302 SCIENCE PROGRESS. 



The specific conductivity of the sample of water from 

 which Kohlrausch's value was obtained was 0*25 x io _I ° at 

 1 8°. Recently Kohlrausch and Heydweiller (124) have 

 shown more clearly than ever to what a large extent the 

 conductivity is affected by the merest traces of impurity. 

 From observations on samples of different degrees of purity, 

 one of which was the purest yet obtained, they calculated, 

 after making use of the views of the new theory of 

 solutions, that the conductivity of pure water was only 

 0*0361 x io~ 10 . The dissociation deduced from this number 

 is : — 



1*05 x io~ 7 at 2 5 . 

 It is certainly surprising, considering the extreme smallness 

 of the amount, that all these methods indicate that at 25 

 every 10 million litres of pure water contain about 1 gram 

 of hydrogen ions. 



Two curious points arise in a paper by Daniell (125). 

 If a sufficiently thin metallic partition be placed in a volta- 

 meter when the current passes no gas is evolved at the 

 partition. The critical thickness for gold is 0'0 4 9 mm., and 

 for aluminium 0'0 3 5 mm. In the case of a copper sulphate 

 solution the salt itself, and not the copper ions, appears to 

 traverse a gold leaf partition. 



Observations on conductivity are now largely used in 

 determining the constitution of organic acids. It is note- 

 worthy that Werner and Miolati (126) have attempted to 

 use the same method to throw light on the structure of the 

 complex double salts of ammoninum and platinum, cobalt, etc. 



Optical properties. — Most of the work on optical pro- 

 perties may be grouped around the dissociation hypothesis. 

 From observations on the refractive index of aqueous 

 solutions, Hallwachs (127) and Bary (128) find there is 

 no evidence of dissociation. It is to be noted, however, 

 that this result is not surprising as the dissociation of the 

 substances used varies little over the changes in concentra- 

 tion studied. According to Muynck (129) the refractive 

 index of solutions of salts containing iodine gives no proof 

 of the presence of the double and triple molecules indicated 

 by Hittorf's transference ratios. 



