6o6 



NA TURE 



[April 27, 1905 



are new. Experiments were niadi- on the use of an 

 anilin cive chrysordine, for the cxtcTiiiination of mos- 

 suilo laivie and pupie. It was found to act satis- 

 factorily in a dilution of i in 30,000, but for I-racU.al 

 purposes its use in tliis slronKth would bj- proh.b.fv.. 

 on account both of cost and ot .ts yellow colour. 

 Bitinirand noxious insects other than mosquitoes is the 

 subiect of the next article, the most interestinR hnd 

 'U\U C. xwrsitans, the tsetse lly which carries 

 napana, on the Pon^o River, Bahr-el-Gh.-.zal and a 

 few pajics are devoted to insects and vcRetable para- 

 sites injurious to crops, the most important bc.nK an 

 ■lohis destructive to the dura crop described by Mr. 

 Th.obald as Afhis sorbin (nov. sp.) CyanoRenesis 

 hydrocyanic production, in the dura (Sorghum viilgan) 

 is another subject briefly dealt with, and of importance, 

 since considerable loss of horses and cattle has some- 

 times been occ.isioned thereby. The dura coiilains a 

 glucoside wliich yields hydrocyanic acid on decom- 

 position, the cause of whicli has been ascribed to abnor- 

 mal f^rowth, but may be duo to the dura aphis as 

 demonstrated by Dr. Balfour. 



Lastly, the >,'eneral routine work, pathological and 

 chemical, of the laboratories is summarised, some in- 

 teresting notes are given of the various diseases met 

 with in the Sudan, and the occurrence of eosinophilia 

 in Bilhar/ia disease and dracontiasis is discussed. 



We congr.itulate Dr. Balfour on his first year's 

 work contained in this report, which is copiously illus- 

 trated, some of the coloured plates of mosquitoes and 

 other insects being beautifully executed. 



R. T. Hewlett. 



Till Ihe Sun Grows Cold. By Maurice Grindon. 

 Pp. 1 1 (. (London: Simpkin, Marshall, Hamilton, 

 Kent and Co., Ltd., 1904.) Price 2s. 6d. net. 

 'ftiouci! this story is, so far as its main incidents 

 are concerned, of a familiar kind, it differs from 

 others in that several of the persons described are 

 interested in science. For instance, there is a Sir 

 John Harpur. who " was making important alter- 

 ations in his Observatory; he was an ardent 

 .Astronomer, and F.R.A.S."; Lady Harpur, again, 

 " had a love of flowers beyond tliat of a botanist, 

 although she was adept in the science"; and the 

 hero, Ralph Hillary, at one time of his life had a 

 workroom " in which he could follow up chemical 

 •and other researches to his heart's content." More- 

 over, after Ralph takes as a second wife his early 

 sweetheart, they engage together in scientific re- 

 search, and discover a substance of " extraordinary 

 radio-activity " to which they give the name 

 Helenium— after Ralph's sister. We cannot say that 

 Ihe author has been successful in blending fact rmd 

 fiction together so that one can scarcely be distin- 

 guished from the other; yet this art is essential to 

 the writer of scientific romance or romantic science. 



A Short Introduction to the Theory of Electrolytic 

 Di.'isociation. By J. C. Gregory. Pp. 76. (Lon- 

 don : Longmans and Co., 1005.) Price i.t. 6d. 

 This is a useful little book for those students who, 

 after taking a course of systematic chemistry, wish 

 to know something of the behaviour of electrolytic 

 solutions. The language and mode of jjresent.'ition 

 are simple, and although one might take exception to 

 many points of detail, the bonk, nn Ihe whole, should 

 prove a trustworthy guide. The headinirs of the four 

 chapters into which the bfKik is divided afford a 

 sufTicient indic.ilion of its contents: — chajjler i., the 

 condition of dissolved substances; chapter ii., iorus 

 and precipitation; chapter iii., hydrogen and hydroxyl 

 ions; chapter iv., electrolytic and general consider- 

 ations. 



NO. 1852, VOL. 71] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neitlier can tie undertake 

 to return, or to correspond with the -jirilers of, rejected 

 iiiatiuscripts intended for this or any other part of Nature. 

 jVo notice is taken of anonymous communications.] 



Electromagnetics in a Moving Dielectric. 



S0.M1; time ago, when coiibidering the assiimpiion that 

 the ether Inside a body is quite stationary wlien a body is 

 moved, and that in the application to Maxwell's ethereal 

 equations this involves the use of a fixed time differenti- 

 ation for the ether, and a moving one for the matter, 1 

 argued that the same applied not only to the electric 

 polarisation, as done by Lorcntz and by Larmor, but also 

 lu Ihe magnetic polarisation. I told the late Prof. 

 I'itzGerald that to make the extension seemed to l»e a sort 

 of categorical imperative. For it involves no assumption 

 .IS to how the magnetic polarisation is produced. .\l 

 tile time I made the application to plane waves only. 

 .Since then I have extended it to the general case. The 

 principal interest at presi'ol lies in the mechanical activity, 

 I fundamentally involved in the question of the pressure of 

 radiation, and electromagnetic moving forces in general. 

 The results confirm the desirability of applying similar 

 reasoning to the magnetic and to the electric polarisation, 

 in so far as they are relatively simple, and cast light 

 upon the subject. 



Thus, let M = VDB be the complete qHasi-momentum 

 per unit volume, and M„ = Vd„B., the ethereal part. Then 

 if the velocity of the matter is u, and of the ether q, the 

 motional activity (in the absence of free electrification, or 

 variation of the electrical constants in space) comes to 



\\x{d/,/f) + v(u.n ;(M - M„) + 'Md/dt) + v(q.q);M„ ; ( 1 ) 

 or, in a more developed form, 



u\i//i/f i uV + Vu + Vu.uJ(M -M||) + q][//'(// + qV + Vq + V.;.qlM„. (2) 

 Here the factor of u is the moving force on the matter, and 

 that of q the force on the ether. It will be seen that in 

 the material part we simply deduct that part of the 

 complete M which does not move with the matter. This 

 makes a great simplification of ideas. Vo avoid miscon- 

 ception, the V in (i) acts upon all that follows, whereas in 

 (2) the first V acts on the M's, but the second and third 

 on the velocities only, as may be seen on coniparismi 

 with (1). 



It is necessary, however, 10 point out distinctly Ihe data 

 involved in the above, as the simplification comes about 

 in a special way. Divide the displ.acement D into 

 D„ = f„E in the ether, and D,-i:,E| in the matter, where 

 E,=E + o, and e = V(U"q)B„. Similarly, divide the in- 

 duction B into B|| = ^„H and B, =^|H|, where H, = H + h, and 

 h = VD„(o-q). The electric energy is U„+ U, = .iED„-t- 

 lE|D,, and the magnetic energy is T„-t-T, = JHB„+ JH,B,. 

 .\lso, let there be four pcolotnipic pressures, of Maxwellian 

 type, say P„, P, electric, and Q„, Q, magnetic. E.g. 

 P, = L',- E,.D|, meaning a tension U, p.irallel to E, com- 

 bined with equal lateral pressure. The rest are similar. 

 Kinally, the two circuital equations are 



V'v(H-h„ -h,) = i>. Vv{E-e„-e|) = B, (3) 



where the motional electric and magnetic forces arc de- 

 fined by h„ = VD„q, h, = VD,u, o„ = VqB„, o,=VuB,. This 

 completes the data, and from them may be derived the 

 equation of activity 



~ vIVEH +q(U„-l-T„ + P„ + Q.) + u(a, + T, +• Pj -l-Q,)! 

 = U+t.|(U„',„),-„+(U,/r,)^,+(T„'M,.)M„+(T,/^,)Ai + Po«+Pi»». (4) 

 where F„ and F, are the forces displayed in (2). The 

 meaning is that the left side of (4) is the convergence of 

 the flux of energy made up of the Poynting (lux, the 

 convective flux, and the activity of the pressures, whilst 

 the right side shows the result in increasing the stored 

 energv iiii<i in work done upon the matter and ether, cither. 

 l>oth or neither, according to the size of the two velocities. 



The terms involving c, &-c.. in (4) represent residual 

 activity which mav be of dilTerent sorts. The commonest 

 is when the constants vary in sp.ace, especially at .1 

 boundiirv. For example, <f, = -uv.c, if c, docs not varv 



