Nitrogen is variable, but, in general, the older peats contain the most. To this topic we shall shortly recur, and now pass on to notice-- _The ultimate composition of the compounds of which peat consists._ Below are tabulated analyses of the organic acids of peat:-- _Carbon._ _Hydrogen._ _Oxygen._ Ulmic acid, artificial from sugar 67.10 4.20 28.70 Humic acid, from Frisian peat 61.10 4.30 34.60 Crenic acid 56.47 2.74 40.78 Apocrenic acid 45.70 4.80 49.50 It is seen that the amount of carbon diminishes from ulmic acid to apocrenic, that of oxygen increases in the same direction and to the same extent, viz.: about 21 _per cent._, while the hydrogen remains nearly the same in all. b. _The mineral part of peat, which remains as ashes_ when the organic matters are burned away, is variable in quantity and composition. Usually a portion of sand or soil is found in it, and this not unfrequently constitutes its larger portion. Some peats leave on burning much carbonate of lime; others chiefly sulphate of lime; the ash of others again is mostly oxyd of iron; silicic, and phosphoric acids, magnesia, potash, soda, alumina and chlorine, also occur in small quantities in the ash of all peats. With one exception (alumina) all these bodies are important ingredients of agricultural plants. In some rare instances, peats are found, which are so impregnated with soluble sulphates of iron and alumina, as to yield these salts to water in large quantity; and sulphate of iron (green vitriol,) has actually been manufactured from such peats, which in consequence have been characterized as _vitriol peats_. Those bases (lime, oxide of iron, etc.,) which are found as carbonates or simple oxides in the ashes, exist in the peat itself in combination with the humic and other organic acids. When these compounds are destroyed by burning, the bases remain united to carbonic acid. 5.--_Chemical Changes that occur in the formation of Peat._ When a plant perishes, its conversion into humus usually begins at once. When exposed to the atmosphere, the oxygen of the air attacks it, uniting with its carbon producing carbonic acid gas, and with its hydrogen generating water. This action goes on, though slowly, even at some depth under water, because the latter dissolves oxygen from the air in small quantity,[2] and constantly resupplies itself as rapidly as the gas is consumed.