JOURNAL 



OP THE 



ASIATIC SOCIETY OF BENGAL. 



Vol. LXIV. Part II.— NATURAL SCIENCE, 

 No. II.— 1895. 



Notes on the bleaching action of light on colouring matters. — By 

 Alexander Pedler, F.R. S., &c. 

 [Read, 6th Feb.] 



That many colours fade when exposed to sunlight is a fact which 

 is only too frequently observed, and which admits of no doubt. The 

 colours which are thus bleached are almost invariably of organic 

 nature, while coloured substances of inorganic character are, as a rule, 

 practically unaffected by the action of light. The exact cause of this 

 bleaching action of sunlight on organic colouring matter is, however, 

 not well understood, and the experiments summarized in this note were 

 conducted to add to the sum of our knowledge on this subject. They 

 are, therefore, published not with the hope that they will set the question 

 of the cause of the bleaching action of light at rest, but rather because 

 they strengthen the conclusions which appear to have been arrived at 

 by previous workers on this subject, and to exist in a more or less in- 

 definite form in chemical literature. 



That the subject of the bleaching of colours by light is not yet in a 

 satisfactory condition may be judged by the following quotation from a 

 work published as recently as 1890, by Professor E. Hjeltof Helsingfors, 

 the well known Sweedish chemist, who in his work on " General Organic 

 Chemistry," in the chapter on the " Chemical Action of Light," writes*: — 



" A considerable number of organic colouring matters lose their 

 colours and become bleached by the action of sunlight ; the process by 



* General Organic Chemistry, by Hjelt. Translated by Dr. Tingle, 1890. 



J. ii. 18 



( //I 



