﻿904 Mr. R. A. Mallet on the Failure of 



intrinsic pressure, and Walden * has calculated the value of 

 the ratio from determinations made at the boiling-point. If 

 surface tension is really proportional to intrinsic pressure, 

 the same ratio should hold at ordinary temperatures, nor is 

 there anything in Walden's calculations inconsistent with 

 this. Accordingly we may write 



Sn = 75-3S<r, 



BH being the increment of intrinsic pressure due to the non- 

 aggregated solute particles and oV the corresponding in- 

 crement of surface tension in dynes per centimetre. Since 

 1000 atmospheres increase of pressure depress the freezing- 

 point of water 8°'5, the depression, 3D, corresponding to 

 the increase of internal pressure SIT would be 



S D _ So-x75-3x8°-5 



1000 

 = 0-64&r°. . . " (v) 



The correspondence between the observed increases in the 

 depression and the calculated values of 0*64 8a, shown in 

 the last column of the table is remarkable. It appears, 

 therefore, that the observed increases in the depression 

 of the freezing-point of aqueous solutions of salts above 

 that due to a normal solute, are caused by the enhanced 

 intrinsic pressures resulting from the greater molecular 

 fields of the solutes. For non-aggregated salts the increased 

 depression is given by the equation (v.). Incidentally this 

 may be regarded as an experimental verification of Walden's 

 relation. 



The Science Museum, 

 South Kensington, 

 London, S.W. 7. 



LXXVI. On the Failure of the Reciprocity Law 

 in Photography. By R. A. Mallet, B.A.f 



TP1HE failure of the photographic plate with a silver 

 JL bromide-gelatine emulsion to obey the " Reciprocity 

 Law" of Bunsen and Roscoe was first observed by Abney f. 

 The first quantitative work on the subject was done by 

 Schwarzschild §, who proposed as an empirical " Law of 



* ZeM.phijsik. Chem. vol. lxvi. p. 385 (1909). 

 t Communicated by Prof. T. R. Merton, F.R.S, 

 t Proc. Roy. Soc. liv. p. 143 (1893). 

 § Astrophysical Journ. xi. p. 89 (1900). 



