PROPERTIES OF WATER AND SOURCES OF SUPPLY. 7 



removed more or less successfully by adding lime in 

 solution. This will combine with the CO 2 and be- 

 come chalk, which will settle. 



For those engineers who have sufficient quantity 

 of work in connexion with water supplies to warrant 

 the outlay, a water tester is a very useful asset. The 

 one which has been brought under the writer's notice 

 as being especially useful in this direction, is the 

 Dionic water tester, an invention of Messrs. Digby 

 & Biggs, made by Messrs. Evershed & Vignoles. 

 The principle upon which the apparatus acts is based 



FIG. i. 



on the fact that the conductivity of pure water con- 

 taining any electrolyte substances in solution is due 

 almost entirely to the dissolved substances, and only 

 to a very small extent to the water itself. This 

 theory is established by Kohlrausch. The apparatus 

 is described by the makers as follows. 



The complete apparatus is shown in fig. i, where 

 G is a bent glass tube to contain the water under 

 test, and the A and B are the electrodes for passing 

 the electric current through the water. The elect- 

 rodes are connected by wires to a direct-reading 

 conductivity meter M, and a continuous-current 



