Farmyard Manure. 25 



sulphuric acid is driven out from the gypsum, and phosphoric 

 acid takes its place. 



Similar chemical reactions, dependent on changes of tempera- 

 ture, are continually taking place in dung-heaps in an active state 

 of fermentation, as well as called into play by heating drainings 

 of dung-heaps. 



I have kept for days a reddened litmus-paper inserted into the 

 neck of a bottle, in which such drainings were placed, without 

 perceiving the slightest change in the colour of the paper, thus 

 proving distinctly that these drainings do not contain a trace of 

 free ammonia. But when the temperature of the drainings is 

 slightly elevated ammonia is given off at once, and continues to 

 escape as long as the liquid is kept boiling, and a good deal of 

 water is left in the vessel in which the liquid is boiled. For 

 this reason it is necessary in determinations of free ammonia in 

 this and similar liquids containing humus-acids, to continue the 

 process of distillation until the liquid is nearly evaporated to 

 dryness. In boiling the drainings of dung-heaps the volatiliza- 

 tion of ammonia is accompanied by the deposition of flakes of 

 humic and ulmic acids, as well as carbonate of lime, held in 

 solution by carbonic acid,^which in boiling is likewise expelled. 



It thus appears that although the affinity of humus acids for 

 ammonia is sufficiently strong completely to prevent its escape 

 at the ordinary temperature, it suffers a change at a slightly 

 elevated temperature, in consequence of which ammonia 

 escapes. Drainings of dung-heaps contain in solution bi-car- 

 bonate of lime, which, at the ordinary temperature of the air, has 

 no effect upon humate and ulmate of ammonia ; on heating, the 

 bi-carbonate of lime loses carbonic acid, and becomes neutral car- 

 bonate of lime, a combination which is capable of decomposing 

 humates and ulmates of ammonia. The ulmic and humic acid 

 of the latter uniting with the lime, with which they form 

 insoluble compounds, leave the ammonia in a free state, and 

 on boiling of liquid it gradually evaporates with the watery 

 vapours. 



The examination of the chemical constitution of the drainings 

 of dung-heaps thus leads at once to the explanation of the reason 

 why hot dung has a pungent smell, caused by the escape of 

 ammonia, and why even rotten dung when cold does not give off 

 any free ammonia. In relation to the amount of ammonia farm- 

 yard manure always contains a great excess of these humus acids, 

 hence the free ammonia proceeding from the interior portions of 

 dung-heaps, which are in an active state of fermentation, is 

 arrested by the humus substances contained in the cold external 

 layers of dung-heaps. In contact with air any undecomposed 



