160 ON THE WATER IN THE CHALK 



Colney Hatch, where an analysis shows Ca. 57 and Mg. 69, but 

 Na. only 70, and CO2, 126. 



The result is that the Harrow water is excessively hard, and 

 that so large a part of its hardness is permanent, that after the 

 application of Clark's process, it still remains hard. Its hardness 

 is also of a peculiar character. As it comes from the well, it has 

 1 4 "8° of hardness and is therefore not a very hard water, consider- 

 ably softer in fact than any of the London companies' waters 

 derived from the Thames. When heated however to about 160^ 

 to 180° Fahrenheit a change takes place and its hardness increases 

 to a maximum of 38 "8°. By actually boiling the water the chalk 

 in solution is precipitated and its hardness then becomes reduced 

 to about 27^ to i8|° If treated by Clark's process, its hardness 

 becomes about 18*^ to 20*^, or about that of Thames water, and 

 at this it remains under all conditions of temperature. 



The well at the Harrow Waterworks was sunk with such 

 care to exclude the water from the sands in the beds above the 

 chalk, that there can be no doubt that we are dealing in this case 

 with water from the chalk alone. I have no information as to the 

 well at Sheepcote Farm, which presents such a relatively large 

 amount of magnesium and sodium sulphates as to render it quite 

 unfit for domestic use. 



The remarkable difference in so limited an area in the waters 

 derived from apparently the same source need explanation. I can 

 offer none better than the supposition that, within this area, the 

 water from the sands above the chalk intermingle more freely than 

 elsewhere from the absence of impervious separating beds, with 

 the chalk water proper below. 



It seems probable from the lines on Mr. Lucas's map that the 

 sand-waters in the neighbourhood of Harrow are under a pressure, 

 which would bring them in a well up to the level of 1 20 ft. above 

 the O.D., while the chalk waters only rise to 102 ft., so that if 

 there were free communication the sand-waters would penetrate 

 into the chalk with a pressure due to a head of about 20 ft. 

 Unfortunatel}', I have been able to get no analysis of these sand- 

 waters so as to judge whether their constitution is such that their 

 intermingling with the chalk waters would be likely to produce a 

 water with such constituents as we actually find. 



