August 9, 1888] 



NATURE 



343 



Cloud Electric Potential. 



Under the above heading, in Nature of July 19 (p. 269), 

 which has just come into my hands, Mr. E. Douglas Archibald 

 •criticizes a statement of mine in Part III. of " Deschanel," re- 

 specting electrified drops of water in a cloud. The following is 

 the statement : — 



"The coalescence of small drops to form large ones, though 

 it increases the electrical density on the surfaces of the drops, 

 does not increase the total quantity, and therefore cannot '-directly 

 influence the observed potential." 



At the word "therefore" I give a reference to a previous 

 section, in which it is shown that the potential at a point is the 

 sum of all the quotients qjr, q denoting an element of the 

 electricity to which the potential is due, and r the distance of 

 this element from the point in question. Since the coalescence 

 of drops is without effect on the value of each q and its corre- 

 sponding r, it cannot affect any one of the quotients q/r, whose 

 sum constitutes the potential. 



Mr. Archibald's criticism is : — 



"Surely this entirely omits the fact that the capacity of a sphere 

 is equal to its radius, and thus, in the case of eight equal spheres 

 coalescing into one, not merely would the density be doubled, 

 but the potential of the same quantity would be increasedy^«r 

 times. " 



This criticism rests on two false assumptions : — 



First, that the potential of a drop depends on its own charge only, 

 and can therefore be computed by dividing its charge by its radius. 



Secondly, that the potential of the drops (which on this sup- 

 position would be very different from the potential at a point 

 midway between two drops) can be identified with "the observed 

 potential." J. D. Everett. 



Cushendall, Co. Antrim, August 3. 



THE ABSORPTION SPECTRA OF CRYSTALS. 



A LL who are interested in the difficult work now going 

 ■£*- forward in so many chemical laboratories, in connec- 

 tion with the nature and constitution of those most 

 complex mixtures known as " rare earths," and who 

 recognize the extremely important influence which the 

 solution of this subject must exert upon the very basis of 

 our modern chemistry, will gladly welcome a new and 

 exquisite means of investigation which M. Becquerel has 

 recently brought to light. 



As the reward of a most exhaustive study of the 

 changes which are brought about in a beam of light by 

 its passage through a crystal, M. Becquerel has discovered 

 the key by means of which he is enabled to interpret the 

 subtle indications which the issuing rays afford as to the 

 nature of the molecules among which they have been 

 threading their way. It appears at first sight more than 

 wonderful that these delicate indications can have led to 

 precisely the same weighty conclusions as those arrived 

 at from the renowned physico-chemical researches of 

 Auer von Welsbach, Lecoq de Boisbaudran, Demarqay, 

 Soret, Crookes, and Kriiss and Nilson. Yet such indeed 

 is the case, and it even appears likely that the new method 

 may be carried still further into the region beyond that 

 which has up to the present been reached by these 

 experimenters. 



In order to explain the nature of this discovery, it will 

 be necessary to describe the experimental steps which 

 have led M. Becquerel towards it. In the year 1866 

 Bunsen found what now appears to be the germ of a great 

 principle — that when a crystal of the sulphate of the 

 substance didymium, now known to be a most complex 

 mixture, was traversed by a beam of plane-polarized light 

 vibrating at an angle of 20° to the horizontal diagonal of 

 the crystal, the absorption spectrum was slightly different 

 from that which was obtained when the ray was polarized in 

 a plane at right angles. This observation did not attract 

 much attention at the time, it being considered merely as 

 a curious manifestation of the phenomenon of pleochroism. 



Sorby, however, in 1869 again reopened the question, 

 having found that in zircons the ordinary and extra- 



ordinary rays presented different bands of absorption. 

 Since that time Becquerel himself has shown that the 

 same applies to all birefractive crystals which give 

 absorption spectra. 



With so much premised, we are now in a position to 

 consider the main results of this more recent investigation. 

 They may be very briefly summarized as follows : — 



(1) The bands in the absorption spectra of all crystals 

 have fixed positions : the intensity alone varies with the 

 direction of propagation of the light. 



(2) In most crystals, the principal directions of absorp- 

 tion coincide with the directions of optical elasticity. 



(3) In certain crystals the directions appear to be very 

 different for different bands, but they always remain 

 subject to the conditions imposed by the crystalline 

 symmetry ; thus in monoclinic crystals one of the principal 

 axes of absorption always coincides with the axis or 

 symmetry, and the other two rectangular axes are situated 

 in the plane of symmetry. 



Hence it appears to be a fact that the absorption of 

 luminous radiations of fixed wave-length admits of three 

 directions of symmetry. These directions appear generally 

 to coincide with the principal directions of optical 

 elasticity, with the exception of certain remarkable 

 anomalies in particular crystals. Here, however, is the 

 whole gist of the matter. Why these anomalies? Just 

 as from a consideration of the deviations from Boyle's 

 law physicists have learned how to measure the size of 

 those wonderfully minute entities familiar to us as mole- 

 cules, so has M. Becquerel extracted a most important 

 principle out of the anomalies to the law of absorption 

 in crystals. 



It appears probable that absorption may be due to a 

 physical phenomenon dependent upon the intermolecular 

 movements. The intimate relation between phosphor- 

 escence and absorption, notably in the compounds of 

 uranium and certain of the rare earths, appears to show 

 that in solids and liquids the radiations absorbed are 

 those which vibrate in unison with the molecular move- 

 ments. This conception is in fact nothing more than an 

 extension to solids and liquids of the well-verified law of 

 the absorption by incandescent vapours. 



As the molecular elasticity varies in different directions 

 in crystals not isotropic — that is, not belonging to the cubic 

 system — so will the absorption vary ; and if, in two 

 isomorphous substances, the directions of molecular 

 elasticity do not exactly correspond, the directions of 

 different absorption in the two substances will vary in 

 like manner. Now it is quite true that many crystals of 

 isomorphous substances— that is to say, substances of 

 analogous chemical constitution crystallizing in similar 

 forms — have their optic axes unequally inclined. 



If we crystallize two such substances together, in 

 gradually increasing proportions of one of them, we find 

 that the angle between the optic axes in the mixed crystals 

 diminishes progressively until it reaches zero, after which 

 the two axes again diverge in a plane perpendicular to 

 their original plane. Thus can we cause the influence of 

 each in turn to preponderate. 



Each chemical substance therefore exerts its own in- 

 fluence, and the molecules retain the optical properties 

 which they manifest when the substance crystallizes alone. 

 Hence the propagation of luminous waves is the resultant 

 of the actions which each of the molecules composing 

 the crystal exerts upon the luminous vibrations. If the 

 directions of absorption do not coincide with the axes of 

 optical elasticity, it indicates the presence of molecules of 

 different substances in the crystal. From these considera- 

 tions it will be evident that the anomalies are probably 

 due to the coexistence in the same crystal of different 

 matters, geometrically isomorphous, but optically unlike, 

 and which from the absorption point of view behave as if 

 each were alone. The use of the spectroscope will there- 

 fore enable us to recognize the individuality of differently 



