So much for the fact of soil absorption; as to the cause or causes of this absorption, a great number of theories have been put forward. Those may be divided into two classes--those accounting for it as due to physical properties of the soil; and those, on the other hand, explaining it as due to chemical action. To the latter class Way's belonged. He explained it as due to the formation in the soil of hydrated double silicates, consisting of a silicate of alumina, along with a silicate of the base fixed. Bruestlein and Peters, on the other hand, were of the opinion that it was purely physical in its nature. A theory has been advanced that it is due to the formation of insoluble ulmates and humates, formed by the union of ulmic and humic acids, along with the bases fixed. Among others who devoted investigation to this interesting question, may be mentioned Rautenberg and Heiden. On reviewing the evidence, it seems to be pretty well established that it really is mainly a chemical act, due chiefly to the formation of double silicates, and doubtless to a certain extent to the formation of insoluble humates and ulmates. Heiden's experiments would seem to indicate, however, that it is also partly of a physical nature. With regard to the absorption of phosphoric acid, this has been shown to be a chemical act, and depends on the formation of insoluble phosphates of calcium, iron, aluminium, and magnesium, the percentage of iron especially determining this. Much analytical work has been accomplished of late years with a view of ascertaining the amount of ash in different kinds of plants, and in the different parts of the plant. _Action of Manures._ The department of agricultural chemistry which has been most largely developed of late years is that connected with the problems of _manuring_. It is, from a practical point of view, of most value. It is some considerable time since we have recognised that the only three ingredients it is, as a rule, expedient to apply as artificial manures, are _nitrogen_, _phosphoric acid_, and _potash_. The nature, mode of action of the different compounds, and properties of these three substances, and their comparative influence in fostering plant-growth, together with the economic question of which form is, under various circumstances, the most economical for the farmer to use, have together given rise to a large number of "field" and "pot" experiments. As the principles underlying this practice form the subject of the following treatise, any further discussion of the question must be left to the following chapters. _Note._--The reader interested in the historical development of agricultural chemistry is referred to Sir J. H. Gilbert's Presidential Address to the Chemical Section of the British Association, 1880. FOOTNOTES: [1] The History of the Chemical Elements. By Sir Henry E. Roscoe, F.R.S. (Wm. Collins, Sons, & Co.)