198 



Proportion of 

 impurities. 



Insoluble 

 matter. 



Earthy mu« 

 riates. 



ANALYSIS OF BRITISH AND FOREIGN SALT. 



I. The total amount of impurities, and the quantity of 

 real muriate of soda, contained in each variety of common 

 salt, may be learned by inspecting the last two columns of 

 the table. From these it appears, that the foreign bay salt 

 is purer, generally speaking, than salt which is prepared by 

 the rapid evaporation of seawater; but that it is contami- 

 nated with about three times the amount of impurities dis«. 

 coverable in an equal weight of the Cheshire large-grained 

 salt, and with more than twice those that are. found in the 

 stored and common sail of the same district. 



II. The insoluble matter in the foreign salt, after the ac- 

 tion of boiling water, appears to be chiefly argillaceous earth 

 coloured by oxide of iron; and is probably derived in part 

 from the pits, in which the seawater is submitted to evapo- 

 ration. We may, perhaps, assign the same origin to the 

 very minute portion of rnuriate of lime, which is not found 

 in the saU prepared by evaporating seawater in metallic ves- 

 sels, or evea in the mother liquov. or uucrystallizable residue. 

 In seasali prepared by rapid evaporation, the insoluble por- 

 tion is a mixture of carhouate of lime with carbonate of 

 magnesia, and a fine siliciom, sand ; and in the salt prepared 

 from Cheshire brine, it is almost entirely carbonate of lime. 

 The insoluble part of the less pure pieces o<" rock salt is 

 chiefiV a marly earth, with some sulphate of lime. The 

 quantity of this impurity, as it is stated in the table, is con- 

 siderably below the average, which in my experiments has 

 varied from 10 to 45 parts in 1000. Some estimate of its 

 general proportion, when ascertained on a larger scale, may 

 be formed from the fact, that government, in levying the 

 duties, allow* 6.5lb. to the bushel of rock salt, instead of 

 561b., the usual weight of a bushel of salt. 



III. The earthy m uriates, and especially that with base 

 of magnesia, abound most in salt which is prepared by the 

 rapid evaporation of seawater. Now since common salt, in 

 all its forms, contains, as will afterward appear, very little 

 water of crystallization, it is probable, that the muriate of 

 magnesia, discovered by the analysis of seasalt, is derived 

 entirely from that portion ofthe mother liquor, which adheres 

 to the salt after being drained, and which amounts to about 

 pne seventh bf its weight. The larger the size of the grain, 



tke 



