The action of plaster is often puzzling. From the fact that where land has been strongly limed, a small quantity of plaster applied shows such decided benefit, there would seem plausibility in Liebig's theory that its effects must be traceable not to the lime, but to the sulphuric acid. The ammonia in rain-water in the form of carbonate (a volatile salt) is decomposed by plaster, the sulphuric acid having greater affinity for it, thus forming two new compounds, sulphate of ammonia and carbonate of lime. But as arable soil has the same property of absorbing ammonia from the air and rain-water, and fixing it in the same or even a higher degree than lime, there is only the sulphuric acid left to look to for an explanation of the favorable action of plaster on the growth of plants. It is found that plaster in contact with soil undergoes decomposition, part of the lime separating from the sulphuric acid, and magnesia and potash taking its place, quite contrary to the ordinary affinities. These facts show that the action of plaster is very complex, and that it promotes the distribution of both magnesia and potash in the ground, exercising a chemical action upon the soil which extends to any depth of it; and that, in consequence of the chemical and mechanical modifications of the earth, particles of certain nutritive elements become accessible and available to plants that were not so before. It is said plaster is of most benefit in wet seasons; such is not always the case. It is certainly beneficial to clover, wet or dry; so of potatoes. A few years since, when the drought was so intense in this section as to render the general potato crop almost a total failure, the writer produced a plentiful crop by the use of plaster alone. On examination at the dryest time, the bottoms of the hills were found to be literally dust, yet in this dust the tubers were swelling finely: the leaves and vines were of a deep rich green, and remained so until frost, while other fields in sight, planted with the same variety, but not treated with plaster, were brown, dead, and not worth digging. That gypsum attracts moisture may be proved by plastering a hill of corn and leaving a hill by it unplastered; the dew will be found deposited in greater abundance on the plastered hill. But, according to Liebig, certain products of the chemical action of plaster enter into and are incorporated with the structure of the plant, closing its breathing pores to such an extent that the plant is enabled to withstand a drought which would prove fatal to it unassisted.