Mr. Williams on the Destructive Distillation of Boghead Coal. 223 
which the maximum increase occurs. In order to determine whether 
this common property of the curves arises from the general mode of 
action of affinity, or whether the light plays an essential part, we 
have made experiments upon idio-chemical induction, 7. e. action 
in which pure chemical attractions alone effect the alteration. For 
this purpose we employed a dilute aqueous solution of bromine with 
tartaric acid, which mixture, when left to itself in the dark, wnder- 
goes decomposition, hydrobromic acid being formed. By determi- 
ning the amount of free bromine contained in the liquid at different 
times, we became acquainted with the rate at which the decompo- 
sition occurred. Analysis showed that the amount of hydrobromic 
acid formed was not the same in equal spaces of time; and curves 
representing this increase were found to have the form obtained for 
the photo-chemical induction. Hence the cause of this maximum 
increase appears not to lie in any peculiar property of light, but 
rather in the mode of action of affinity itself. 
One of the many interesting applications of the laws of photo- 
chemical induction relates to the phenomena of photography. As 
an instance of this application we cite the remarkable observations 
of Becquerel, which induced him to assume the existence of certain 
rays which can continue, but not commence, chemical action. In 
order to explain the phenomenon observed by the French physicist, 
we do not need to suppose the existence of a new property of light, as 
the facts are easily explained by the laws of photo-chemical induction ; 
and we are satisfied that these relations, which we have examined 
only in the case of chlorine and hydrogen, occur in a slightly modi- 
fied form in other photo-chemical processes. 
Having determined in this part of our investigation the most im- 
portant phenomena of photo-chemical induction, we shall in the 
next section consider the laws which regulate the chemical action of 
light after the induction is completed. 
January 22.—Dr. W. A. Miller, V.P., in the Chair. 
The following communications were read :— 
“On some of the Products of the Destructive Distillation of Bog- 
head Coal.’”’—Part I. By C. Greville Williams, Esq. 
The paper, of which the following is a brief abstract, constitutes 
the first part of the author’s examination of the hydrocarbons con- 
tained in boghead naphtha. In it he gives the results of his experi- 
ments on that portion of the fluid which resists the action of mono- 
hydrated nitric and sulphuric acids. He had previously stated the 
fact of his having obtained a substance possessing the composition 
and vapour-density of butyle*, and had expressed a belief that he 
should succeed in isolating uot only that radical, but also propyle, 
amyle, and caproyle. The composition of the radicals varies so little, 
that to determine the boiling-points it was necessary to take the 
density of the vapour of all those fractions which distilled anywhere 
near their known boiling-points ; and in each case he regarded that 
* Chem, Gaz., vol. xiv. p. 19. 
