ON DUST AND DISEASE. 



173 



chalk being so considerable, and the quantity of water 

 which they yield so well known. 



But this water, so admirable as regards freedom from 

 mechanical impurity, labours under the disadvantage of 

 being rendered very hard by the carbonate of lime 

 which it holds in solution. The chalk-water in the 

 neighbourhood of Watford contains about seventeen 

 grains of carbonate of lime per gallon. This, in the 

 old terminology, used to be called seventeen degrees of 

 hardness. This hard water is bad for tea, bad for 

 washing, and it furs our boilers, because the lime held 

 in solution is precipitated by boiling. If the water be 

 used cold, its hardness must be neutralised at the 

 expense of soap, before it will give a lather. These are 

 serious objections to the use of chalk-water in London. 

 But they are successfully met by the fact that such 

 water can be softened inexpensively, and on a grand 

 scale. I had long known the method of softening water 

 called Clark's process, but not until recently, under the 

 guidance of Mr. Homersham, did I see proof of its 

 larger applications. The chalk-water is softened for the 

 supply of the city of Canterbury ; and at the Chiltern 

 Hills it is softened for the supply of Tring and Ayles- 

 bury. Caterham also enjoys the luxury. 



I have visited all these places, and made myself 

 acquainted with the works. At Canterbury there are 

 three reservoirs covered in and protected, by a concrete 

 roof and layers of pebbles, both from the summer's heat 

 and the winter's cold. Each reservoir holds 120,000 

 gallons of water. Adjacent to these reservoirs are 

 others containing pure slaked lime the so-called 

 * cream of lime.' These being filled with water, the 

 lime and water are thoroughly mixed by air forced by 

 an engine through apertures in the bottom of the re- 

 servoir. The water soon dissolves all the lime it is 



