ios Dr. Henry's Analysis of several Varieties 
At an early period of the enquiry, it appeared to me pro- 
bable that 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 212" 
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 heat, ( = 4 0 or 5 0 of 
Wedgwood’s pyrometer) lost absolutely nothing 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 decrepitation * which is 
produced by the similar treatment of all the varieties of arti- 
ficial salt. Even these varieties, however, exposed during 
equal times to a low red heat, do not lose more than from 
half a grain to three grains in one hundred. This comparison 
cannot be extended to the salt prepared at a boiling tempera- 
ture from sea water ; because the muriate of magnesia, which 
proportion of muriate of magnesia, that were discovered in the several varieties of 
Scotch salt, sent to me by Dr. Thomson. For this reason, in stating 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 pro- 
bably 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 extraordinary degree of 
purity ; a fact of which I satisfied myself by repeated experiments. 
* Decrepitation is occasioned by the sudden conversion into vapour of the water 
contained in salts, when its quantity is insufficient to effect the watery fusion. It is 
a property peculiar to salts which hold only a very small proportion of water in com- 
bination, as muriate of soda, nitrate of lead, and sulphate of potash. 
