344 SCIENCE OF AGRICULTURE. Part II. 



it appears that in vegetation compound forms are uniformly produced from simple ones ; 

 and the elements in the soil, the atmosphere, and the earth absorbed and made parts of 

 beautiful and diversified structures. The views which have been just developed lead to 

 correct ideas of the operation of those manures which are not necessarily the result of 

 decayed organised bodies, and which are not composed of different proportions of carbon, 

 hydrogen, oxygen, and azote. They must produce their effect, either by becoming a 

 constituent part of the plant, or by acting upon its more essential food, so as to render it 

 more fitted for the purposes of vegetable life. 



SuBSECT. 2. Of the different Species of Mineral Manures. 



2283. Alkaline earths, or alkalies and their combinations, which are found unmixed with 

 the remains of any organised beings, are the only substances which can with propriety be 

 called fossil manures. The only alkaline earths which have been hitherto applied in this 

 way are lime and magnesia ; though potassa and soda, the two fixed alkalies, are both 

 used to a limited extent in certain of their chemical compounds. 



2284. The most common form in which lime is found on the surface of the earth, is in a 

 state of combination with carbonic acid or fixed air. If a piece of limestone or chalk be 

 thrown into a fluid acid, there will be an effervescence. This is owing to the escape of 

 the carbonic acid gas. The lime becomes dissolved in the liquor. When limestone is 

 strongly heated, the carbonic acid gas is expelled, and then nothing remains but the pure 

 alkaline earth ; in this case there is a loss of weight ; and if the fire has been very high, 

 it approaches to one half the weight of the stone ; but in common cases, limestones, if 

 well dried before burning, do not lose much more than 35 to 40 per cent, or from seven 

 to eight parts out of twenty. 



2285. When burnt lime is exposed to the atmosphere, in a certain time it becomes mild, 

 and is the same substance as that precipitated from lime-water ; it is combined with car- 

 bonic acid gas. Quicklime, when first made, is caustic and burning to the tongue, 

 renders vegetable blues green, and is soluble in water ; but when combined with carbonic 

 acid, it loses all these properties, its solubility, and its taste : it regains its power of effer- 

 vescing, and becomes the same chemical substance as chalk or limestone. Very few 

 limestones or chalks consist entirely of lime and carbonic acid. The statuary marbles, 

 or certain of the rhomboidal spars, are almost the only pure species ; and the different 

 properties of limestones, both as manures and cements, depend upon the nature of the in- 

 gredient mixed in the limestone ; for the true calcareous element, the carbonate of lime, 

 is uniformly the same in nature, properties, and effects, and consists of one proportion of 

 carbonic acid 41 "4, and one of lime 55. When a limestone does not copiously effervesce 

 in acids, and is sufficiently hard to scratch glass, it contains silicious, and probably 

 aluminous earth ; when it is deep brown or red, or strongly coloured of any of the 

 shades of brown or yellow, it contains oxide of iron ; when it is not sufficiently hard 

 to scratch glass, but effervesces slowly, and makes the acid in which it effervesces milky, 

 it contains magnesia ; and when it is black, and emits a fetid smell if rubbed, it contains 

 coaly or bituminous matter. Before any opinion can be formed of the manner in which 

 the different ingredients in limestones modify their properties, it will be necessary to con- 

 sider the operation of pure lime as a manure. 



2286. Quicklime, in its pure state, whether in powder or dissolved in water, is injurious 

 to plants. In several instances grass has been killed by watering it with lime-water. 

 But lime, in its state of combination with carbonic acid, is a useful ingredient in soils. 

 Calcareous earth is found in the ashes of the greater number of plants ; and exposed 

 to the air, lime cannot long continue caustic, for the reasons that were just now assigned, 

 but soon becomes united to carbonic acid. When newly burnt lime is exposed to air, it 

 soon falls into powder : in this case it is called slacked lime ; and the same effect is 

 immediately produced by throwing water upon it, when it heats violently, and the water 

 disappears. Slacked lime is merely a combination of lime, with about one third of its 

 weight of water ; i. e. fifty-five parts of lime absorb seventeen parts of water ; and 

 in this case it is composed of a definite proportion of water, and is called by chemists 

 hydrate of lime ; and when hydrate of lime becomes carbonate of lime by long exposure 

 to air, the water is expelled, and the carbonic acid gas takes its place. When lime, 

 whether freshly burnt or slacked, is mixed with any moist fibrous vegetable matter, there 

 is a strong action between the lime and the vegetable matter, and they form a kind 

 of compost together, of which a part is usually soluble in water. By this kind of oper- 

 ation, lime renders matter which was before comparatively inert, nutritive ; and as 

 charcoal and oxygen abound in all vegetable matters, it becomes at the same time con- 

 verted into carbonate of lime. 



2287. Mild lime, powdered limestone, marls, or chalks, have no action of this kind 

 upon vegetable matter ; they prevent the too rapid decomposition of substances already 

 dissolved ; but they have no tendency to form soluble matters. It is obvious from these 

 circumstances, that the operations of quicklime, and marl, or chalk, depend upon prin- 



