454 Comparative Vegetable Chromatology. [October, 



relative quantity of each kind can be easily determined by 

 having the solutions in two tubes of equal diameter, and 

 diluting one or both until the depth of colour is the same, 

 or still better, until the spectra exactly correspond when 

 compared side by side. The relative amount of each is then 

 known by measuring the length of the columns of solution 

 in the tubes. It is also often easy to ascertain the relative 

 amount of more than one constituent in similar solutions ; 

 for the absorption-bands of one may first be made equal, 

 and then those of another, measuring the relative volumes 

 when the solution of each colouring matter is thus found to 

 be of equal strength. For the future I intend to try to 

 carry out this sort of analysis by means of standard solu- 

 tions, sealed up in tubes free from air; and, if they remain 

 permanent, the relative composition of mixed solutions 

 could be determined without actually comparing them 

 together, which would make it possible to ascertain the 

 changes that take place in plants at different seasons of the 

 year, and to otherwise develope the subject to a far greater 

 extent than heretofore. I have also lately adopted another 

 method, which makes it to some extent possible to dispense 

 with such comparisons. I endeavour to determine the 

 relative proportion of the various coloured constituents in 

 terms of the amount of light absorbed by each. When this 

 is in nearly the same part of the spectrum, the comparison 

 can be made with considerable accuracy; but when it is in 

 very different parts, only approximately, but yet in such a 

 manner as to yield far better results than any other method. 



In applying these principles, of course there are many 

 questions of detail, depending on particular circumstances ; 

 and what I have now described must be looked upon merely 

 as such a general account of the methods I have adopted as 

 seemed to me desirable, since otherwise the possibility of 

 determining some of the facts might have appeared doubtful. 

 I now, therefore, proceed to the consideration of the subject 

 more immediately claiming attention. 



Comparative vegetable chromatology may be divided into 

 two principal parts, viz., that in which we compare leaves 

 or fronds of the same kind of plant growing in different 

 conditions, in order to learn the effects due to external 

 influences, and that in which we compare different plants 

 growing in similar conditions, in order to learn the effects 

 due to internal organisation. Some of the effects of a dif- 

 ference in the amount of light are well known. When it is 

 almost absent, the leaves are yellow and pale, owing to 

 chlorophyll and some other colouring matters not being 



