British and Foreign Salt. 1 1 5 



evaporation advanced, the greatest part of this earthy com- 

 pound being deposited at an early stage of the process. 

 Different specimens of the same kind of salt may, there- 

 fore, differ in chemical purity as much from each other as 

 from other varieties. But when the impurities contained 

 in a solution of muriate of soda are of a different species 

 from those of Cheshire brine, and consist chiefly of the 

 earthy muriates, the order will be reversed, and the purest 

 salt, as I have already suggested, will be that which is first 

 deposited, the contamination with the muriate of lime or 

 of magnesia continuing to increase as the process advances 

 to a conclusion *. 



At an early period of the inquiry, it appeared to me pro- 

 bable thai the differences between the several varieties of 

 culinary salt might depend, in some degree, on their con- 

 taining variable proportions of water of crystallization. It 

 was found, however, by experiment, that the proportion of 

 water in any variety of common salt, after being dried at 

 2 12 J Fahrenheit, is not much greater or less than that which 

 is contained in any other variety. Pure transparent rock- 

 salt, calcined for half an hour in a low red heM, ( = 4° or 

 5° of Wedgwood's pyrometer,) lost absolutely uothing of 

 its weight. It is remarkable, also, that the pure native salt, 

 if free from adventitious moisture, may be suddenly and 

 strongly heated, with scarcely any of that sound called de- 

 crepUationf, which is produced bv the similar treatment of 

 all the varieties of artificial salt. Even these varieties, how- 

 ever, exposed during equal times to a low red heat, do not 

 lose more than from half a grain to three grains in one hun- 

 dred. This comparison cannot be extended to the salt 

 prepared at a boiling temperature from sea water ; because 

 the muriate of magnesia which these varieties contain, is 

 decomposed at a red heat, and deprived of its acid. 



* I cannot on any other principle explain the considerable differences, as 

 to the proportion of muriate of magnesia, that were discovered in the se- 

 veral varieties of Scotch salt sent tome hy Dr. Thomson. For this reason, 

 in seating the analysis of Scotch salt, I have given, in the table, that result 

 which was most frequently obtained; and have withheld the names of the 

 manufacturers, because the differences were probably in a great measure 

 accidental, and not the result of greater or less skill in the preparation. One 

 specimen of Lymington salt which I examined, contained fully as much 

 muriate of magnesia as any of the Scotch samples. The cat salt of that 

 place, however, contrary to my expectation, proved to possess a very ex- 

 traordinary degree of purity; a fact of vvlu'ch I satisfied myself by repeated 

 experiments. 



f Decrepitation is occasioned by the sudden conversion into vapour of 

 the v.arer contained in salts, when its quantity is ir.sCiflicient to effect the 

 watery fusion. It is a property peculiar to salts which hold only a very 

 small proportion of water in combination ; as muriate of soda, nitrate of 

 lead, and sulphate of potash. H 2 The 



