10 VETERINARY HYGIENE 



be added so as to form the hydrate of magnesium, which is less 

 soluble than the carbonate. 



The lime, in addition to removing the calcium and magnesium, 

 also causes the precipitation and settlement in the tanks of most 

 of the iron and manganese that may be present, and, what is of 

 great importance, a great deal of the organic matter and water 

 fauna and flora. Clark's process, therefore, purifies as well as 

 softens the water. 



As the treated water readily absorbs CO 2 from the air and as 

 the sedimentation is naturally a slow process, it is hastened by 

 drawing the water through filter cloths as in the Porter- Clark 

 modified process. 



Permanent hardness due to the presence of sulphates and 

 chlorides of calcium and magnesium may be reduced by the addition 

 of washing soda (carbonate of soda), so that there are formed 

 sodium sulphate, sodium chloride and calcium carbonate : 



CaSO 4 + Na 2 CO 3 = Na 2 SO 4 + CaCO 3 

 CaCl 2 +Na a CO,=2NaCl +CaCO 3 



Permutit Process. The name given to this process is taken 

 from the Latin permutare, to exchange. It has been found that 

 when hard waters pass through certain soils containing zeolitic 

 minerals, chiefly silicates of aluminium and calcium, they become 

 softened. Dr. Thresh* found that very hard chalk waters when 

 they percolate through the Thanet sands under Essex and London 

 lose their calcium carbonate and magnesium sulphate, which are 

 replaced by sodium carbonate and sodium sulphate. 



This exchange is effected by a mineral contained in the soil. 

 Cans, of Berlin, made artificial zeolites, " sodium permutit," by 

 fusing silica, alumina, and sodium carbonate, " a hydrous meta- 

 silicate of sodium and aluminium," being the product to which he 

 gave the name of " Permutit. "f 



Permutit possesses the power of exchanging its base for another 

 base. For instance, if water containing calcium carbonate is passed 

 through sodium-permutit the permutit parts with its sodium and 

 takes in exchange the calcium. Thus the permutit becomes calcium- 

 permutit, and the water contains sodium carbonate. When the 

 permutit has parted with all its sodium it can be regenerated by 

 passing through it a solution of common salt, the permutit taking 

 the sodium and liberating in the water a soluble calcium salt. By 

 this way the permutit can be used over and over again. Very hard 



* The Examination of Waters and Water Supplies, 1913. 

 t Water Supplies, Rideal, 1914. 



