WATER AND ITS PURIFICATION 257 



storage, and (C) filtration, and to apply to Parliament in April 

 for a Provisional Order." 



Experiments on Thames River Water, which forms a large 

 part of the supply to London, have led to similar encouraging 

 conclusions. These are expressed by Dr. Houston in the Report 

 (p. 19) in the following words : " No hesitation is felt in ex- 

 pressing the opinion that river water no matter how impure, 

 may be brought into a condition of absolute safety bacteriologically. 

 and of great relative purity chemically by means of lime." 



The process of water purification sketched in the foregoing 

 lines is applicable on a small scale to the water derived from 

 wells or pools or streams in isolated country districts unprovided 

 with a common supply. But it necessarily requires the use of 

 several rather large tanks, even when the service of one house 

 only has to be provided for, and a little trouble is involved in 

 the regular operations of the preparation of the lime, its inter- 

 mixture under proper conditions with the water, and the disposal 

 of the sludge which gradually accumulates. In such cases it is 

 very convenient to have a means of softening the very hard 

 waters derived from wells in chalky or limestone districts by an 

 operation which is simple and requires only the use of common 

 and familiar materials. The use of the material known as 

 " permutit " affords a very efficient way out of the difficulty 

 which besets the householder in many country houses whether 

 the water supply is used raw, without treatment, or if recourse 

 is had to the lime process. 



Permutit is an interesting case of the application of a mere 

 laboratory product to practical purposes. The permutits are 

 complex silicates, artificially produced, which have the property 

 of exchanging their basic constituents when immersed in appro 

 priate solutions. By melting together china clay (an aluminium 

 silicate) and soda, a compound is formed which after being 

 crushed and washed with water contains the constituents soda, 

 alumina, silica, and water in proportions represented approxi- 

 mately by the formula Na 2 O.Al 2 3 .2Si0 2 .6H 2 0. Its use is for 

 the softening of waters which owe their " hardness " to the 

 presence of lime and magnesia in the form not only of carbonate, 

 but of sulphate, or chloride. The presence of these compounds 

 in any considerable proportion is the cause of the formation of 

 scale in steam boilers, and the destruction of much soap with 

 formation of an insoluble curd when used for washing. If a 



