Dr Davy's Agricultural Discourse. 337 



smaller proportion, in a very finely-divided state, when derived, it 

 may be inferred, like alumine, from the decomposition of certain 

 minerals containing it, such as felspar. In this state it is soluble 

 either by means of carbonated alkali, or carbonic acid alone, as I be- 

 lieve, or water alone, according to a distinguished Swedish chemist. 

 Owing to this quality it is capable of entering into the composition 

 of vegetable textures. When deposited from its solution, it is not 

 in the manner of alumine, but either in minute adhering crystals, or 

 uncrystallized in the form of a compact hard stony crust. 



Lime exists in the soil most generally in the state of carbonate 

 of lime; even if introduced in the caustic state, owing to its strong 

 affinity for carbonic acid, it rapidly absorbs this gas from the atmo- 

 sphere. The carbonate has a strong tendency to crystallize : it un- 

 dergoes crystallization in the act of its formation, when the lime is 

 absorbing carbonic acid. If you precipitate lime from a solution of 

 one of its salts in water by an alkaline carbonate, the carbonate of 

 lime thus obtained will be in minute crystalline grains, — minute, 

 according to our ordinary ideas of bulk, but coarse indeed, if com- 

 pared under the microscope with the precipitate of alumine. Nor 

 has it the property of alumine, as you may satisfy yourselves by a 

 very easy experiment, of retaining or preventing the flow of water. 



Magnesia, like lime, having a considerable affinity for carbonic 

 acid, commonly exists in the soil in the state of carbonate. But it 

 has not the same disposition to crystallize, and in consequence, per- 

 haps, its particles are finer ; at least this may be inferred from the 

 examination of the carbonate, artificially obtained by precipitation 

 by a carbonated alkali, added to the solution of a magnesian salt. 

 These, though finer than those of carbonate of lime procured in the 

 same manner, are visible individually under the microscope ; and 

 are therefore very much larger than those of alumine. And tested 

 by water, the carbonate of magnesia is found to retard, not entirely 

 prevent, its flow and transmission. 



The relative minuteness of the particles of these three earths is 

 well shewn by the time required by each to subside after suspension 

 in water by agitation. It will be found that the carbonate of lime 

 will descend and find its place of rest rapidly ; the carbonate of mag- 

 nesia in slower time; and the alumine by far the slowest. And 

 hence the wide dift'usion of this last-mentioned earth — a happy cir- 

 cumstance in the economy of nature. Washed out of the naked 

 disintegrating rocks by rain, with mineral particles of other kinds, 

 not so minute, but hardly less diffusible, they ai'e carried by rivers 

 into seas, and by tides and currents transported even into the ocean ; 

 there they subside and form beds, destined, it may be, to become 

 fertile soils on islands or even continents, should the rocky founda- 

 tions on which they rest be elevated into the atmosphere, as this 

 island has been, and so many others — covered with beds of clay and 

 soil, which we are sure from their nature are of distant origin. 



VOL. XfJV. NO. LXXXVIII. — APKIL 1848. V 



