Chemical Composition of the Thames Water, 123 



This last Table exhibits, I believe, a fair view of the question, 

 and the numbers are in themselves so eloquent, that I leave them 

 without any comment for the reader to make his own deductions 

 from them. 



In conclusion, I must observe that my friend Mr. Simpson is 

 of opinion, from his own practical experience in the matter, that 

 the results detailed above can only be considered as representing 

 the mean composition for the year during which the experiments 

 were made j that, in fact, from the much greater increase in the 

 volume of the upland water which comes down the river in some 

 years, when the rainfall is sometimes almost double what it was 

 during the past year ; and its diminution during others, when 

 the rainfall does not exceed one-half what it then was, — there 

 is every reason to believe that far greater fluctuations in com- 

 position sometimes occur than have been demonstrated by this 

 investigation. 



Appendix to paper '^ On a peculiar Power possessed by Porous 

 Media of removing Matter from Solution in Water." 



Allow me to correct an error in figures which occurred at 

 page 2 of my former paper, relative to the dimensions and volume 

 of water passing through the filters at the Chelsea Waterworks. 

 The total filtering surface there is about three-fourths of an acre, 

 or 32,670 square feet, and the filtration taking place at the rate 

 of one foot per hour, yields about 204,187 gallons of filtered 

 water per hour. 



Since the publication of this paper, I have received the follow- 

 ing letter from Mr. Robert Hunt, F.ll.S., Keeper of the Mining 

 Records ; and the facts therein mentioned may serve to confirm 

 my views respecting the peculiar power of porous media. 



" Museum of Practical Geology, Jermyn Street, 

 July 12, 1856. 



" Dear Sir, — I have read your paper with pleasure and profit. 

 Allow me to give you two examples, within my own knowledge, 

 confirmatory of your views. 



"At Perran-poHh, some six or seven miles north of Truro, 

 the heaps of w^aste [deads) from the old mines contain consider- 

 able quantities of copper pyrites, which is, in the process of de- 

 composition, converted into sulphate of copper. This salt is 

 washed out by the rains, and the solution flows through the 

 sands {blown sand) widely spread over that district. The sul- 

 phate of copper is separated by the sand, and the sand contain- 

 ing the copper is collected from time to time and sold to the 

 copper smelters. Again, at Botallack Mine, the water which 



