212 Prof. J. Tyndall on the Influence exercised [Dec. 19 



bromine,* but not by iodine. Finally, while hydrochloric and hydro- 

 bromic acid in aqueous solution each resist decomposition when insolated 

 in the presence of free oxygen, it is known that hydriodic acid under 

 like conditions is rapidly destroyed. j* This destruction, according to 

 our experiments, is promoted by all the rays, but is much less active 

 behind red glass than behind blue. It occurs also, but more slowly, 

 in the dark. 



Here we appear to have a phenomenon analogous to the oxidation of 

 the hydrogen of oxalic acid. 



The question arises how far a preliminary dissociation of the con- 

 stituent atoms of the molecule may influence the reaction. It has been 

 clearly shown by M. LemoineJ that hydriodic acid gas is completely 

 dissociated by light; but the same observer states that in aqueous 

 solution no such dissociation in sunlight can be demonstrated-— a fact 

 observed also by M. Berthelot. It may be, however, that the phe-. 

 nomena of dissociation and oxidation under light may go on side by ? 

 side, the presence of oxygen promoting the splitting of hydriodic 

 acid by its determining affinity. In like manner it may be that in the. 

 decomposition of oxalic acid the oxygen plays a similar part, deter- 

 mining the dissociation of C 2 4 .H 2 , and replacing the dissociated -, 

 radicle C 2 4 . The analogy of chlorine, however, leads us to the belief 

 that, in its relations to hydrogen under the influence of light, oxygen 

 may be classed with that element ; but the, reactions above noted 

 would seem to indicate that, under these conditions, its affinity for 

 hydrogen is inferior to that of either chlorine or bromine. § 



We would note also the following known reactions which occur in 

 air and sunlight : 



(1.) The decomposition of arsenamine with formation of water and 

 deposition of arsenic. 



(2.) The absorption of oxygen by and precipitation of sulphur from 

 sulphuretted hydrogen; — reactions which, although occurring in the 

 dark, are accelerated by sunlight. 



V. " Note on the Influence exercised by Light on Organic In- 

 fusions." By John Tyndall, D.C.L., F.R.S., Professor of 

 Natural Philosophy in -the Royal Institution. Received 

 December 17, 1878. ;- ' " 



Early last June I took with;me to the Alps 50 small hermetically 

 sealed flasks containing infusion of cucumber, and 50 containing 



* Br 2 +<EE 2 = 2ECB* + O. 

 t 2JiI + 0=a 2 + H 2 0. 

 % " Annales de Chim. et de Phys.," [5], t. xi. 

 § Under ordinary conditions the direct combination of oxygen and hydrogen 

 gases does not occur in sunlight. 



