Page:Proceedings of the Royal Society of London Vol 1.djvu/378

Rh tion, it is pretty evident that the earthy muriates discovered in the analysis of sea salt are derived from the portion of the mother water which adheres to the salt after being drained; and accordingly, those salts prepared from sea water that are smallest grained, and conse- quently have the largest proportion of interstice, are debased by the largest proportional quantity of this species of impurity. But of this impurity the Cheshire salts are nearly free, as they do not contain one part in 1000 of earthy muriates; and indeed it is scarcely possi- ble that any portion of Cheshire prepared salt can contain more, since the rock itself does not contain more than 5 in 1000; while on the contrary, in sea Water, the earthy muriates amount to no less than ﬁlﬁrths of the entire quantity of salt contained. Dr. Henry pays particular attention to these muriates, because the propensity of common salt to deliquesce by attracting moisture from the atmosphere, depends in great measure (though not entirely) on the presence of these deliquescent compounds.

Since in the analysis of salts nominally the same, great diﬁ'erence often occurred even in examination by the same process, Dr. Henry endeavoured to trace the origin of this disagreement of his results. And, as he conceived it might arise from the different degrees of pu- rity of the liquor in different stages of its evaporation, he procured three samples of common salt, of which one was taken from the boiler two hours after the ﬁrst application of heat; the second at the end of four hours; and the third at the end of six hours : and he found

The ﬁrst to contain 16 parts of sulphate of lime in 1000 ;

The second to contain 1 l ; and,

The third only 3%.

But on the contrary, when the impurities are of a different species, and are highly soluble, these will be found to abound most in the salt last drawn, on account of the large proportion they then bear to the aggregate contents of the mother liquor.

The author also ascertained the quantities of water contained in the several varieties of salt; but this he found to he very small, and not constant in any one, appearing rather as an accidental than a necessary ingredient in any of them.

Since the diﬁ'erences of chemical composition discoverable by ex- periment are not suﬂicient to account for those properties which are imputed to the several varieties of muriate of soda, the author is of opinion they must depend upon some mechanical property; and the most obvious are the magnitude of the crystals, and their degree of compactness or hardness, which must each retard the process of so- lution; since a given weight of the salt will expose less surface for solution, even from mere magnitude of its particles; and hence will remain more permanently between the different layers of provisions, and furnish a constant supply of saturated brine during the gradual exudation of the ﬂuids originally contained.

For the purpose of estimating the compactness of several different varieties of salt, Dr. Henry took some pains to measure their speciﬁc