18/3.] 



Comparative Vegetable Chromatology . 



449 



(A), very slightly affected by light, the result might be expressed 

 thus : — 



Original state lOA + 10b + lQc 



Subsequent states 



Comparative Quantitative Analysis. 



By applying the above-described general methods to the study of the 

 mixed coloured solutions obtained from plants, there is usually no serious 

 difficulty in carrying out a qualitative analysis, even though, as often 

 happens, half a dozen or more coloured substances are present. It is, 

 however, often very desirable to determine them quantitatively; not, indeed, 

 their relative weights in one case, but the relative amount of each 

 particular colouring-matter in different plants or in the same when 

 grown in different conditions — a kind of comparative quantitative 

 analysis. In some cases this can be done without difficulty. The various 

 coloured constituents can be separated with sufficient accuracy, and the 

 relative volumes of equally strong solution determined. The most con- 

 venient plan is to have test-tubes of the same internal diameter, and to 

 dilute the solution in one or in both until the intensity of the colour is 

 exactly the same, or, still better, until the characteristic absorption is equal, 

 as seen in the spectroscope, with equal illumination. The relative length 

 of the column in the tubes then, of course, gives the relative amount of 

 the particular colouring-matter. We may also thus ascertain the relative 

 quantity of two or even more in the same solution, by making the re- 

 spective bands equal, first in the case of one and then in that of the 

 other. I intend to construct a form of spectrum-microscope and pre- 

 pare apparatus specially suited for accurate comparisons, which require 

 perfectly equal illumination ; but even those analyses which I have been 

 able to make by means of my present instruments are sufficient to show 

 the value of such quantitative methods. I have hitherto chiefly com- 

 pared the different specimens together, but for the future I shall prepare 

 suitable standard solutions, sealed up in tubes with as little air as possible, 

 protected by oil of citronelle ; and if it turn out that they remain un- 

 altered by long keeping, it will thus be possible to determine the re- 

 lative amount of the various colouring-matters in plants growing at 

 different seasons of the year. 



Fundamental Colouring-matters. 



Trusting that the above account will be sufficient to indicate the 

 general characters of the methods I have employed, I now proceed to the 

 consideration of the individual colouring-matters. The total number 

 met with in plants is very great, possibly some hundreds ; but a large 

 proportion occur only in the petals, fruits, seeds, and roots. The study 

 of these will no doubt throw much light on particular questions, but 



J 9A + 5c 

 { 8A 



