CHEMICAL SCIENCE. 191 



Dextrine and cane-sugar are unaffected by light. 



There is a curious action on oxalic acid. If a 4 per cent, solution 

 of the acid be mixed with a 1 per cent, solution of nitrate of ura- 

 nium, and the mixture boiled for even a considerable length of time, 

 provided this is done in the dark, no change takes place. But if 

 the light, even of a clouded sky, have but a momentary action, a 

 decomposition, evidenced by the disengagement of gas, at once sets 

 in ; and if the mixture be placed in the sun, a quantity of carbonic 

 oxide may be collected. That this action is due neither to the tem- 

 perature nor to the free acid, is evident from the fact that at a tem- 

 perature of zero, and with the employment of oxide of uranium, 

 the same results are obtained. 



Direct experiments have shown that animal starch (glycogenous 

 substances) is more rapidly changed into sugar in the light than in 

 the dark ; and remarkably enough, nitrate of uranium decreases 

 instead of increases the action. 



It is remarkable that animal starch in frogs' liver is not changed 

 into sugar in winter, which is also the case with the vegetable 

 starch. 



This might explain why the sugar-forming substances which are so 

 abundant in the membrane of the foetus immediately disappear after 

 birth. 



It can scarcely be doubted that light plays a slow but very 

 powerful part in effecting changes in the animal body ; and it is 

 evident that a knowledge of the substances which accelerate 

 or lessen this action is of great importance in medicine. The 

 symptoms of diabetes, and the action which light has been ob- 

 served to exert on scrofulous persons, may be adduced as cases in 

 point. — Philos. Mag. 



COMPOSITION OF THE PHOTOGRAPHIC IMAGE. 



Mr. Jon:; Spiller, F.C.S., of the War Department, has com- 

 municated to the Philosophical Magazine, No. 126, a paper, of 

 which the following are the opening and closing passages : — 



The composition and chemical nature of the Photographic Image, 

 as produced by the action of light upon the chloride of silver, is 

 even at the present moment, notwithstanding the numerous experi- 

 ments recorded on the subject, one upon which authorities are 

 divided. While there is abundant evidence to show that the darken- 

 ing consequent on exposure to the sun's rays is a process of reduction 

 accompanied with the evolution of chlorine, there are yet two opinions 

 entertained as to the extent to which this reducing action ordinarily 

 proceeds. In accordance with one hypothesis, the white or proto- 

 chloride of silver (Ag CI) is assumed to suffer the full decomposition 

 into its elements, becoming, therefore, reduced to the state of metal ; 

 while, according to a second view, the progress of this reducing 

 action is limited to an intermediate stage, whereby a compound is 

 produced containing less chlorine by one-half than the original sub- 

 stance, and to which the name and formula, subchloride of silver 

 (Ag* CI), have been applied. As a contribution towards a fuller 



