358 



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 -xVVVths 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 difference 

 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 1 ; 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 be very small, and 

 not constant in any one, appearing rather as an accidental than a 

 necessary ingredient in any of them. 



Since the differences of chemical composition discoverable by ex- 

 periment are not sufficient 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 



