C 58 ] 
ifland, falitron j which is the name given by them 
to falt-petre ; and that it is fold for about five pence 
a pound. 
It appears to be the natron or nitrum of the anti- 
ents, or, as it is fometimes called, the foflil alkali, 
which is the bafis of fea-falt: the fame which is 
procured from the Spanifii barilla, and from our own 
kelp. 
The mineral alkali differs from the vegetable in 
its ready cryftallizing without any addition of fixed 
air, which is neceffary to make the latter take the 
form of cryftals * ; and in its not melting in a moift 
air ; and on this laft account it is a much more com- 
modious ingredient in medicinal powders, than the 
vegetable alkali ; as it is not like this apt to run per 
deliquium ; but on the contrary, .inftead of attracting 
moifture from the air, it is robbed by the air of its 
own moifture, fo that its cryftals foon lofe their 
tranfparence, and are turned to powder. The natron 
liquefies in a very gentle heat : it refembles the vege- 
table alkali in tafte and fixednefs, and like that is 
ufcd in making foap and glafs ; and they are both ap- 
' plicable to moft of the fame purpofes. 
Of the cryftals of natron, when very dry, but yet 
with fcarce any white powder on them, i oo grains 
may be diffolved in 384 gr. of water, when Farhen- 
heifs thermometer is at 37. Such cryftals quite dry, 
and juft- inclining to grow white, will lofe of 
their weight if dried, with a heat fufficient to fcorch 
paper. 
* See Dr, Black’s experiments in the Edinburgh Eflays, 
Vol. II. page a 1 8. 
2 ‘‘ The 
