304 LIMING. 



The limestone that is used for burning is seldom pure; it fre- 

 quently contains clay, quartzy sand, metallic oxides, and occasionally 

 carbonaceous matter ; frequently too it is so largely mixed with 

 magnesia that it acquires peculiar characters ; this is the magnesian 

 limestone or dolomite. The purest carbonate of lime, by exposure 

 for some time to a white heat, loses 43.7 of carbonic acid, and con- 

 sequently contains 56.3 of caustic lime. Limestone is one of the 

 most common of rocks ; in the crystalline and saccharoid state, or 

 of closer and finer grain, it often constitutes mountain masses, and 

 is met with in every part of the geological series ; it meet«« us as 

 chalk in beds of enormous thickness, filling up extensive basins in 

 the tertiary series ; such are the chalk beds of the south and west 

 coasts of England, extending through the counties of Kent and 

 Sussex, &c. 



The only mineral substance with which chalk, limestone, or car- 

 bonate of lime is likely to be confounded, is gypsum or sulphate of 

 lime. But it is easy to distinguish either of these salts from the 

 other : carbonate of lime dissolves with eflfervescence in dilute 

 hydrochloric acid ; sulphate of lime is insoluble in this liquid. 

 Carbonate of lime is quite insoluble in water ; sulphate of lime is 

 very sensibly soluble, and a copious precipitate falls on the addition 

 of a solution of oxalic acid or of oxalate of ammonia. Gypsum is 

 always so soft that it can be scratched with the nail ; limestone, 

 save in the state of chalk, is generally so hard that it resists the 

 nail. 



The i)urning of lime for agricultural uses is carried on in the same 

 way as for building and other economical purposes. Burnt or quick- 

 lime is a very different article from chalk or limestone ; it is power- 

 fully caustic or destructive of the organic tissue, and instead of 

 being altogether insoluble, it is now soluble in about 630 parts of 

 cold water. All the world knows how lime from the kiln, when 

 watered, rises in temperature, breaks first into larger and then into 

 smaller pieces, and finally falls down into fine powder ; but every 

 one is not aware that there is a true chemical union of water with 

 the earth, and that the resulting powder is in chemical language a 

 hydrate of lime, a substance which is much less caustic than pure 

 lime, but s^till distinctly alkaline in its reaction. 



It is generally admitted that the soil which is without a certain, 

 and that a considerable proportion of the calcareous element, never 

 possesses a high degree of fertility. This in particular is the opin- 

 ion of English agriculturists, who apply lime with a kind of profu- 

 sion ; and the great improvement it frequently produces on the crops 

 of grain, leaves no doubt as to the advantages of the procedure. 

 Still it is now generally recognized that liming ceases to be useful 

 upon lands that are already sufliciently calcareous, or that rest on a 

 sub-soil of chalk. It is, therefore, by supplying the calcareous ele- 

 ment which land requires to constitute it a soil adapted to the growth 

 of corn, that the application of line becomes useful ; liming, in 

 fa«t, enables us to make this neces&iry addition at least cost. Like 

 other mineral manures, lime of itself produces little or no eflfect; 



