A SYSTEM OF 
3. They have a ftrong attradion t 6 the 
alcaline falts and earths, whence they al- 
ways unite with them with an efFervef- 
cence, and fometimes with a ftrong heat : 
by this mixture bodies are produced^ 
which are employed in common life under 
the names of vitriols, neutral falts, gyp^ 
fum^ &c. 
4. They change moft of the expreffed blue 
juices of vegetables into red. 
the queftlon remains ftill undecided, if the nitrous, vegetable, 
and urinous acids are primitive fubftances ? or if they owe 
their origin to one and the fame principle? and, if this laft 
be the cafe, of what nature this principle is i* But howfoever 
this may prove, the confideratioft of thefe acids feems more 
properly to belong to another fcience. The fame may be 
faid of the do6lrine which holds, that the nitre is produced 
from the principles of the fea-falt, by a certain peculiar mo- 
dification. 
The above-mentioned two mineral acids, whofe qualities 
we know nothing of, until they have been by art extraded 
from the vitriols, and the fea-falt, are indeed never found 
pure in nature, becaufe as foon as they, on any occafion, are, 
either by a natural or artificial heat, Separated from any fub- 
ilance, they inftantly attack and unite with anotheri Ne- 
verthelefs, as they may, and perhaps fometimes really do 
Cxift in form of vapours, which efcape our fight ; and that 
the theory of the falts, and the faline ores, is founded upon 
qualities already difcovered in thefe acids ; I have thought it 
necelTary to defcribe them fuch as they are, when mixed with 
pure water alone ; and this the rather, fince the water is their 
moft common vehicle, in the exercife of their eifeifts in the 
mineral kingdom. 
It has been obferved before (Se£l:. xl.), that the qualities of 
arfenic in form of a calx may agree with the definition of 
the falts, and at the fame time be reckoned among the femi- 
metals, which cannot be any otherwife explained, than that 
the arfenic confidered in a certain refpedl and form, is a falt| 
and when confidered in other circumftances, a metal. This 
is the cafe with feveral other bodies of the mineral kingdom. 
5. They 
