Wheat, clover, turnips, for example, each require certain elements from the soil; they will not flourish where the appropriate elements are absent.  Science teaches us what elements are essential to every species of plants by an analysis of their ashes.  If therefore a soil is found wanting in any of those elements, we discover at once the cause of its barrenness, and its removal may now be readily accomplished.

The empiric attributes all his success to the mechanical operations of agriculture; he experiences and recognises their value, without inquiring what are the causes of their utility, their mode of action:  and yet this scientific knowledge is of the highest importance for regulating the application of power and the expenditure of capital,—­for insuring its economical expenditure and the prevention of waste.  Can it be imagined that the mere passing of the ploughshare or the harrow through the soil—­the mere contact of the iron—­can impart fertility miraculously?  Nobody, perhaps, seriously entertains such an opinion.  Nevertheless, the modus operandi of these mechanical operations is by no means generally understood.  The fact is quite certain, that careful ploughing exerts the most favourable influence:  the surface is thus mechanically divided, changed, increased, and renovated; but the ploughing is only auxiliary to the end sought.

In the effects of time, in what in Agriculture are technically called fallows—­the repose of the fields—­we recognise by science certain chemical actions, which are continually exercised by the elements of the atmosphere upon the whole surface of our globe.  By the action of its oxygen and its carbonic acid, aided by water, rain, changes of temperature, &c., certain elementary constituents of rocks, or of their ruins, which form the soil capable of cultivation, are rendered soluble in water, and conseqently become separable from all their insoluble parts.

These chemical actions, poetically denominates the “tooth of time,” destroy all the works of man, and gradually reduce the hardest rocks to the condition of dust.  By their influence the necessary elements of the soil become fitted for assimilation by plants; and it is precisely the end which is obtained by the mechanical operations of farming.  They accelerate the decomposition of the soil, in order to provide a new generation of plants with the necesary elements in a condition favourable to their assimilation.  It is obvious that the rapidity of the decomposition of a solid body must increase with the extension of its surface; the more points of contact we offer in a given time to the external chemical agent, the more rapid will be its action.

The chemist, in order to prepare a mineral for analysis, to decompose it, or to increase the solubility of its elements, proceeds in the same way as the farmer deals with his fields—­he spares no labour in order to reduce it to the finest powder; he separates the impalpable from the coarser parts by washing, and repeats his mechanical bruising and trituration, being assured his whole process will fail if he is inattentive to this essential and preliminary part of it.