112 Farmyard Manure. 



rotten duno^ to be fixed, the addition of dilute sulphuric acid, a 

 solution of green vitriol, and other chemical agents which change 

 volatile compounds of ammonia into non-volatile combinations, is 

 unnecessary and useless. At any rate, tliese and other fixers of 

 ammonia are useful additions to dung-heaps only in so far as 

 they themselves possess fertilising properties. In the paper to 

 which reference has been made, I furnished experimentally the 

 proof that, simultaneously with the formation of ammonia — which 

 always proceeds when organic substances containing nitrogen 

 enter into putrefaction — under ordinaiy circumstances, ulmic, 

 humic, and similar organic acids are produced, whic:h, on account 

 of their great affinity for ammonia, lay hold of any free ammonia 

 generated from excrementitious matters and effectually fix it, 

 provided the temperature of the heap is kept down sufficiently 

 low. In the interior of a dung-heap, where the heat rises often 

 to a temperature of from 120° to 150° F., ammonia, indeed, is 

 given off so abundantly that its presence here becomes patent 

 by its characteristic pungent smell. Such a smell is always 

 observed on turning a manure-heap in an active state of fer- 

 mentation. Fortunately, the external cold layers of dung-heaps 

 act as a chemical filter, and retain the ammonia proceeding 

 from the heated interior portions of the heap so effectually that 

 even a delicate red litmus paper is not altered in the least. As 

 the faintest traces of ammonia turn reddened litmus paper dis- 

 tinctly blue, it is plain that, however strong the smell of a dung- 

 heap may be, it cannot be due to the escape of ammonia if the 

 red colour of the paper is not turned blue by holding it, pre- 

 viously moistened with water, close to a dung-heap. Some 

 doubts having been expressed of the accuracy of this observation, 

 I have repeatedly examined manure-heaps for free ammonia. 

 Numerous experiments, which need not be described in detail, 

 have fully confirmed my former observations. It is true a 

 manure-heap which has just been turned, or which is examined 

 the day after, gives off a small quantity of ammonia. Although 

 this amounts to a m.ere trace, yet it distinctly affects red litmus 

 paper ; but when a dung-heap is allowed to consolidate for a 

 week or so, and is then examined with litmus paper, not a trace 

 of free ammonia can be detected in the air close to the dung- 

 heap, whilst no difficulty is experienced in detecting free am- 

 monia in the interior heated portions of the same heap. I have 

 since found that farmyard manure, perfectly free from volatile 

 carbonate or uncombined ammonia, when macerated in boiling 

 water, gives off a slightly pungent smell, which, as far as its 

 pungency is concerned, is caused by the escape of ammonia. 

 It appeared to me a matter of some interest to investigate the 



