On the Action of Light upon the Soluble Iodides. 393 



But it may be fairly assumed that M r is greater or less than 

 Q q, according as N M is greater or less than M Q. Conse- 

 quently, if the rings supplying the annual strata increase in 

 width, the velocity of the ice near the surface also increases, 

 as the pole is approached, and vice versa. 



The permanence of height in the ice-cap must depend upon 

 the fact, that the resolved part of the annual velocity near the 

 surface in the vertical direction is equal to the depth of the 

 annual snowfall. Consequently, for a given path, a widening 

 of the rings corresponding to a greater velocity would be 

 consonant with a thinner ice-cap, and a narrowing of them 

 with a thicker one. 



LXII. Action of Light upon the Soluble Iodides, with the Out- 

 lines of a New Method in Actinometry . By Albert R. 

 Leeds, Ph.D.* 



THE question as to whether potassium iodide, in dilute so- 

 lution, is decomposed by free sulphuric acid, has fre- 

 quently been made a matter of controversy. Schonbein 

 contended that it was not, and, in an acrimonious reply to Prof. 

 Fischer {Journ. fur prakt. Chem. 1845, xxxiv. p. 492), im- 

 pugned the purity of the latter' s chemicals. The same ground 

 was taken by Houzeau, in a discussion with M. L. Sauvage 

 (Comptes Rendus, 1868, lxvii. pp. 633, 714, 1138), the former 

 going so far as to state that, when the solutions were a thou- 

 sand times dilute, no decomposition took place even on pro- 

 tracted boiling. These discrepancies appear to have originated 

 from an oversight of the essential part played by air or oxygen 

 in the reaction. This is represented by the general equation 

 MI + HA + = MA + H 2 + 1, where M indicates the basic and 

 A the acid radical, coefficients being omitted. This holds true 

 not only of the ordinary mineral acids, but has been verified 

 in the three of the organic acids experimented upon — oxalic, 

 tartaric, and acetic acids. In the dark the decomposition di- 

 minishes with the increase of dilution, being indeterminable, 

 when the dilution has reached the one-thousandth, at the end 

 of twelve hours, but plainly recognizable when the dilation 

 has attained the one four-thousandth at the expiration of five 

 days. These figures apply more especially to the potassium- 

 iodide and sulphuric-acid solutions, upon which very nume- 

 rous quantitative determinations of the liberated iodine were 

 made. In sunlight the amount of iodine set free increases in 

 the same ratio as the increase in surface of exposure to the 



* Communicated by the Author. 



