280 ^Ir Philip Sewell on the [sess. lit. 



perature, exposure of soil, and also according to the nature 

 of the solvent in which it is held. Although coi-rections are 

 now carefully made, mistakes are only too common, from 

 errors in observation with the spectroscope, but it appears 

 that a similar spectrum is not a certain indication of an iden- 

 tically similar suVistance, inasmuch as Sachsse has observed 

 allied colours differing in the amount of carbon they con- 

 tained, yet exactly alike in their spectra. The analysis, 

 therefore, which alone will be conclusive as to the true 

 relationship of these colours, lies with the chemist. 



B. Chemical Properties. 



Owing to the large mass of literature relating to chloro- 

 phyll, and in view of the certain relationship which has been 

 detected by Schimper between it and the variously coloured 

 chromo})lasts, it is necessary for us to refer in some detail 

 to the chemical properties of this the most important of all 

 tlie colouring- matters. 



The first difficulty met with as to its composition is that 

 about which so considerable a conflict has been waged — • 

 whether it should be regarded as a definite chemical substance 

 or a mixture of several substances. 



According to Vines, Pringsheim, and otliers the probabilities 

 are in favour of its l)eiug a definite substance. Etiolin, how- 

 ever, is always present with it in the cell, and there is most 

 commonly more oi' less of xanthophyll. Etiolin, as we have 

 .seen, is chlorojiliyll in the process of formation — xanthophyll 

 is generally regarded as a product of its decomposition. 

 Etiolin, as asserted by Hansen, exists in wheat-leaves in 

 the proportion of 1 to 1()() of the cldoi-ophyll ; xantlio- 

 ])hyll, at times, as in autumn leaves, or as among the 

 Diatomacea;, may take the place of, or quite obscure, the 

 chloT'o])hyll. 



Many observers, nolaldy Soi-l)y and Sachsse, have, on the 

 separation of the chltir(jj)hyll from the leaf, found several 

 otlier colouring matters in the solutioii which are distinct 

 fi-om it. These are generally regarded as ])roducts of the 

 decomposition of chlorophyll, due to its removal from tlie 

 cell. Sorliy distinguished seven, three of which he called 

 xanthojjhylls ; Sachsse distinguished nine, five of which were 



