Set. II. Agriculture and Vegetation. 143 



all pit-well waters are hard, and contain a 

 nitrous acid joined to an abforbent bafe ; 

 which imperfect -fait, by the addition of an 

 alkaline fait alone, can be converted into 

 real nitre. Thirdly, by boiling hard water, 

 or expofing it to a great degree of heat, 

 the nitrous acid is really volatilifed, and 

 the abforbent earth falls to the bottom. 

 This proves, that the nitrous acid is vola- 

 tile, and exifts in the air. The fpiritus 

 nltri fumam is continually evaporating in 

 the air. Thefe experiments prove, I think, 

 beyond all doubt, that the nitrous acid 

 exifts in the air, diftinft from the vitriolic 

 acid. This nitrous acid appears to be the 

 fructifying principle which we formely di- 

 covered in the air. 



THIS point being fettled, let us now ac- 

 count for the different changes brought 

 about in the making of nitre. All earths 

 are not fit for this purpofe ; only fuch as 

 are attra<5lers of acids, or abforbent earths, 

 lime, marl, and the other abforbents -, 



or 



