270 Dr. Bostock on the Purification of Thames Water. 



sulphur and ammonia, neither of which could be detected after 

 depuration. 



This depurating process may be denominated a species of 

 fermentation ; /. e. an operation, where a substance, without 

 any addition, undergoes a change in the arrangement of its 

 component parts, and a new compound or compounds are pro- 

 duced. The newly formed compounds were, in this case, en- 

 tirely gaseous, and, except a part of the carbonic acid, were 

 discharged. The saline bodies, being not affected by this pro- 

 cess, remained in solution, leaving the fluid free indeed from 

 what are considered as impurities, yet so much loaded with 

 earthy and neutral salts, as to be converted from a soft into a 

 hard water*. The source of the saline bodies may be supposed 

 to be the organic substances, principally of an animal origin, 

 which are so copiously deposited in the Thames ; of these the 

 most abundant are the excrementitious matters, as well as the 

 parts of various undecomposed animal bodies. The different 

 species of the softer and more soluble animal compounds act 

 as the ferment, and are themselves destroyed, while the salts 

 which were attached to them are left behind. It may be con- 

 ceived therefore, that the more foul is the water, the more com- 

 plete will be the subsequent process of depuration ; and we 

 have hence an explanation of the popular opinion, that the 

 Thames water is peculiarly valuable for sea stores, its extreme 

 impurity inducing the fermentative process, and thus removing 

 from it all those substances which can cause it to undergo any 

 further alteration. 



The brown colour which the water exhibited after its depu- 

 ration appeared to depend on the solution of a minute quantity 

 of what is generally termed extractive matter, and which is 

 observed in water that contains decayed vegetable substances; 

 it is almost always present in the beginning of winter in the 

 water of ponds, or of slow streams that have received the fall- 

 ino- leaves. After the heavy rains that occurred in December 

 18!27, the New River water, with which my cistern is supplied, 

 was observed to be very turbid and dark-coloiu'ed. By re- 

 maining some hours at rest, a quantity of earthy matter sub- 

 sided, and left the water nearly transparent, but the dark co- 

 lour still continued! . 



* The terms hard and soft, as applied to water, are obviously relative ; 

 but water which contains as much as 5 grains in the pint of saline matter, is 

 generally regarded as too hard for n)any ceconflmical and manufacturing 

 processes. The water in question contained 4.36 grains per pint. 



f 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 afterwards 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. 



I found 



