ON THE AIR AND WATER OF TOWNS. 25 
_ smell of sulphuretted hydrogen, which filled the neighbouring houses. This 
‘I found the persons accustomed to filter and to drink: the sulphur was con- 
"verted into sulphuric acid, and the water was actually made quite pure. 
_ These are no doubt some of the advantages of a filter; if so, we are then 
to consider that a filter acts according to its cubic dimensions and not by its 
surface only. If the porous rocks have thus the power of oxidizing sulphur 
and nitrogen, we may then ask, have they not also the power of oxidizing 
“carbon? Hydrogen is no doubt oxidized, the ammonia being broken up so 
as to form oxides, as nitric acid and water. 
_ We see that natural filtration, with abundance of room and free move- 
“ments, dissipates the organic matter, and nitric acid too, if ever formed. 
The time allowed for filtration being so short, that is, the time from the 
falling of the rain to the appearance of the pure water from the spring, we 
‘cannot suppose that vegetation accomplishes the purification, whilst there is 
ho deposit of impurity apparent to account for the change. It seems to me 
that the action of the compressed air on the surface of bodies is sufficient to 
answer this question, and that this matter is removed by a process of oxi- 
dation. It was Saussure who showed that humus can unite oxygen and 
hydrogen; Liebig has shown that humus is constantly capable of combining 
' with oxygen, and calls it a constant source of carbonic acid. When then we 
_ see water not very free from organic matter enter a rock and come out free 
from organic matter and sparkling with carbonic acid, leaving no visible 
organic impurities behind it, we may safely conclude that the oxidation of 
_ the carbon has effected it; this then is a higher degree of purification than 
he oxidation of the nitrogen, which is probably allowed to go free. 
__ Processes such as these are going on constantly wherever water is filter- 
ing. On land generally such things must be constantly occurring. The 
 ditch-water of our fields is a very different water from the river-water into 
which it runs, or even of the drains a few feet only below it. Some water 
taken from a ditch in the neighbourhood of Manchester became in a few 
days a complete mass of life, and the many specimens of animalcules in such 
‘water make it a good subject of study. Water froma drain three yards deep 
does not however contain this immense quantity of organic or organizable 
"Matter, depositing only some green matter, partly animal, partly vegetable. 
_ When water flows from hills or elevated land in a river-course, it under- 
‘goes changes according to the nature of the bed, and also according to the 
number of towns on its banks. As an instance of this, I will follow the 
Tiver Thames from its sources to London Bridge without giving the details 
_ of analysis here, but the character of the changes as known to me. 
_ Water from the Seven Springs or from Thames Head or Andover Ford, 
proceeding as it does from the rock, is in the perfectly oxidized state of 
which we have been speaking ; it contains a great deal of carbonic acid and 
_ of lime in solution. When allowed to stand, it preserves its great purity (or 
“clearness of appearance rather) any length of time, not appearing to change. 
Such water as this requires no managing; it would be a good thing if it 
could at once be introduced into houses; it is in fact spring-water from the 
rock, and such water is known to be always good, unless the rock contain de- 
_ Teterious substances. Rocks of course are found which give out a water much 
teer from lime than the water of the Thames sources; such, for example, as 
_ those between Lancashire and Yorkshire. At a place called Swineshaw on 
_ one of these hills a stream gushed out from the hard and insoluble rough 
Tock of this place, having the purity of average distilled water, with a spark- 
ling appearance and agreeableness to the palate which distilled water never 
a 
