ANALYSIS OF BRITISH AND'FOREIGN SALT. gQj 



from other varieties. But when the impurities, contained 



pn 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 and lejjt earthy 



gait, as 1 have already suggested, will be that which is first 



deposited; *he contamination with the muriate of lime or pf 



magnesia continuing to increase, as the process advances tp 



a conclusion*. 



At hi early period of the inquiry it appeared to me pro- Water of cry*. 

 , * , , , ,.«> .1 ^ •■ ifl r tallizatiorinear- 



bable, that the differences between the several varieties of ly sim n ar inal| 



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



• • • ,, • i- r *. ii- *• T . the same he*t. 

 taming variable proportions ot water or crystallization, it 



pal round, however, by experiment, that the proportion of 



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



212° Fahrenheit, is not much greater or less than that 



which is contained id any other vaiiety. Pure transparent 



rock salt, calcined for half an hour in a low red heat (n 4° 



pr 5° of Wedgwood's pyrometer), lost absolutely nothing of 



its weight. It is remarkable, also, that the pure natire 



salt, if free from adventitious moisture, may be suddenly 



and strongly heated, with scarcely any of that sound called 



decrepitation f, which is produced by the similar treatment 



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



* I cannot on any other principle explain the considerable differ- 

 ences, as to the proportion of muriate of magnesia, that were disco- 

 vered in the several varieties of Scotch salt, sent to me by Dr. Thom- 

 son. For this reason, in stating; the analysis of Scotch salt, I have 

 given, in the t^ble, that result which was most frequently obtained ; 

 and have withheld the names of the manufacturers, because the dif- 

 ferences were. probably in a great measure accidental, and not the re- 

 sult of greater or less skill in the preparation. One specimen of Ly- 

 mington salt, which L 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 extraor- 

 dinary degree of purity ; a fact of which I satisfied myself by repeated 

 experiments. 



t Decrepitation is occasioned by the sudden conversion into yapour Decrepitation! 

 of the water contained in salts, when its quantity is insufficient to ef- 

 fect the watery fusion. It is a property peculiar to salts which hold 

 only a very small proportion of water in combination, as muriate of 

 toda, nitrate of lead, and sulphate of potash. 



however. 



