Book III. SPECIES OF MINERAL MANURES. 339 



the same causes ; and always destroys, to a certain extent, the efficacy of animal manures ; 

 either by combining with certain of their elements, or by giving to them new arrange- 

 ments. Lime should never be applied with animal manures, unless they are too rich, or 

 for the purpose of preventing noxious effluvia. It is injurious when mixed with any 

 common dung, and tends to render the extractive matter insoluble. According to 

 Chaptal (Chimie applique^ i^c. i. 153.) lime forms insoluble composts with almost all 

 animal and vegetable substances that are soft, and thus destroys their fermentative proper- 

 ties. Such compounds, however, exposed to the continued action of the air, alter in 

 course of time ; the lime becomes carbonate ; the animal or vegetable matters decompose 

 by degrees, and furnisli new products as vegetable nourishment. In this view, lime 

 presents two great advantages for the nutrition of plants ; the first, that of disposing 

 certain insoluble bodies to form soluble compounds ; the second, that of prolonging the 

 action and nutritive qualities of substances, beyond the term which they would retain 

 them if they were not made to enter into combination with lime. Thus the nutritive 

 qualities of blood, as it exists in the compound of lime and blood known as sugar baker's 

 scum, is moderated, prolonged, and given out by degrees : blood alone applied directly 

 to the roots of plants will destroy them with few or no exceptions. 



2224. Lime promotes fermentation. In thosie cases in which fermentation is useful to 

 produce nutriment from vegetable substances, lime is always efficacious. Some moist 

 tanners' spent bark was mixed with one fifth of its weight of quick-lime, and suffered to 

 remain together in a close vessel for three months ; the lime had become colored, and 

 was effervescent : when water was boiled upon the mixture, it gained a tint of fawn-color, 

 and by evaporation furnished a fawn-colored powder, which must have consisted of lime 

 united to vegetable matter, for it burnt when strongly heated, and left a residuum of 

 mild lime. 



2225. Different kinds of limestones have different effects. The limestones containing 

 alumina and silica are less fitted for the purposes of manure than pure limestones; but 

 the lime formed from them has no noxious quality. Such stones are less efficacious, 

 merely l)ecause they furnish a smaller quantity of quick-lime. There is very seldom 

 any considerable portion of coaly matter in bituminous limestones ; never as much as 

 five parts in 100 ; but such limestones make very good lime. The carbonaceous matter 

 can do no injury to tlie land, and may, under certain circumstances, become a food of 

 the plant. 



2226. The subject of the application of the magnesian limestone is one of great interest. 

 It had been long known to farmers in the neighborhood of Doncaster, that lime made 

 from a certain limestone applied to the land, often injured the crops considerably. 

 Tennant, in making a series of experiments upon this peculiar calcareous substance, 

 found that it contained magnesia ; and on mixing some calcined magnesia with soil, in 

 which he sowed different seeds, he found that they either died or vegetated in a very 

 imperfect manner, and the plants were never healthy. And with great justice and 

 ingenuity he referred the bad effects of the peculiar limestone to the magnesian earth it 

 contains. 



2227. Magnesian limestone is used with good effect in some cases. Magnesia has a 

 much weaker attraction for carbonic acid than lime, and will remain in the state of caus- 

 tic or calcined magnesia for many months, though exposed to the air. And as long as 

 any caustic lime remains, the magnesia cannot be combined with carbonic acid, for lime 

 instantly attracts carbonic acid fron> magnesia. When a magnesian limestone is burnt, 

 the magnesia is deprived of carbonic acid much sooner than the lime ; and if there is not 

 much vegetable or animal matter in the soil to supply by its decomposition carbonic acid, 

 the magnesia will remain for a long while in the caustic state ; and in this state acts as a 

 poison to certain vegetables. And that more magnesian lime may be used upon rich 

 soils, seems to be owing to the circumstance that the decomposition of the manure in them 

 supplies carbonic acid. And magnesia, in its mild state, t, e. fully combined with car- 

 bonic acid, seems to be always a useful constituent of soils. Carbonate of magnesia 

 (procured by boiling the solution of magnesia in supercarbonate of potassa,) was thrown 

 upon grass, and upon growing wheat and barley, so as to render the surface white ; but 

 the vegetation was not injured in the slightest degree. And one of the most fertile parts 

 of Cornwall, the Lizard, is a district in which the soil contains mild magnesian earth. 

 It is obvious, from what has been said, that lime from the magnesian limestone may be 

 applied in large quantities to peats; and that where lands have been injured by the 

 application of too large a quantity of magnesian lime, peat will be a proper and efficient 

 remedy. 



2228. A simple test of magnesia in a limestone is its slight effervescence with acids, and 

 its rendering diluted nitric acid, or aqua fortis, milky. PVom the analysis of Tennant, it 

 appears to contain from 20'3 to 22'5 magnesia ; 29*5 to 31 ? lime ; 47-2 carbonic acid ; 

 08 clay and oxide of iron. Magnesia limestones are usually colored brown or pale 

 yellow. They are found in Somersetshire, Leicestershire, Derbysliire, Shropshire, 



Z 2 



