148 Composition of Waters of Land- Drainage and of Rain. 



The next question is, how is this production of nitric acid from 

 manure brought about ? This may possibly happen in several ways. 

 The substances may in the act of decomposition give rise through 

 their nitrogen to nitric acid, which may subsequently be washed 

 through the soil, taking with it any alkali it happens to meet. 



Or, in the progress of its putrefaction the nitrogenous matter 

 may by filtration be converted, as Dr. Angus Smith has shown, 

 into nitric acid. 



Or, lastly, the nitrogen may first form ammonia, which may 

 subsequently be oxidized into nitric acid. I believe the first of 

 these suppositions to be the most reasonable. The production of 

 nitric acid by filtration of putrid animal matter, as shov.'n in the 

 very interesting experiments of Dr. Smith, applies rather to sand 

 (and especially to charcoal) than to soil, because the latter arrests 

 such matters, and so soon as this happens the production of nitric 

 acid would, I believe, be out of the question. In fact I have 

 failed to obtain nitric acid by filtration of putrid matters through 

 soils. This subject, however, requires further examination. 



The conversion of ammonia into nitric acid in the soil I am 

 very unwilling to believe. It is by no means easy to produce 

 this result when the ammonia is out of the sphere of other in- 

 fluences, and the strongest oxidating substances are unable to 

 effect it ; * but when it is united in the soil to substances, such 

 as silica and alumina, this action is still less likely to happen, 

 especially at common temperatures. I have filtered a weak solu- 

 tion of ammonia for days through a column of sand 10 or 12 inches 

 deep, and have not only got no nitric acid in the liquid, but by 

 analysis have recovered all the ammonia originally present. But 

 the production of nitric acid during the decay of highly nitro- 

 genous substances, such as those in question (bones, woollen-rags 

 and such animal matters) when in free contact with the air is 

 well understood, and I entertain no doubt that to this action we 

 must attribute the great quantity of nitric acid in those cases where 

 such manures have been liberally applied. The same remark 

 applies to guano, for it must be remembered that although we 

 are in the habit of speaking of guano as an ammoniacal manure, 

 it is by no means the case that the whole of the nitrogen in it 

 exists in the form of ammonia ; on the contrary, we know that 



upon whicli I am engaged. In one of these, taken from a field from ■which a 

 sample of water was afterwards collected, I found as much as 0-672 grains of 

 nitric acid per lb. This quantity would be equal to 1505 grains (about l-5th lb.) 

 for each ton of soil, or 20 lbs. for each inch in depth of soil on an acre. If we 

 suppose only 12 inches of soil to contain this proportion, we should have 240 lbs. 

 of nitric acid per acre actually in the soil at that time : a quantity quite sufficient 

 to account for that subsequently found in the water. 



* The permanganate of potash does not, I find, even when boiled with a solu- 

 tion of ammonia, give rise to any nitric acid. 



