12$ A SYSTEM OF 



3. They have a flrong attraction to the 

 alcaline falts and earths, whence they al- 

 ways unite with them with an effervef- 

 cence, and fometimes with a flrong heat : 

 by this mixture bodies are produced, 

 which are employed in common life under 

 the names of vitriols, neutral falts, gyp- 

 flinty &C 



4. They change mofl of the expreffed blue 

 juices of vegetables into red. 



the queflion 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 ' But howfoever 

 this may prove, the confideration of thefe acids feems more 

 properly to belong to another fcience. The fame may be 

 faid of the doctrine which holds, that the nitre is produced 

 from the principles of the fea-fak, by a certain peculiar mo- 

 dification. 



The above-mentioned two mineral acids, whofe qualities 

 we know nothing of, until they have been by art extracted 

 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, ie para ted from any fub- 

 ftance, they inftantly attack and unite with another. Ne- 

 verthelefs, as they may, and perhaps fometimes really do 

 exift 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 

 necefTary to defcribe them fuch as they are, when mixed with 

 pure water alone ; and this the rather, fince the water is their 

 moil common vehicle, in the exercife of their effects in the 

 mineral kingdom. 



It has been obferved before (Sect, xi.), 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- 

 metab, which cannot be any otherwife explained, than that 

 the arfenic confidered in a certain refpect and form, is a fait; 

 and when confidered in other circumftances, a metal. This, 

 is the cafe with feveral other bodies of the mineral kingdom. 



5. They 



