io 6 Dr. Henry's Analysis of several Varieties 
All that was necessary for my purpose, was an approximation 
to the truth ; and the introduction of a small error could be 
of no importance, provided it were the same in every case, 
since the comparison would still hold good. 
The specific gravity of rock salt, there can be little diffi- 
culty in determining with precision. A piece of this salt,* of 
such perfect transparency, that I had reserved it as a cabinet 
specimen, weighed in the air 513 grains, and lost, when 
weighed in alcohol, 194 grains. The alcohol, at the tempera- 
ture of 56° Fahrenheit, had the specific gravity of 820, and 
hence that of the salt may be estimated at 2170. Another 
specimen considerably less pure, and more approaching to a 
fibrous fracture, had the specific gravity of 2125 only. 
For ascertaining the specific weights of artificial varieties of 
salts, I used a very simple contrivance. It consisted of a glass 
globe about 3|- diameter, having a stem or neck 10 inches 
long. Sixteen cubic inches of water (each 252 •§• grains at 6 o° 
Fahrenheit,) filled the whole of the globe, and about half 
an inch of the lower part of the neck ; and from the line where 
the water stood in the instrument, it was accurately graduated 
upwards into hundredth parts of a cubical inch. Into this ves- 
sel I poured exactly sixteen cubic inches of a perfectly satu- 
rated solution of common salt ; and then added 400 grains of 
the salt under examination, washing down the particles that 
adhered to the neck by a portion of the liquid, which had been 
previously taken out of the globe for the purpose. As much 
as possible of the air which adhered to the salt, was dislodged 
by agitation ; and the increase of bulk was then observed. 
> - : . , 
Care was taken that the salts were all of equal temperature, 
* Foliated rock salt of Jameson. See his Mineralogy, Vol. II. p. io. 
