156 ON THE WATER IN THE CHALK. 



not covered by overlying strata. I find from the Sixth Report of 

 the Rivers' Pollution Commission (pp. loo-i) that out of 42 

 instances of such wells, excluding 12 as exceptional, either from 

 indications of more or less pollution, or as affected by the infiltra- 

 tion of water from the Thames (as at Grays, Erith, Plumstead, 

 etc.), there are 30 instances of apparently normal character, in 

 Avhich the total solid matter held in solution varies between 233 

 and 384 parts per million, the mean being 331 such parts ; and 

 that in these the hardness varies between 19° and 32", the mean 

 being 25 •8.'' The Watford water may be taken as typical. Its 

 analysis in detail is given in the column headed A in the Table 

 on p. 157. AVith respect to water from deep wells in the 

 chalk beneath the London clay, I find (Rivers' Pollution Commis- 

 sioners, 6th Report, p. 103) that out of 11 instances in the Thames 

 basin, excluding 2 as exceptional (to be presently considered), 

 there remain 9 instances which may be regarded as of normal 

 character, and in these the total solid matter held in solution 

 ranges between 330 and 840 parts per million, the mean being 672 

 such parts J also, that the hardness ranges between 6^ and 172'', 

 the mean being 10". 



A comparison of these two results at once forces on our atten- 

 tion the remarkable fact that while the quantity of total solid 

 matter dissolved is much larger on the average in the water from 

 the chalk where it is covered by the London clay than in that from 

 its outcrop, and also ranges between much wider limits, yet the 

 hardness is on the average very much less, though this also ranges 

 between somewhat wider limits. 



This want of relation between the amount of solid consti- 

 tuents and the hardness of the water indicates a totally different 

 mineral constitution in the two kinds of water. This, then, 

 requires examination in detail, and for this purpose I have put 

 together a number of analyses in the subjoined table (p. 157), 

 derived mainly from tables in Watts's Dictionary of Chemistry. In 

 this table the column headed A gives the normal composition of 

 water from the open chalk ; those headed B the composition of 

 various waters from the chalk beneath the London clay, regarded 

 as fairly normal types ; while those headed C refer to certain 

 exceptional waters from the same. 



