714 Absorption of Ultra-Violet Light by Dilute Solutions. 



dipped. R, S, and T are resistance-boxes, R being large and 

 the sum of S and T being always maintained constant. The 

 wire which is shown connected to C can also be joined to 

 A and B. Let S 1? S 2 , S 3 be the values of S when contact is 

 made with A, B, and C respectively and the electrometer is 

 brought to zero. Let i be the current through the electro- 

 lytic cell, and j that through the boxes S and T. Then we 

 have by Ohm's law 



[resistance of BC] x i = (S 3 — S 2 )/. 



Hence, resistance of BC = — — ~ — . 



fex 



The method can be made very accurate by using about 

 16 volts in the battery, as great sensitiveness is thereby 

 obtained. The effect of polarization is eliminated altogether, 

 and not merely reduced to a minimum, as wdien intermittent 

 or alternate currents are employed. 



The following results for the resistances of the same 

 column of the various liquids were obtained. To get the 

 specific resistance the figures should be divided by about 

 160. The experiments were performed at temperature 13° C. 



Solution. Resistance in ohms. 

 Tap-water 2'73xl0 5 



Do. boiled 2*90xl0 5 



Artificial tap- water 2*66 x 10 5 



Do. without CaCO« ... 1-38 x 10 6 



Distilled water used 2*63 x 10 7 



Now it is doubtless a coincidence that the first and third 

 figures should agree so nearly, but it is evident that no 

 marked difference of dissociation is indicated. As regards 

 the disagreement of the values for boiled tap-water and 

 artificial tap-water without CaC0 3 , it should be mentioned 

 that the boiling was not prolonged, as it was not desired to 

 concentrate the remaining constituents more than necessary. 

 Much of the carbonate may thus have remained in solution. 



We now see that the most probable cause of the superior 

 absorption of real tap-water is the existence of large numbers 

 of different salts in very small quantities. The more varied 

 the constituents the more chance there is of covering the 

 whole range of the ultra-violet spectrum, and a glance at the 

 curve in fig. 1 shows that small concentrations are relatively 

 very effective. Absorption is to be particularly anticipated 

 if coloured salts are present. Now the pipes of the labora- 

 tory water-supply are partly iron ones ; it was consequently 



