1873.] 



Comparative Vegetable Chromatology . 



447 



applications of this method will be given when describing special 

 cases. 



General Action of Light. 

 Independent of its value in such practical analyses, the study of the 

 action of light on vegetable colouring-matters is of great interest in con- 

 nexion with changes that occur in the plants themselves ; but since so 

 much still remains to be learned, I will now only give a general summary 

 of what I have already ascertained. As a general rule, when solutions 

 of such substances are exposed to the open sun, their colour is destroyed 

 without the formation of any intermediate coloured product ; but in a 

 few cases entirely different coloured compounds are generated, and after- 

 wards themselves destroyed on further exposure. This bleaching effect 

 depends upon the combined presence of light and air ; for if kept in the 

 dark, or if sealed up in tubes quite free from air, and then exposed to the 

 sun, very little or no such alteration occurs. Other things being equal, 

 the rate of change varies enormously according to the nature of the 

 substance, and, with the same substance, varies much according to the 

 solvent. 



Sensitizers. 



The presence of minute quantities of certain oils very greatly increases 

 the rate of change in a solution exposed to the light, and it is convenient 

 to call this their sensitizing action. Probably in many cases it depends 

 upon their ozonizing influence. Such substances occur in certain plants, 

 and the result is that the effect of an equal amount of light is sometimes 

 increased threefold or even more. If this substance could be procured 

 pure in moderate quantity it would be a most valuable reagent, since it 

 has very little or no effect on colouring-matters unless exposed to the 

 light. In the mean time I have found turpentine a very useful sensitizer, 

 the chief objection being that it sometimes acts as an oxidizer, even in the 

 dark, and is too energetic, so that very little should be used. Oil of 

 citronelle has the most powerful reverse action of any that I have ex- 

 amined, and greatly reduces the effect of light. I therefore often use it 

 to protect from change. 



Correlation of Optical and. Chemical Characters. 

 "We might naturally expect that those rays of light which are absorbed 

 most strongly would be most instrumental in effecting decomposition. 

 If this be a law, it is subject to modifying conditions, not yet apparent ; 

 and since many of the changes do not take place without the presence of 

 air, they are due not to light itself alone, but to oxidization intensified by 

 its presence. At all events it is not merely the blue end of the spectrum 

 (the so called chemical rays) that act, for some substances are rapidly 

 destroyed in red light, as was shown by Herschel and subsequently by 

 Gerland*. I have met with so many cases where a number of substances 

 * Archives Neerlandaises, 1872, vol. vii. p. 1 et seq. 



