24 MAGNESIAN EARTH. 



4. It seems doubtful whether magnesia, in any form or condition, 

 should be countec^ among the earths of agriculture, or physical 

 constituents of soils. Though very generally diffused through soils, 

 it is usually in extremely small proportions. In this country, so 

 far as my personal observation or other information has extended, 

 no soil is known to contain magnesia, in any form, as a physical 

 or considerable constituent ; and even as a chemical or manuring 

 agent, the quantities present in soil have been so small, and, more- 

 over, so associated with larger proportions of the kindred earth 

 lime, that the effects of the magnesia alone could not be appre- 

 ciated. Nor are the chemical effects of magnesia much better 

 known in Europe, where they are more obvious to observation, and 

 have been more or less remarked upon by all agricultural chemists. 

 They have been considered by most writers as injurious to the fer- 

 tility and productiveness of soils. But, though without any 

 evidence of facts, I would infer the reverse operation of magnesia 

 in small proportions. The grounds of this inference are presented 

 in the general similarity of chemical character of magnesia to lime 

 and also the very general diffusion of magnesia, in some form of 

 combination (though not often as carbonate), in soils, and espe- 

 cially the richest soils.* 



In other parts of the world, however, magnesia is much more 

 abundant. It is present in large and (as there supposed) injurious 

 quantity in the Gatinais (between the rivers Seine and Yonne), in 

 France,")" and also in Cornwall, in England. J 



Magnesia very much resembles lime in most of their known 

 qualities, and especially in their respective chemical affinities to 

 other bodies. The resemblance is perfect in this important respect, 

 that the pure chemical earth magnesia has no natural existence, 

 because of its strong attraction for acids. If made pure by art, it 

 is then the " calcined magnesia" of druggists. In that artificial 

 state, and in which only the pure chemical earth ever exists at all, 

 if exposed to the atmosphere, it soon attracts carbonic acid, and so 



* In a specimen of the celebrated rich alluvial soil of Red River, 

 Louisiana, I found from 1 to 2 per cent, of carbonate of magnesia ; and 

 something less in the equally rich deposit of the Mississippi River, on the 

 Arkansas shore. The rich mud of the Nile contains 4 per cent, of this 

 earth. (Regnault, quoted by Boussingault), Rural Economy, &c., p. 338, 

 (1st Am. ed., 1845.) 



f These peculiar soils were described at length in the "Annales d' Agri- 

 culture Fran$aifie" by M. Puvis, whose article was translated for and pub- 

 lished in the Farmer's Register, vol. iv., p. 212, accompanied by my reasons 

 for doubting the conclusions of the author as to the magnesia being the 

 cause of sterility. 



J The Lizard Downs. (Davy.) This soil is formed in part by the disen- 

 tegration of the underlying serpentine, a magnesian rock. (J. F. "W. 

 Johnston.) 



