COMMON AND MAGNESIAN LIME-STONES. 365 



mountain lime-stones are generally of this kind. In those of inferior 

 quality it may amount to 12 or 20 per cent, while nmny calcareous beds 

 are met with in which the proportion of lime is so small that they will 

 not burn into agricultural or ordinary building lime — refusing to slake 

 or to fall to powder when moistened with water. Of this kind is the 

 Irish calp and the lime-stone nodules which are burned for the man- 

 ufacture of hydraulic limes or cements.* It is easy to ascertain the 

 quantity of earthy matter contained in lime-stone, by simply intro- 

 ducing a known weight of it into cold diluted muriatic acid, and ob- 

 serving or weighing the part which, after 12 hours, refuses to dis- 

 solve or to exhibit any efibrvescence. It is to the presence of these 

 insoluble impurities that lime-stones in general owe their color, pure 

 carbonate of lime being perfectly white. 



2°. Magnesian lime-stone. — Though often nearly white, the mag- 

 nesian lime-stones of our island are generally of a yellow color. They 

 cannot by the eye be distinguished from common lime-stones of a simi- 

 lar color, but they are characterised by containing a greater or less 

 proportion of carbonate of magnesia', which is more or less easily de- 

 tected by analysis. Pure carbonate of magnesia consists of 

 per cent. 



Carbonic acid. .51-7 "^ 



Magnesia 48-3 ! or one ion of pure dry carbonate of magnfesia 



/ contains 9| cwts. of magnesia. 



100 J 



It contains, therefore, a considerably larger proportion of carbonic 

 acid than is present in carbonate of lime. 



Magnesian lime-stone is very abundant, is indeed the prevailing 

 rock in many parts of England (Lee. XL, sec. 4,) but the proportion 

 of carbonate of magnesia it contains is very various in different lo- 

 calities. Even in the same quarry different beds contain very unlike 

 proportions of magnesia, and are therefore more or less fitted for 

 agricultural purposes. Thus several varieties of this lime-stone, ex- 

 amined by myself, from different parts of the county of Durham, 

 contained the two carbonates in the following proportions : 



Garmondswav 97-5 25 trace trace Hard compact grey. 



Stony-gate..'. 98 161 027 012 Crystalline fine grained yellow 



Ful well 95 2-1 3 2-6 \ Honey-combed crystalline 



{ yellow. 



Seaham (A) 96-5 2 3 0-2 10 Hard fine-grained compact. 



(B) 950 1-3 0-2 3 5 Hard porous brown. 



Hartlepool 54.5 4493 033 024 Oolitic yellow. 



HumbledonHill(A)57-9 41-8 7 28 Perfect encrinal columns. 



" " (B) 60-41 38 78 ? 081 Consistingin part encrinal col. 



Ferry Hill 54-1 4472 1-58 46 Yellowish compact. 



Some of these varieties, as we see, contain very little carbonate of 



• Thus that of Aberlhaw contains about 86 of carbonate of lime and 11 of clay, &c. ; 

 that of Yorkshire 62 of carbonate of lime and 34 of clay ; of Sheppy 66 of carbonate of 

 lime and 32 of clay. These lime-stones are burned, and then crushed to an impalpable 

 powder, which sets almost immediately when mixed up with water. 



