346 SCIENCE OF AGr.ICULTURE. Part II. 



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

 retained if these substances were not made to enter into combination with lime. Thus 

 tlie nutritive qualities of blood, as it exists in the compound of lime and blood known as 

 sugarbaker's scum, are moderated, prolonged, and given out by degrees ; blood alone, 

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



2291. Lime promotes fermentation. In those cases in which fermentation is useful to 

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

 spent tanners' bark was mixed with one fifth of its weight of quicklime, and suffered 

 to remain in a close vessel for three months ; the lime had become coloured, and was 

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

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

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

 of mild lime. 



2292. 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 because they furnish a smaller quantity of quicklime. 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 the land, and may, under certain circumstances, become a food of 

 the plant. 



2293. The subject of the application of the magnestan limestone is one of great interest. 

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

 from a certain limestone, when 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. With great justice and 

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

 it contains. 



2294. 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 

 caustic 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 from magnesia. When a magnesian limestone is burnt, 

 the magnesia is deprived of car])onic 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, in which state it acts as a 

 poison to certain vegetables j 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. Magnesia in its mild state, i. 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. 



2295. A simple test of magnesia in a limestone is its slight effervescence with acids, and 

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

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

 0*8 clay and oxide of iron, Magnesian limestones are usually of a brown or pale yellow 

 colour. They are found in Somersetshire, Leicestershire, Derbyshire, Shropshire, 

 Durham, and Yorkshire ; and in many parts of Ireland, particularly near Belfast. In 

 general, when limestones are not magnesian, their purity will be indicated by their loss 

 of weight in burning ; the more they lose, the larger is the quantity of calcareous 

 matter they contain. The magnesian limestones contain more carbonic acid than the 

 common limestones ; and I have found all of them lose more than half their weight by 

 calcination. 



2296. Gypsum. Besides being used in the forms of lime and carbonate of lime, cal- 

 careous matter is applied for the purposes of agriculture in other combinations. One of 

 these bodies is gypsum or sulphate of lime. This substance consists of sulphuric acid 

 (the same body that exists combined with water in oil of vitriol) and lime ; and when 

 dry it is composed of 55 parts of lime and 15 parts of sulphuric acid. Common gypsum 

 or selenite, such as that found at Shotover Hill, near Oxford, contains, besides sul- 



