24 REPORT—1848, 
as no vegetation is to be perceived in them, even by a microscope, after a long 
period. If however a mere trace of nitric acid be found, as in the well of 
Tower Hill, a green matter deposits on standing. This perfect freedom from 
animal or vegetable growth is a ground for suspicion also of nitrates being 
present, as there generally is a little green matter found in the purest waters, 
unless they pass through great depths of sand or gravel, as in the new red 
sandstone, where the water, if taken from a deep well, is entirely free from 
everything but inorganic salts. 
The fact of some of the wells being freer than others from these salts is a 
proof of a dependence on the state of the soil, and I doubt not that the drain- 
age has the greatest effect on the change made. Some of the mud taken from 
under a street in Manchester, where a sewer had been allowing some moisture 
to ooze out, was found to contain nitrates also in considerable quantities, but 
the sand and gravel below was nearly dry and perfectly free from nitrates. 
Although it is very probable that nitric acid is formed most readily in the 
sand, yet it is also more rapidly carried away, and after much rain we cannot 
expect to find such a soluble salt remaining, 
As to the source of this acid, I made some experiments last year for the 
Metropolitan Sanitary Commission, which I may here relate. My object was 
to get an idea of the nature of filtration. A jar, open at both ends, such as 
is used with an air-pump, was filled with sand, and some putrid yeast, which 
contained no nitric acid, was mixed with pure water and poured on the sand, 
allowing it to filter through. The product of nitric acid was abundant, so 
much so, as when boiled down to give it out at once, on the addition of pro- 
tosulphate of iron and sulphuric acid, making the red fumes of the peroxide 
of nitrogen apparent without the aid of any very refined test. 
Charcoal was tried for the same end; it did not answer, although allowed 
to act for two months; it was put into a large Hessian crucible, and the 
liquid allowed to trickle through the crucible and charcoal together. 
Ox-fiesh was in this manner oxidized into nitric acid, after allowing it to 
putrefy. This result could be obtained by means of an ordinary household 
filter, if the time allowed were long enough and other conditions favourable. 
The same was done on a smaller scale, by allowing nitrogenous organic 
matter to stand over spongy platinum. 
No doubt this is a very important provision of nature for the prevention 
of the evil consequences of putrefaction ; it is the complete destruction of all 
dangerous gases and the perfect purification of the most impure substances ; 
whether it be advisable to drink water having much of this oxidized matter in 
it is another question. We see however in this the two great agents of sanitary 
improvement at work for us, the air and the water acting through the soil; 
whatever goes through such an ordeal is made pure. The drainage of a 
country is therefore that which removes the evil effects of decomposition, as 
well as the excess of moisture. 
The action of air and water on surface is then a powerful one, and pro-_ 
bably is capable of doing many marvellous things with the substances given 
to it to treat. The effect produced on sulphuretted hydrogen is no less de- 
cided. A bottle of strong sulphuretted hydrogen was poured upon the sand- 
filter, and sulphuric acid was the result, with sulphates formed of bases 
which it had washed out of the sand. Sulphuret of ammonium filtered 
through sand contains sulphuretted hydrogen no longer, and will not blacken 
lead, so powerful is this kind of oxidation. 
Water from a pump ina yard not far from me, gave out a disagreeable 
