290 
DR. ROSTOCK ON THE PURIFICATION OF THAMES WATER. 
table substances ; it is almost always present in the beginning of winter in the 
water of ponds, or of slow streams that have received the falling leaves. After 
the heavy rains that occurred in December 1827, the New River water, with 
which my cistern is supplied, was observed to be very turbid and dark-coloured. 
By remaining some hours at rest, a quantity of earthy matter subsided, and 
left the water nearly transparent, but the dark colour still continued*. 
I found that this colouring matter was not removed by boiling, nor by filtra- 
tion through sand and charcoal, but that alum and certain metallic salts, 
esoecially when heated with it, threw down a precipitate, and left the water 
without colour. Of the metallic salts the most effectual appeared to be the 
sulphate of iron ; a drop of the solution of this salt, boiled with 500 times its 
bulk of the water, threw down a flocculent, orange-coloured precipitate, and 
left the water perfectly colourless. I obtained the same results, only much 
less in degree, when these re-agents were added to the Thames water after its 
depuration. 
The sediment which was removed from the water by filtration, as mentioned 
above, appeared to be a heterogeneous mass of various substances, about ruths 
of which was siliceous sand ; it also contained a black matter, which gave the 
whole a dark gray colour, and which was removed by a red heat, a number of 
fine fibres that looked like animal down, and some large fibres probably of 
vegetable origin ; there were also bits of wood, fragments of coal, and small 
shining particles of a metallic nature, which seemed to be sulphuret of iron. 
The mass indeed consisted of all those substances which were casually intro- 
duced into the Thames, and which had not been decomposed by the fermen- 
tative process. They must of course differ, both in quantity and in quality, 
in every different portion of the water, so to render it unnecessary to attempt 
a more minute examination of them ; in the present instance, the sediment, 
when completely dried at a temperature of 200°, was in the proportion of about 
9 grains in 10,000 grains of the water. 
' It is not easy to institute any exact comparative scale of the shades of brown. An infusion 
formed by digesting, for 10 days, powdered galls in twenty times their weight of water, and after- 
wards diluting the infusion with an equal bulk of water, will exhibit a colour nearly similar to that of 
the New River water in the state in which I examined it. 
