Gypsum, or Plaster-stone, a native combination of calcareous earth with vitriolic acid. It is more loose and friable than limestone, and does not effervesce with acids, either in a crude or calcined state. But, though easily reduced to powder in the fire, it is, according to Cronstedt, nearly as difficult of fusion as lime-stone.

There are various species of gyp-sum discovered in Saxony, Spain, Italy, and other parts of Europe ; and substances of a gypsous nature also abound in several parts of this country. Those found in the counties of Derby and Nottingham, are so fine as to admit of being polished, and manufactured into vases, etc. in a manner similar to alabaster.

The chief use of gypsum, however, is as a material for small ornaments and figures, as well as moulds for casting wax-work, etc. But, within a few years, it has been advantageously employed for fertilizing the soil; and various experiments have been made by different agriculturists, to ascertain its efficacy. From these, it appears to be a most valuable manure ; and a correspondent in the 5th volume of the "Letters of the Bath and West of England Society," states, that he covered a piece of grassland two inches thick, with barn-manure ; while, on another part of the same exhausted land, he scattered gypsum or plaster of Paris, in order to compare its effects with those of the dung. Both spots were mowed twice in the same year, and once in the succeeding : in every crop the land covered with gypsum was more productive. - The effects of the latter manure on cabbages and tur-nips were equally beneficial; and particularly uplands, which were completely exhausted, and abandoned on account of their sandy nature, have thus been rendered fertile. These experiments have been conducted on a very extensive plan in the United States of America, especially in Pennsylvania ; where two crops of grass were annually cut from sandy heights, the first of which yielded upon an average two tons per acre, and the latter, one ; nor has this produce decreased after a succession of six years. In the same State, an old wheat-field was manured with gypsum about ten days after the harvest ; in the ensuing March it was sown with clover ; and early in September more than two tons of rich clover were obtained from each acre. Nine additional bushels of corn per acre were, likewise, produced in that country by a similar treatment of the soil.

Although the numerous experiments made in Britain hare not succeeded in every instance, yet the superiority of gypsum over every other manure, for chalky and dry-calcareous lands, has been clearly evinced.

In the year 1791, Mr. ARTHUR Young scattered on a field of good turnip loam with a gravelly bottom, at the rate of five bushels of gypsum per acre, part of which was afterwards sown with clover, and the rest with wheat. The en-suing summer was uncommonly dry; and, though both the wheat and clover were eventually burnt up, yet previous to the drought, the latter was not only considerably higher, but also thicker, of a deeper, and far more luxuriant colour, and of a broader leaf than any other clover that had not been thus manured. No alteration, however, was discernible in the wheat. Mr. Young concludes his account ("Annals of Agriculture," vol. 10) with observing, that neither a si-milar quantity of night-soil, pigeons' dung, peat-ashes, nor any other substance with which he is acquainted, would have had an equal effect.

In the 17th vol. of the work last quoted, there is an account ex-tracted from a provincial paper, concerning the effects of gypsum ; from which it appears that, if oats be immersed in water, drained, and then gradually mixed with plaster of Paris, till the former were sufficiently dry to be sown evenly, the produce of such prepared oats will be much finer, and far more luxuriant, than from unprepared seed. One bushel of gypsum only was mixed with eight of oats, from which were produced 122 bushels, while 96 only were obtained from an equal quantity without any pre vious preparation. The clear profit, therefore, was 26 bushels of fine oats, and, if the increased weight of l1/4lb. be allowed, it will amount to thirty bushels and a half !,

Sainfoin, grass, and clover, seem to receive the greatest benefit from gypsum, which, for the purpose of manure, ought to be previously broken, either by the hand with hammers, or by mill-stones, and then sifted : in this pulverized state, it may be scattered on the land, at any season of the year, in the proportion of eight or nine bushels per acre. The best time, however, for strewing this dry manure, is previous to gentle showers, by the aid of which its efficacy will be considerably increased.

Mr. Kirwan affirms, in his excellent "Treatise on Manures," that the gypsum successfully employed in agriculture is of a fibrous texture ; and in his opinion clay-soils are more improved by it than the calcareous. This assertion appears to contradict the experience of those who have employed that sub-stance on a large scale, and espe-cially the American farmers. - We shall not attempt to reconcile these differences, because the same manure may be attended with opposite effects on soils variously mixed and combined. The proper season for scattering the plaster of Paris is, according to Mr. Kir-wan, in the month of February or March, when it should be strewed on grass-land, at the rate of eight bushels par acre; as a larger proportion would be detrimental to the soil. He farther observes, that the theory of the effects of gypsum is to be deduced from its uncommon septic property; because it accelerates putrefaction in a higher degree than any other substance. Hence it ought not to be ploughed in, but merely deposited on the surface of the land, in order that the old grass may he speedily converted into coal, to nourish the young vegetables.

Dr. Darwin, however, ques-tions these deductions concerning, bodies promoting putrefaction ; as the advancement of that process has, in general, been judged of simply by the exhaling odour ; which is liable to be altered, or destroyed, by its union with many bodies, without otherwise affecting the tendency to dissolution.

For the prevention of fatal accidents from either swallowing, or inhaling, gypsous matter, we refer the reader to the article LIME, which requires similar precautions and antidotes.