1873.] Comparative Vegetable Chromatology . 445 



carbon or fixed oils are much more numerous, and often very partially 

 and, as it were, accidentally present : those of the former class may 

 often be separated from one another by means of bisulphide of carbon, 

 alcohol, and water, mixed in proper proportions. As an illustration I 

 take the very simple case of olive Alga?, like Fucus or Laminaria. They 

 should be well crushed, slightly dried, and heated in spirits of wine of 

 the usual strength. Absolute alcohol must not be used, since it dissolves 

 out the chlorophyll in such a manner that it is not carried down as 

 usual, when agitated with bisulphide of carbon. When cold the solu- 

 tion should be agitated in a test-tube with so much excess of the bisul- 

 phide that a considerable quantity subsides to the bottom, carrying with 

 it the whole of the orange xanthophyll* and the greater part of the blue 

 chlorophyll, leaving some of the latter in the alcohol along with nearly 

 all the fucoxanthine and chlorofucine. This solution should then be re- 

 moved by a pipette, and agitated with a fresh quantity of bisulphide, the 

 process being repeated until it subsides with only a very slight tinge of 

 green. The alcoholic solution, on being evaporated and treated with 

 bisulphide, yields approximately pure fucoxanthine. Uniting the different 

 lots of bisulphide containing chlorophyll, and agitating them over and 

 over again with fresh spirit, taking care that a small excess of bisulphide 

 is always present, the whole of the chlorophyll may be removed, and the 

 orange xanthophyll left in the final residue. On adding a little water to 

 the washings containing the chlorophyll, the disolved bisulphide is pre- 

 cipitated, carrying down all the chlorophyll. By proceeding in a similar 

 manner, with or without the addition of weak acids or alkalies, various 

 other colouring-matters found in plants may be more or less perfectly 

 separated. All these processes should be carried on without exposure 

 to strong daylight, or else some of the constituents might be in great 

 measure destroyed. 



Pit o to ch emical Analysis. 



There are many cases in which certain colouring-matters cannot be 

 separated in a satisfactory manner by merely chemical methods, and then 

 fortunately we are able to resort to what I propose to call photochemical 

 analysis. The action of light on the coloured constituents of plants was 

 made the subject of an excellent memoir by Herschel, published in the 

 Philosophical Transactions for 1842, p. 181 ; but the object he had in 

 view and the methods he employed were altogether different to mine. 

 He adopted no means for separating the different constituents of the 

 plants, and some of his solutions probably contained as many as ten 

 different coloured substances ; and, moreover, at that time no attention 

 had been paid to their spectra. He exposed paper coloured with such 

 mixtures to the solar spectrum, so as to learn the effect of each portion ; 

 and his results are of great interest, as showing that the decomposition 



* The exact use of all these terms will be explained in the sequel. 



