PURIFICATION OF WATER FOR DRINKING PURPOSES 191 



rapid that under favourable circumstances the upper 

 layers of water are, after three hours' time, fit for dis- 

 tribution. Samples of the unsoftened and softened water 

 were examined x with the following results : 



Number of Micro- 

 organisms in 

 1 c.c. of water 



Unsoftened water 322 



Water after softening and two days' subsidence 



drawn from the main service -pipe . . 4 



Reduction in the number of micro-organisms = 99 per cent. 



Gaillet and Huefs Process. In this ingenious modi- 

 fication of Clark's process the water under treatment is 

 mixed with the requisite proportion of lime-water and 

 caustic soda (this effects the precipitation of some of 

 the lime present as sulphate or chloride), and the whole 

 is then made to pass upwards in a sinuous channel 

 through a tower provided with a series of oblique 

 diaphragms, which latter accelerate the deposition of 

 the carbonate of lime. The passage through this tower 

 occupies a period of two hours, during which the organ- 

 isms become entangled in the carbonate of lime pre- 

 cipitate, and are separated with the latter from the 

 water, as is seen from the following examinations made 

 by one of us of the water before and after treatment : 



Number of organisms 

 in 1 c.c. of water 



Artesian Well at Clyde Wharf, London ; untreated 



water from tanks . . . . . 182 



Ditto after treatment by Gaillet and Huet's 



process 4 



Reduction in the number of micro-organisms = 98 per cent. 



The great practical importance of these results is 

 sufficiently obvious, demonstrating, as they do, how by 

 means of Clark's process the most remarkable diminu- 



1 Proceedings of Institution of Civil Engineers, 1886, Percy Frank- 

 land, on * Water Purification.' For a fuller description of these methods, 

 see article by the same on ' Water,' in Thorpe's Dictionary of Chemistry 

 applied to the Arts, dc., 1893. 



