1. The gradual conversion into gases of a large portion of the organic elements in the manure. Of these gaseous products the most abundant is _carbonic acid gas_ (CO_2). It is in this form that the carbonaceous matter which constitutes the chief portion of the manure escapes into the air. Carbon also escapes into the air, combined with hydrogen, in the form of _carburetted hydrogen_ or _marsh-gas_ (CH_4), a product of the decomposition of organic matter in the presence of a large quantity of water. This gas is consequently found bubbling up through stagnant water. Next to carbonic acid gas, _water_ (H_2O) is the most abundant gaseous product of decomposition. The nitrogen present in the manure, in different forms, is converted by the process of decomposition chiefly into _ammonia_, which, combining with the carbonic acid, forms carbonate of ammonia, a very volatile salt. It is to this fact that one of the great sources of loss in the decomposition of farmyard manure is due. If the temperature of the manure-heap be permitted to rise too high, the carbonate of ammonia volatilises. It is probable, also, that a not inconsiderable portion of the nitrogen escapes into the air in the free state. The last of the most important gaseous products of decomposition are _sulphuretted_ and _phosphoretted hydrogen_. It is to these gases that much of the smell of rotting farmyard manure is due. 2. The second class of substances formed are _soluble organic acids_, such as _humic_ and _ulmic acids_. The function performed by these acids is a very important one. They unite with the ammonia and the alkali substances in the mineral portion of the manure, forming humates and ulmates of ammonia, potash, &c. It is these ulmates that form the black liquor which oozes out from the manure-heap. In very rotten farmyard manure traces of _nitric acid_ may be found; but it must be remembered that the formation of nitrates is practically impossible under the ordinary conditions of active fermentation of farmyard manure, except perhaps in its very last stages. 3. The third class of changes taking place have to do with the mineral portion of the manure. The result of the formation of so much carbonic and other organic acids is to increase the amount of _soluble_ mineral matter very considerably. _Analyses of Farmyard Manure._ It is chiefly to the valuable researches of the late Dr Augustus Voelcker that we owe our knowledge of the composition of old and fresh farmyard manure. All interested in this important question should peruse the original papers on this subject contributed to the 'Journal of the Royal Agricultural Society' by Dr Voelcker. Typical analyses illustrating the variation in the composition of farmyard manure at different stages of decomposition will be found in the Appendix.[164] From what has been already said, it is obvious chat the composition of farmyard manure is of a very variable nature.