712 



Mr. F. B. Pidduck on the Absorption of 



as a much weaker solution produces measurable increase of 

 absorption. 



Potassium Bromide. 



Concentration 

 in 10 -6 normal. 



Thickness 

 15 mm. 



Thickness 

 7*5 mm. 



5 



•910 





10 





•884 



12-5 



•750 





20 





•785 



25 



•573 



•696 



33-3 





•590 



50 



•407 



•505 



100 



•334 



•342 



250 



•245 







1000 





•199 



50,000 



•191 







it was not found possible to obtain consistent results with 

 tap- water, different specimens giving, with 15 mm. thickness, 

 absorption ratios ranging between 'lid and *173. The 

 approximate analysis of Oxford tap-water, as regards the 

 four principal constituents, is as follows : — 



NaCl 2'$ parts by weight in 100,000 



MgS0 4 2-1 „ 



CaSO, 2-8 



CaC0 3 22 



On making up an artificial tap-water of the above composi- 

 tion, it was found that the ratio of electrometer deflexion as 

 compared with distilled water was *674, very much higher 

 than in the natural product. Before entering into the 

 question of the origin of the discrepancy, it is desirable to 

 indicate the relative efficacy of the constituents. The ratio 

 giving the absorption for the separate constituents at the 

 above strengths were 



NaCl -804 



MgSO, -966 



CaS0 4 *853 



CaC0 3 '785 



A mixture of the first three, an artificial tap- water with 

 the calcium carbonate absent, gave a ratio *747. The effect 

 of this last constituent is therefore less in combination than 

 might be inferred from its separate effect. A moment's 

 reflection will, however, show that this should be so, for the 



