5 68 



NA TURE 



[April 1 6, 1885 



The more complicated spectra which are frequently observed 

 seem referable to the existence of more than two twin planes in 

 close proximity. There is no reason to think that the explanation 

 of these spectra would involve any new principle not already 

 contained in the explanation of the appearance presented when 

 there are only two twin planes, though the necessary formulae 

 would doubtless be more complicated. 



Corresponding to a wave incident in any direction, in one 

 component of a twin, on the twin plane, there are in general two 

 refracted waves in the second component in planes slightly in- 

 clined to each other, and two reflected waves which alsT have 

 their planes slightly inclined to each other, the angle of inclin- 

 ation, however, being by no means very small, as chlorate of 

 potash is strongly double refracting. The planes of polarisation 

 of the two refracted waves are approximately perpendicular to 

 each other, as are also those of the two reflected waves ; but on 

 account of the different orientation of the two components of the 

 twin, the planes of polarisation of the two refracted waves are 

 in general altogether different from those of the incident wave 

 and of its fellow, the trace of which on the twin plane would 

 travel with the same velocity. In the plane of symmetry at any 

 incidence, and for a small angle of incidence at any azimuth of 

 the plane of incidence, the directions of the planes of polarisation 

 of the two refracted waves agree accurately or nearly with thee 

 of the incident wave and its fellow. In these cases, therefore, 

 an incident wave would produce hardly more than one refracted 

 wave, namely, that one which nearly agrees with the incident 

 wave in direction of polarisation. In these ca^es the colours are 

 not produced. It appears, therefore, that their production 

 demands that the incident wave shall be very determinate!}' 

 divided into two refracted wave , accompanied of course by 

 reflected waves 



It seems evident that the thickness of the stratum affects the 

 result through the difference of phase which it entails in the two 

 refracted waves on arriving at the second twin plane. But 

 whereas in the ordinary case of the production of colour by the 

 interposition of a crystalline plate between a polariser and an 

 analyser, we are concerned only with the difference of retard- 

 ation of the differently polarised pencils which are transmitted 

 across the plate, and not with the absolute retardation, it is 

 possible that in this case we must take into account not only the 

 difference of retardation for the differently polarised pencils 

 which traverse the stratum, but also the absolute retardation ; 

 that is, the retardation of t!-e light reflected from the second 

 relatively to that reflected from the first twin plane. 



21. I have not up to the present seen my way to going further. 

 It is certainly very extraordinary and paradoxical that light 

 sh uld suffer total or all but total reflection at a transparent 

 stratum of the very same substance, merely differing in orient- 

 ation, in which the light had been travelling, and that, inde- 

 pendently of its polarisation. It can have l.othing to do with 

 ordinary total internal reflection, since it is observed at quite 

 moderate incidences, and only within very naivovo limits of the 

 angle of incidence. 



RECENT PROGRESS IN CHEMISTRY^ 

 'THE progress of chemistry during the last year has been con- 

 siderable, and a great deal of interesting and important 

 work has been done. Nevertheless it cannot be said to have 

 been a yeaT productive of any very special discoveries. In 

 physical chemistry the subjects connected with heat have occu- 

 pied a good deal of attention, such as the heat of formation of 

 chemical compounds, &c. Experiments on the liquefaction and 

 solidification of gases by pressure and low temperature have also 

 been continued, and, in addition to the results which were ob- 

 tained some time since, we now know chlorine, not only as a 

 liquid, but also as a crystalline solid. The same is true of hydro- 

 chloric acid, carbonic oxide, silicon fluoride, and assinuretted 

 hydrogen. 



Last year I referred to the work which was being done with 

 hydroxylamine, and also mentioned that another analytical re- 

 agent of equal importance was claiming attention, viz. Emil 

 Fischer's phenylhydrazine. The promise of new work which 

 this substance gave has been fully realised, and it has proved 

 useful, not only as an analytical reagent, but has been the means 

 of producing a number of new and important products. 



Work is still actively pursued on the pyrroline, pyridine, and 



1 From the Annual Address of the President of the Chemical Society, 

 Mr. \V. H. Perkin, F.R.S., March 30. 1885. 



quinoline series, and it is remarkable to see how new methods 

 for the production of bodies of this description are being con- 

 stantly discovered. Those of A. Behrmann and Hofmann, who 

 obtain pyridine derivatives from citramide, and of H. v. Pech- 

 mann, who obtains them from malic acid, may be taken as 

 illustrations. 



It is interesting to notice, in reference to the pyridine 

 series, Ladenburg's experiments (Bet:, xvii. 772-74), who finds 

 that the compounds formed by the union of these bases with 

 the iodides of the alcohol radicals, when strongly heated, yield 

 substituted pyridines in the same way as Hofmann showed some 

 time since that aniline, under like circumstances, yielded sub- 

 stituted anilines, such as toluidine, &c. Hofmann (Ber., xvii. 

 1200) has also found that conine hydrochloride, when distilled 

 with zinc dust, yields a base he has named conyrine, which he 

 believes to be a propyl or isopropyl pyridine ; and this, by 

 treatment with hydriodic acid at 28o°-30o°, regenerates conine, 

 which has exactly the same physiological action as the natural 

 (though it is probably optically inactive). Ladenburg {Ber., 

 xvii. 1196) has obtained a prppylpyridine which, when treated 

 with sodium and alcohol, yields a base smelling very much like 

 conine ; it has many properties in common with conine, and, 

 like it, is poisonous, acting in the same manner and to the same 

 degree. It is, however, optically inactive, as might be expected. 

 It will be remembered that Schiff (.4>w. Ch. Pkirm., clvii. 352) 

 obtained a base very similar to conine from isobutyric aldehyde 

 and ammonia some years ago, but it did not appear to agree in 

 all its properties with that body. From the new work which 

 has been done in this subject we may now soon expect to have 

 the constitution of this base definitely established. Ladenberg 

 has also succeeded in producing piperidine from pyridine. The 

 identity of this product with that obtained from piperine from 

 pepper has been established (Ber., xvii. 5 13-5 15)- 



Hofmann, while continuing his work on the action of bromine 

 in alkaline solutions in amides, has found the curious fact that 

 nitriles are produced in considerable quantities containing one 

 atom of carbon less than the amide — in fact, corresponding with 

 the amines formed in the reaction, and are, in all probability, 

 produced from them by the removal of the hydrogen atoms. As 

 these nitriles can be converted into amides by sulphuric acid, 

 and again treated with bromine and alkali, it is evident that by 

 this means we can gradually work down step by step from one 

 member of the homologous series to another. 



It will be remembered that Pechmann and Duisberg (Ber., 

 xvi. 21 19-2128) succeeded in obtaining substituted coumarins 

 and their hydroxy derivatives by acting on aceto and benzoyl- 

 acetic acids with phenols. Pechmann (Bei ., xvii. 929 936) has 

 now succeeded in obtaining coumarins by treating malic acid and 

 phenols with sulphuric acid or chloride of zinc ; with ordinary 

 phenol he has obtained coumarin ; with resorcinol, umbelliferone 

 and will; pyragallol, daphnetin, which gives all the reactions 

 of the natural body. 



Some very curious results have lately been obtained in refer- 

 ence to the destructive action of aluminium chloride on hydro- 

 carbons. Friedel and Crafts communicated a paper on this 

 subject to this Society in 1882 ; it has now been further studied 

 by Auschiitz, Immerdorff, and by Jacobson (Ber., xviii. 657). 

 They have found that this action consists in "a transference of 

 the alcohol radical from one molecule of a hydrocarbon to 

 another molecule of the same hydrocarbon." Thus toluene 

 yields, on the one hand, benzene, and on the other, xylene 

 and more highly methylated benzenes, orthoderivatives being 

 very rarely found among the products. 



Last year I referred to the discovery of thiophene, or, more 

 properly, thiophen, and its homologues by Victor Meyer. 

 During the yfar our knowledge of this interesting body has 

 been considerably extended, and its preparation rendered com- 

 paratively easy. H. E. Schulze (Ber., xviii. 497) has recently 

 drawn that it is contained — as might be expected — in the sul- 

 phuric acid used to purify crude benzene, and that if its de- 

 composition be prevented by diluting the acid with an equal 

 bulk of water as soon as it is separated from the benzene, the 

 thiophen which is doubtless present in the form of a sulpho acid 

 nay easily be recovered by hydrolysing, by merely passing steam 

 into the acid liquid. 



The synthesis of thiophen, recently effected by J. Volhard 

 and H. Erdmann (Ber., xviii. 454) by merely distilling sodium 

 succinate with phosphorus trisulphide (by which about 50 per 

 cent, of the theoretical yield is obtained), is also of interest, as 

 well as the production of methylthiophen from sodium pyro- 



