Book III. SPECIES OF MINERAL MANURES. 337 



(Salsola soda)f when it grows in inland situations, afford the vegetable alkali ; when it 

 grows on the sea-shore, where compounds which afford the fossil or marine alkali are 

 more abundant, it yields that substance. Du Hamel found that plants which usually 

 grow on the sea-shore, made small progress when planted in soils containing little com- 

 mon salt. The sun-flower, when growing in lands containing no nitre, does not afford 

 that substance ; though when watered by a solutio:^ of nitre, it yields nitre abundantly. 

 The tables of De Saussure show that the ashes of plants are similar in constitution to the 

 soils in which they have vegetated. De Saussure made plants grow in solutions of dif- 

 ferent salts ; and he ascertained that, in all cases, certain portions of the salts were 

 absorbed by tlie plants, and found unaltered in their organs. Even animals do not 

 appear to possess the power of forming the alkaline and earthy substances. Dr. Fordyce 

 found, that when canary-birds, at the time they were laying eggs, were deprived of access 

 to carbonate of lime, their eggs had soft shells ; and if there is any process for which na- 

 ture may be conceived most likely to supply resources of this kind, it is that connected 

 with the reproduction of the species. 



2216. It seems a fair conclusion, as the evidence on the subject now stands, that the dif- 

 ferent earths and saline substances found in the organs of plants, are supplied by the soils 

 in which they grow ; and in no cases composed by new arrangements of the elements in 

 air or water. What may be our ultimate view of the laws of chemistry, or how far our 

 ideas of elementary principles may be simplified, it is impossible to say. We can only- 

 reason from facts. We cannot imitate the powers of composition belonging to vegetable 

 structures ; but at least we can understand them : and as far as our researches have gone, 

 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 de- 

 cayed 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. 



2217. 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 fossile 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. 



2218. The most commonform 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 Qarbonic acid gas is expelled, and then notliing 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. 



2219. 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. > Quick-lime, 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 rhomboidjil 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 tlie 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 suflSciently hard to scratch glass, it contains siliceous, and probably 

 aluminous earth. When it is deep brown or red, or strongly colored 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 thai 

 different ingredients in limestones modify their properties, it will be necessary to consider 

 the operation of pure lime as a manure. 



Z 



