526 Prof. Magnus on the Allotropic Condiiom of Sulphur, 



light.'' It is on the change of the values of m, m', &c. that the 

 difference between the composition of prismatic colours and the 

 composition of the colours of substances depends. 



I willingly admit, on Professor Stokes's authority, that " ab- 

 sorption is not always, nor even generally, accompanied by epi- 

 polic dispersion." Speaking of " certain absorbing media," I 

 said that " absorption generates in greater or less degree epipolic 

 dispersion," meaning, as already explained, internal dispersion. 



I quite concur in referring the natural colours of bodies to 

 absorption. This doctrine I taught long since in optical lectures, 

 and never doubted of its truth. 



From the foregoing consideration of Prof. Stokes's Remarks, it 

 appears that there is no material difference in our views except on 

 two points : the real character of the phsenomenon presented by 

 certain absorbing substances which has been ascribed to subject- 

 ive illusion, and the question of fact whether blue and yellow 

 lights in any instance produce a green compound. 



Cambridge Observatory, 

 December 12, 1856. 



LXVIII. On the Allotropic Conditions of Sulphur, 

 By G. Magnus *. 



MITSCHERLICH has recently shown f that sulphur com- 

 bined with a fat or an oil, dissolves in sulphur and imparts 

 to it an extremely strong colour, so much so that one part of 

 tallow heated with 3000 parts of sulphur furnishes an intensely 

 red mass. I have repeated these experiments, and have con- 

 vinced myself that not only the fats, but also a great number of 

 other substances produce a similar action. Besides the fats, the 

 following substances are particularly distinguished for the strong 

 colour they produce: — stearic acidj paraffine, wax, spermaceti^ 

 fossil wax (ozocerite) ; colophony, mastic, gutta percha, and 

 caoutchouc produce a somewhat less strong colour ; and amber, 

 sugar, starch, and cotton colour still weaker, but even then 

 intensely. The latter only exercise a colouring action when 

 brought into the sulphur heated to 300° C. and well mixed 

 with it. 



Some of these substances may have still contained small 

 quantities of fat ; of many of them this can scarcely be main- 

 tained, but it may be thought that they have acquired a 

 colouring action from having been touched with greasy fingers. 

 In order to meet any objection of this kind, I placed some 

 crystals of white sugar-candy in distilled water, and when the 



* Translated from PogcendoHTs Annalen, vol. xcix. p. 145. 

 t Journal fur Praktische Chemie, vol. Ixvii. p. 369. 



