Observations on Liehig's " Organic Chemistry. ^^ 139 



to the temperature and rapidity of the fermentation ; it is this, and 

 the sulphuretted hydrogen formed by animal substances putrefy- 

 ing, that give the nauseous smells of the dung heap : the last is 

 the most nauseous, and like rotten eggs. Part of the carbonic acid 

 also is lost. Undoubtedly there is loss in allowing fermentation to 

 proceed. The whole quantity, however, stated by Sir H. Davy's 

 experiments was not loss : the water of the dung would contain 

 great part of the carbonic acid and ammonia which was found in 

 his retort. The loss may be greatly prevented by preserving the 

 urine, &c., as much as possible in a separate state, and covering 

 the dung heap with loam. The question seems to lie in, whether 

 the decomposition will go on as well, or to as much advantage, 

 in the soil, as in the dung heap. I am afraid not : the heat 

 and moisture are much greater in the dung heap; and, when fer- 

 mentation is once commenced, it will proceed much better by 

 keeping in a heap, than spreading out. From the vast quantity 

 of undecomposed organic matter in a fertile soil (I think Dr. 

 Madden estimated it at 10 percent in some fertile soils), the pro- 

 cess of ei'emacausis, or conversion into carbonic acid, must be 

 very slow, when the constituents of the manure are spread in 

 the soil; and putrefaction must render much organic matter soluble 

 in the dung heap, which would not be so for a very long time in 

 the soil, and would not benefit the crops it was intended for. 



The dung heap should not be very deep, and ought to be turned 

 frequently, that as much of the surface may be exposed to the oxy- 

 gen of the air as can be done safely, to carry on the fermentation, 

 and not evaporate too much ; the oxygen unites to the free carbon, 

 forming carbonic acid, and is most needed when the substances 

 are least soluble. It should be bedded above loam, on purpose 

 that the dissolved matter of the dung in the water may not be 

 run off and lost. Very heavy rains should be thrown off by 

 coverings, open at the ends to admit air. In very warm weather 

 the sun should be kept off, and the heat in the manure, so neces- 

 sary to carry on fermentation, should not be allowed to go far : 

 I have seen it, when neglected, evaporate the dung to a white 

 dry straw, nearly in the state of charcoal. Sir H. Davy says, 

 fermentation goes on from S^° to 80°; but there will not be much 

 given off till above 100°. The heap should be turned to prevent 

 the heat going too far, and cool dry air admitted. In dry warm 

 weather, stable manure will sometimes need watering. A layer 

 of loamy earth should be put above all, to absorb as much as 

 possible of the carbonic acid and ammonia given off. If the 

 putrefaction were maintained in a very violent condition, or 

 continued too long, the whole of the soluble matters would be 

 evaporated into gases, and nothing but the earthy salts remain ; 

 and we may err by too much, as well as too little, fermentation. 

 The time cannot be specified ; from three to six months should 



