CAUSTIC OR CALCINED 



jvAgnesia. 201 



bination with about 30 per cent, of water. When magnesian lime- 

 stones or dolomites are burned, the quick-lime obtained often contains 

 caustic magnesia also in considerable quantity. This mixture is fre- 

 quently appHed to the land, and, as is well known in many parts of 

 England, with injurious effects, if laid on in too large quantities. The 

 cause of this hot or burning nature, as it is called, of magnesian lime, is 

 not very satisfactorily ascertained. I shall, however, slate two or three 

 facts, which may assist in conducting us to the true cause. 



1°. Quick-lime dissolves in 750 times its weight of water, at the or- 

 dinary temperature of the atmosphere, while pure magnesia requires 

 5142 times its weight. The magnesia, therefore, is not likely to injure 

 living plants directly by entering into their roots in its caustic state, since 

 lime which is seven times more soluble produces no injurious effect. 



2°. It seems to be the result of experience, that magnesia in the state 

 of carbonate is but slightly injurious to the land ; some deny that in this 

 state it has any injurious effect at all. This I fear is doubtful ; we may 

 infer, however, with some degree of probability, that it is from some 

 property possessed by magnesia in the caustic state, and not possessed, 

 or at least in an equal degree, either by quick-lime or by carbonate of 

 magnesia, that its evil influence is chiefly to be ascribed. 



3°. When exposed to the air, quick-lime speedily absorbs water and 

 carbonic acid from the air, forming first a hydrate* in fine powder, and 

 then a carbonate. Caustic magnesia absorbs both of these more slowly 

 than lirnedoes, and in the presence of the latter, or wjien mixed with it, 

 must absorb them more slowly still, since the lime will seize on the 

 greater portion of the moisture and carbonic acid which exists in tlie air, 

 immediately surrounding both. When slaked in the air also, the lime 

 may be transformed in great part into carbonate, while the magnesia 

 still remains in the state of hydrate, and it is a property of this hydrate 

 to attract carbonic acid more feebly and slowly, even than the newly 

 burned magnesia as it comes from the kiln. Hence when buried in the 

 soil, after the lime has become nearly all transformed into carbonate, the 

 magnesia may still be all either in the dry caustic state, or in that of a 

 hydrate only. 



4°. Now there exist in the soil, and probably are exuded from the 

 living roots, various add substances, both of organic and of inorganic 

 origin, which it is one of the functions of lime, when applied to the land, 

 to combine with and render innoxious. But these acid cojn pounds unite 

 rather with the caustic magnesia, than with the lime which is already 

 in combination with carbonic acid — and {orm salts,] which generally- are 

 much more soluble in ivater than the compounds of lime with the same 

 acids. Hence the water that goes to the roots reaches them more or 

 less loaded with magnesian salts, and carries into the vegetable circula- 

 tion more magnesia than is consistent with the Jiealthy growth of the 

 j)lant. 



It is hazardous to reason from the phenomena of animal to those of 



• Compounds of substances with watftr are called hydrates (from the Greek word for wa 

 t^.) Thus slaked lime, a compound of lime with water, is called hydrate oflime—Sind the 

 native compound of magnesia with water, alluded to in the text, is called hydrate of mag- 

 nesia. 



• t Compounds of the bases, — potash, soda, lime, magnesia, &c., — with acids, — sulphuric, 

 muriatic, nitric, acetic (or vinegar), &c.,— are called salts. 



9* 



