Ultra- Violet Light hy Dilute Solutions. 



713 



source of light is composite, and it is possible and even 

 probable that the wave-lengths chiefly absorbed by calcium 

 carbonate are acted on by one or other of the other salts, and 

 thus the addition of single extra salts is relatively ineffective. 

 This phenomenon is also manifested when experiments on 

 boiled and filtered tap-water are performed. The two fluids 

 exhibit practically the same ratio, the original tap-water 

 being in fact very slightly more transparent. 



The large difference of absorption between natural and 

 made-up tap-water may be conceived as due to three causes. 

 Firstly, we have the possible efficacy of small suspended 

 particles or even of micro-organisms in scattering the light. 

 Then there may be a difference of dissociation of the salts in 

 the two cases, or the preponderating cause of the greater 

 absorption of natural tap- water may be the presence of a 

 large number of salts in small quantities. We have seen 

 that ordinary filtration has little effect. If a specimen of tap- 

 water is sterilized and then passed through a porcelain filter, 

 it is found to be rendered more instead of less opaque, 

 although the process of sterilization removes the carbonate of 

 calcium. It is quite evident, therefore, that the first of the 

 three causes of the discrepancy is not of great power. 



In order to find out the difference of dissociation, if any, 

 the electrical resistances of the two fluids were found. I 

 take this opportunity of describing the method employed, 

 which is one that has been in use in the laboratory for some 

 years, and seems free from the objections that can be urged 

 against the use of the telephone. The connexions are shown 



Fiff. 2. 



££± 



in the accompanying figure. The electrolytic cell is provided 

 with two side tubes B, C into which a platinum rod can be 



