May II, 1876J 



NATURE 



27 



through which the sun's heat is conveyed to and from the 

 earth, the lower and denser strata absorb the greatest 

 amount, and are necessarily the warmer ; " a sentence of 

 which a teacher would score almost every word. Again, 

 on the subject of dew, we read that " substances like 

 glass, &c., which rapidly lose their own heat and slowly 

 acquire that of others are susceptible of being copiously 

 bedewed." The italics are ours. And once more, " when 

 the temperature of the air is reduced below that of the 

 invisible vapour it contains, the moisture becomes visible." 

 These extracts could be multiplied till we might wonder 

 if it is really a book on Physical Geography we are read- 

 ing. But these are serious defects, and we wish they 

 could be altered. By the side of them it is of less conse- 

 quence that while we read in the Preface that " this 

 revision embraces all that is important in recent disco- 

 very ; ■' yet on turning to the temperature of the sea, 

 where the most important changes have taken place in 

 our knowledge, we are still referred to Sir James Clarke 

 Ross, and told that the ocean has below the surface a 

 uniform temperature of 39^°, for which at the equator we 

 must descend deeper than anywhere else. We can 

 scarcely imagine that any amount of clearness will atone 

 for these things ; let us hope they will be seen to before 

 edition the ninth is required. 



The Flora of South Australia. By R. Schomburgk, 

 Ph.D., Director of the Botanic Gardens, Adelaide. 

 (W. C. Cox, 1875.) 



We have here a complete list of the indigenous flora of 

 South Australia, both tropical and extra-tropical, with 

 some general remarks prefixed. The most predominant 

 natural orders in the colony are Leguminosae, Myrtacese, 

 Compositas, Proteaceae, Cruciferas, Rubiacese, and Gra- 

 mineae. The genera and species are remarkably circum- 

 scribed in area ; many are found in one spot alone. The 

 colony is singularly devoid of native edible fruits and 

 roots ; on the other hand it produces abundance of valu- 

 able timber-trees and of plants suitable for the manufac- 

 ture of paper and other fibres, and for the production of 

 dyes ; but most of the valuable crops are naturalised 

 plants, introduced from Europe or other parts of the 

 world. A. W. B. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. \ 



Theory of Electrical Induction 



In Nature, vol. xiii. pp. 437, 475, Prof. Paul Volpicelli gives 

 an exposition of the two theories of electric induction, containing 

 copious references to the writings of electricians, and numerous 

 experiments of his own. It is remarkable, however, that he 

 has not only omitted all reference to the works of Poisson, 

 Green, Thomson, Beer, Betti, &c., who have studied the mathe- 

 matical theory of induction, but he has not even introduced the 

 word potential into his exposition, unless we are to take the word 

 tension in the sense of potential, where he says that a certain 

 portion of electricity possesses tension while another portion 

 does not. 



The result of this mode of treating the subject without calling 

 in the aid of those ideas and phrases which the progress of 

 science has developed, is to convey the impression that the whole 

 theory of induction of electrification on the surface of conductors 

 is still in a very imperfect and vague condition, whereas there is 

 no part of electrical science in which we can trace more distinctly 

 the correspondence, quantitative as well as qualitative, of the 

 phenomena with the general laws of electricity. It appears, 

 however, from what M. Volpicelli says, that an erroneous theory 

 is still generally adopted in treatises on physics and electricity, 

 and that it ought to be superseded by a more correct theory first 

 proposed by Melloni. 



Both theories admit that if an insulated conductor, without 



charge, is acted on by a charged inductor, the siuface becoines 

 electrified, oppositely to the charge of the inductor on the parts 

 nearest the inductor, and similarly to the charge of the inductor 

 on the parts farthest from it. The first of the two theories, how- 

 ever, asserts that both these electiicities are "erdowed with 

 tension," whereas the second, that of Melloni, asserts that the 

 electricity of the same kind with that of the inductor is alone 

 " endowed with tension," while the other kind of electricity is 

 entirely "latent or dissimulated." 



The only sense which we can attach to the word "tension" 

 as thus used, is that which modem writers mean by " potential," 

 or potential function, the difference being that the word tension 

 is often used in a vague manner, whereas potential is strictly 

 defined. 



Thus a point in space is said to have a certain electric potential, 

 and since all points of a conductor in electrical equilibrium have 

 the same potential, we speak of the potential of the conductor. 

 But we do not speak of the potential of a charge of elertricity, 

 or of electricity being endowed or not endowed with potential. 

 Such language would only lead us into error. 



Let U3 suppose the inductor to be charged positively and the 

 induced body to be insulated and originally without charge. 

 Then, since its insulation prevents any electric communication 

 with other bodies, its total electrification must remain zero, or 

 there must be as much positive electrification as there is negative. 



Hence for every line of electric force which proceeds from the 

 inductor and falls on the induced body, there is another which 

 proceeds from the induced body and falls on the walls of the 

 room, or on some other body whose potential is zero. The 

 potential of the induced body must therefore be intermediate 

 between that of the inductor and that of the walls of the room, 

 which is generally taken as zero. The potential of the induced 

 body is therefore positive. 



There is thus on the surface of the induced body a region nearer 

 the inductor which is negatively electrified, and a region further 

 from the inductor which is positively electrified. These regions 

 are divided by a neutral line on the surface, which is the section 

 of the surface by an equipotential surface in space which has the 

 same potential as the induced body. The total charges on these 

 two regions are exactly equal but of opposite signs. 



If a small insulated conductor is placed in contact with any 

 part of the surface and removed, it will be found to be electrified 

 in the same way as the part of the surface with which it was in 

 contact. A fine short needle point, or a burning pastille, placed 

 on any part of the surface will dissipate the kind of electricity 

 which exists on that part of the surface. See Riess, " Reibungs 

 Elektricitat," Art. 247. 



If any part of the induced body is placed in electrical connec- 

 tion with the earth by touching it with a fine wire, positive elec- 

 tricity will be discharged, and the potential of the induced body 

 will be reduced to zero. This will be the case whether the part 

 touched be positively or negatively electrified. The quantity of 

 electricity discharged will be the product of the potential of the 

 induced body into its electric capacity. , 



After this discharge every part of the surface of the induced 

 body will be negatively electrified, but the parts nearer the 

 inductor more than those which are further from it. 



In the mathematical treatment of the subject Thomson has 

 found it convenient to divide the electrification into two parts, 

 each distributed over the induced body according to its own law. 



(o) The induced electrification when the induced body is con- 

 nected to earth, and the charge of the inductor is E. This 

 electrification is negative on every part of the smface, but the 

 density is greatest next the inductor. 



(;8) The electrification when the induced body has a potential 

 P, and the inductor, still in the same place, has no charge. This 

 electrification is positive on every part of the surface. 



From a knowledge of these two distributions it is easy to 

 determine a third, in which the total electrification is the alge- 

 braical sum of (a) and (^8), and in which the value of /* is such 

 that the total electrification is zero. 



We might then assert that the electrification (iS) is free, because 

 it will be discharged if the body is connected to earth, but that 

 the electrification (o) is latent or dissimulated, because it will not 

 be discharged to earth. 



The only danger of this mode of exposition is that it may 

 suggest to a beginner the notion that electricity, Uke water and 

 other substances, may exist in different physical states, in some 

 of which it is more mobile than in others. 



This idea of variation of quaUty once introduced into the 



