I was some years since assured by Mr. Maclure of Philadelphia, that whenever any doubt or hesitation betrayed itself with respect to its fertilizing agency, it was only necessary to sprinkle a small quantity in a meadow, to satisfy the most sceptical; and that this was usually done in the form of letters or characters, which in a short time became so much more luxuriant than the surrounding grass, as to be visible at a considerable distance. It is, I understand, chiefly applied to grass lands as a _top-dressing_; and the American farmers[112] explain its operation upon its solubility in water, and its consequent absorption by the roots of the grass. Davy, in examining the ashes of sainfoin, clover, and rye-grass, which had grown in soils manured by gypsum, found considerable quantities of that substance; and he thinks it probable that it was intimately connected with their woody fibre. He attempts to explain the reason why the application of gypsum is not generally efficacious, by supposing that most of the cultivated soils may already contain it in sufficient quantities for the use of the grasses. I strongly suspect, however, that it will be hereafter discovered to depend upon the nature of the soil in its hygrometric relations. From the facts already recorded, it would appear that it never answers near the sea, nor in wet lands. In consequence of its solubility, it is enabled to penetrate and pervade the whole vegetable structure; and the experiments of Davy have proved its presence in the ashes of plants exposed to its operation, and have rendered it probable that it enters into union with their woody fibre, by which the density of their textures will be increased, and consequently the evaporation from their leaves diminished; I am from such considerations induced to think that gypsum does not act by effecting any chemical change in the soil, but solely by diminishing the plants evaporation. This idea seems to be borne out by the evidence furnished by the different circumstances attending the operation of this manure: we find, for example, that succulent vegetables, planted on dry soils, are those which are principally benefited by its application, and that the various grasses so manured retain their verdure, even in the dryest season and on the most arid lands; at the same time, we find that these crops, especially clover, acquire a proportionate increase in the density of their fibres, that is to say, that they become much more rank and stubborn, and often to such a degree does this take place, that in America, where its effects are best understood, sheep not uncommonly refuse to feed upon them. Upon the same principle we find that, under circumstances or in situations where the evaporation of a plant is provided for by a constant supply of moisture, the effects of gypsum cease to be apparent.