122 



Mr. H. M. Witt on the Variations in the 



Observe, — that the excess is chiefly in the suspended matter, 

 and among the dissolved salts in the chloride of sodium ; but 

 that the increase in the dissolved organic matter is but slight, 

 nearly the whole of the excess of organic matter separating with 

 the mud in an insoluble state. 



It must, however, be remembered, that in this Table the com- 

 position of the water as its exists in the river is compared, not 

 the composition when q/lfe/- filtration it is supplied to the public. 



But having previously investigated the effect of filtration upon 

 the Thames water both at Chelsea and at Kingston*, I am 

 enabled to draw a similar comparison of the composition of the 

 water at these two points after filtration; which being likewise 

 deduced from a large number of experiments, extending in fact 

 over the whole year, may, I think, be relied upon as placing the 

 question in an impartial light. 



On referring to my former paper in the Philosophical Maga- 

 zine, it will be seen that during nine months the composition of 

 the water at Chelsea after filtration was as follows : — 



Table VIII. — Comparison of the average Composition of the 

 Thames Water at Chelsea and at Kingston after filtration, 

 i. e, in the state in which it is supplied for public consumption. 



* See Phil. Mag. vol. xii. p. 23 (for July). 



t Deduced from Table V. page 8 of former paper. 



