Book III. SPECIES OF MINERAL MANURES. 847 



pliuric acid and lime, a considerable quantity of v/ater ; and its composition may be 

 thus expressed : sulphuric acid one proportion 75 ; lime one proportion 55 ; water two 

 proportions 34. 



2297. The nature of gypsum is easily demonstrated : if oil of vitriol be added to 

 quicklime, there is a violent heat produced ; when the mixture is ignited, water is given 

 off, and gypsum alone is the result, if the acid has been used in sufficient quantity ; and 

 gypsum mixed with quicklime, if the quantity has been deficient. Gypsum, free from 

 water, is sometimes found in nature, when it is called anhydrous selenite ; it is distin- 

 guished from common gypsum by giving off no water when heated. When gypsum, free 

 from water, or deprived of water by heat, is made into a paste with water, it rapidly sets 

 by combining with that fluid. Plaster of Paris is powdered dry gypsum, and its pro- 

 perty as a cement, and its use in making casts, depend upon its solidifying a certain 

 quantity of water, and making with it a coherent mass. Gypsum is soluble in about 500 

 times its weight of cold water, and is more soluble in hot water ; so that when water has 

 been boiled in contact with gypsum, crystals of this substance are deposited as the water 

 cools. Gypsum is easily distinguished by its properties of affording precipitates to 

 solutions of oxalates and of barytic salts. It has been much used in America, where 

 it was first introduced by Franklin on his return from Paris, where he had been much 

 struck with its effects. He sowed the words. This has been sown with gypsum, on a 

 field of lucern, near Washington ; the effects astonished every passenger, and the use 

 of the manure quickly became general, and signally efficacious. It has been advan- 

 tageously used in Kent, but in most counties of England it has failed, though tried in 

 various ways, and upon different crops. 



2298. Very discordant notions have been formed as to the mode of operation of gypsum. 

 It has been supposed by some persons to act by its power of attracting moisture from the 

 air ; but this agency must be comparatively insignificant. When combined with watery 

 it retains that fluid too powerfully to yield it to the roots of the plant, and its adhesive 

 attraction for moisture is inconsiderable ; the small quantity in which it is used likewise 

 is a circumstance hostile to this idea. It has been erroneously said, that gypsum assists 

 the putrefaction of animal substances, and the decomposition of manure. 



2299. The ashes of saintfoin, clover, and rye-grass, afford considerable quantities of 

 gypsum ; and the substance probably is intimately combined as a necessary part of their 

 woody fibre. If this be allowed, it is easy to explain the reason why it operates in such 

 small quantities ; for the whole of a clover crop, or saintfoin crop, on an acre, according 

 to estimation, would afford by incineration only three or four bushels of gypsum. The 

 reason why gypsum is not generally efficacious, is probably because most cultivated soils 

 contain it in sufficient quantities for the use of the grasses. In the common course of 

 cultivation, gypsum is furnished in the manure ; for it is contained in stable dung, and in 

 the dung of all cattle fed on grass : and it is not taken up in com crops, or crops of peas 

 and beans, and in very small quantities in turnip crops ; but where lands are exclusively 

 devoted to pasturage and hay, it will be continually consumed. Should these statements 

 be confirmed by future enquiries, a practical inference of some value may be derived from 

 them. It is possible, that lands which have ceased to bear good crops of clover or artificial 

 grasses, may be restored by being manured with gypsum. This substance is found in 

 Oxfordshire, Gloucestershire, Somersetshire, Derbyshire, Yorkshire, &c., and requires 

 only pulverisation for its preparation. 



2300. Upon the use of sulphate of iron, or green vitriol, which is a salt produced 

 from peat in Bedfordshire, some very interesting docimients have been produced by 

 Dr. Pearson ; and there is little doubt that the peat salt and the vitriolic water acted 

 chiefly by producing gypsum. The soils on which both are efficacious are calcareous ; 

 and sulphate of iron is decomposed by the carbonate of lime in such soils. The sul- 

 phate of iron consists of sulphuric acid and oxide of iron, and is an acid and a very 

 soluble salt ; when a solution of it is mixed with carbonate of lime, the sulphuric acid 

 quits the oxide of iron to unite to the lime, and the compounds produced are insipid and 

 comparatively insoluble. 



2301 . Vitriolic impregnations in soils where there is no calcareous matter are injurious ; 

 but it is probably in consequence of their supplying an excess of ferruginous matter 

 to the sap. Oxide of iron, in small quantities, forms a useful part of soils ; it is found 

 in the ashes of plants, and probably is hurtful only in its acid combinations. The ashes 

 of all peats do not afford gypsum. In general, when a recent peat-ash emits a strong 

 smell, resembling that of rotten eggs when acted upon by vinegar, it will furnish 

 gypsum. There is a curious agency of iron in soils which may here be mentioned. Soils 

 containing iron at a minimum of oxidation decompose carbonic acid : the oleaginous 

 parts of manures, by converting the brown oxide, which occurs in every soil, into that 

 with a minimum of oxygen, form a substance capable of aiding the nutrition of plants, 

 by affording them carbon from carbonic acid. ( T. ) 



2302. Phosphate of lime is a combination of pliosphoric acid and lime, one proportion 



