.DUST AND DISEASE. 321 



conspicuous ; you see the track of the beam, but it is not 

 the thick and muddy track revealed in London waters. It 

 has been a subject much debated whether the supply of 

 excellent water which the chalk holds in store could not be 

 rendered available for London. Many of the most eminent 

 engineers and chemists have ardently recommended this 

 source, and have sought to show that not only is its 

 purity unrivalled, but that its quantity is practically inex- 

 haustible. Data sufficient to test this are now, I believe, 

 in existence ; the number of wells sunk in the chalk is so 

 considerable and the quantity of water which they yield is 

 so well known. 



But this water, so admirable as regards freedom from 

 mechanical impurity, labors under the disadvantage of 

 being very hard. It is rendered hard by the large quantity 

 of carbonate of lime which it holds in solution. The chalk- 

 water in the neighborhood of Watford holds in solution 

 about seventeen grains of carbonate of lime per gallon. 

 This, in the old terminology, used to be called seventeen 

 degrees of hardness. Now this hard water is bad for tea, 

 bad for washing ; it furs your boilers, because the lime 

 held in solution is precipitated by boiling. If the water be 

 used cold, its hardness must be neutralized at the expense 

 of soap before it will give a lather. These are serious ob- 

 jections to the use of chalk-water in London. But they 

 are now successfully met by the experimental demonstration 

 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 ; at the Chiltern Hills it 

 is softened for the supply of Tring and Aylesbury. Carter- 

 ham also enjoys the luxury. 



I have visited all these places, and made myself ac- 



