up on either side of the plane of 

 separation by a Leyden jar action." i 



In the same chapter Mr. Gumming gives his opinion | 

 on the theory of the voltaic cell and electrolysis in" 

 general. We are told how we might imagine the atoms 

 and molecules to be placed ; we are told about polarisa 

 tion ; but Faraday's laws are not even alluded to. 



It would, of course, ba an advantage to science if Ml 

 Gumming were to take an active part in the theoretics 

 and experimental investigation of the subject ; yet w(| 

 doubt whether a text-book on the theory of electricity 

 the best place to bring his views forward, especially 



