Permanent Colours of Opake Bodies. 203 



the vapours difengaged from a mixture of fulphar 

 and iron filings. * 



The origin of phlogifticated air fhews, that its 

 difference, from fixed air, confifts chiefly in its 

 deficiency of water. 



As metals contain no water, fo the phlogifton, 

 which arifes from them during their calcination, 

 does not produce fixed air, but phlogifticated air. 



But vitriols, and all other faline matters, con- 

 , tain water as a conftituent part, and therefore 

 yield fixed air. Calces of metals alfo, which have 

 received aqueous matter, in the procefs of their 

 calcination, as white lead, and other calces which 

 have ahforbed water, together with air, from the 

 atmofphere, alfo yield fixed air. 



The fermentation, and putrefa6lion, of vegetable 

 and animal fubftances, is effeded by means of their 

 moifture, and therefore, by thefe modes of decom-- 

 pofition, fixed air is produced. 



Fixed air is more effectually formed by refpira- 

 tion, than by many other phlogiftic proceffes, in 

 confcquence of the copious fupply of the aqueous, 

 as well as the phlogiftic^ principle, which the air 

 receives from the lungs. 



Fixed air may be formed from vegetable acids, 

 but when it is thus conftituted, it does not differ 

 from that which is produced from alcalies, mag- 

 nefia, calcareous fpar, and various other fubftances, 

 which yield no acid. It is therefore evident, that 

 in each of chcfe infta^es, ii; is formed by the 



combinaiioq 



