JOURNAL 
OF THE 
ASIATIC SOCIETY OF BENGAL. 
Vol. LXIV. Part II.—NATURAL SCIENCE. 
No. II.—1898. 
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 
