444 



Mr. H. C. Sorby on 



[June 19, 



Here, however, I would remark that the exact position of the absorp- 

 tion-bauds given by particular substances varies considerably, according 

 to the nature of the solvent, even though it is quite neutral and exerts 

 no chemical action on the colouring-matter. Thus, for example, the bands 

 may lie much nearer to the red end when dissolved in bisulphide of 

 carbon than when dissolved in benzole or absolute alcohol. This kind of 

 fact has been often noticed by others, but its bearing on some important 

 questions has generally been overlooked. Apparently, as a rule, the 

 bands are more raised towards the blue when the colouring-matter is 

 dissolved than when in a solid state, and this power of raising them 

 varies with the solvent ; but the extent also depends very much on each 

 particular substance. I have carefully determined the order in which 

 various liquids thus raise bands, and compared it with the order of their 

 specific gravity, and of their refractive and dispersive power, but have 

 hitherto failed to recognize any simple connexion between what may be 

 called the band-raising power and any other physical property. In some 

 cases the position of the bands given by the solid substance appears to 

 vary according as it is in a colloid or a crystalline state. Though at 

 first this difference in position may seem to be an objection in the use of 

 the spectrum method, yet it only shows the need of comparing together 

 our specimens in exactly the same condition, and it may be taken advan- 

 tage of as a means for distinguishing closely allied substances which may 

 be influenced in a very different manner and for enabling us to ascertain 

 the condition of a colouring-matter as it exists in an animal or plant. It 

 is thus possible to determine whether it is in a free state or dissolved in 

 an oil or wax, and in the latter case whether it is liquid or solid. In 

 this manner I have been able to detect a difference in the relative 

 quantity of oil in different parts of the same leaf. Various forms of 

 spectrum-microscopes were employed, according to the requirements of 

 the case : and the amount of material at command would often have been 

 too small for examination with any other kind of instrument ; so that I 

 think I may fairly say that comparative vegetable chromatology is the 

 first great subject which has resulted from the use of the spectrum- 

 microscope. 



Separation of Colouring -matters by chemical means. 



On the present occasion I propose to consider almost exclusively those 

 colouring-matters which are soluble in bisulphide of carbon and in fixed 

 oils, but insoluble in water. These appear to be by far the most im- 

 portant in connexion with plant life, or at all events are more uniformly 

 distributed. Those soluble in water and insoluble in bisulphide of 



1869, toI. ix. pp. 43 & 358 ; 1871, vol. xi. p. 215. Monthly Microscopical Journal, 



1870, vol. iii. p. 220 ; 1871, vol. vi. p. 124. Quarterly Journal of Science, 1871, vol i. 

 p. G4. 



