638 PHILOSOPHICAL TRANSACTIONS. [aNNO 1790. 



air, being extricated at the same time, unite before they have acquired the aeri- 

 form state, and constitute fixed air. 



Objects are often too common or too near for our observation. Phlogisticated 

 air presents itself in the decomposition of so many bodies, that its appearance 

 excites no inquiry ; and it is not regarded as essential to the chemical consti- 

 tution of the bodies which yield it, excepting in the instances of nitrous acid 

 and volatile alkali, 2 substances of very small extent in the scale of natural 

 bodies. The calces of metals are well known to contain phlogisticated air ; yet 

 the effect of this air on calcination in general, and how far the very different 

 calces of the same metal are influenced in colour or other properties by the dif- 

 ferent proportions of phlogisticated air, has never been considered. Fixed air is 

 often formed from the calces of metals, mixed with water, or with some other 

 substance containing light inflammable air. Red precipitate mixed with iron 

 filings yielded very pure fixed air. Brass dust mixed with red precipitate, like- 

 wise gave out fixed air, though in less quantity. Turbith mineral and iron 

 filings, treated in the same maimer, afforded much less fixed air than the red 

 precipitate and iron filings. It is probable, that the turbith mineral contains less 

 phlogisticated air, than the red precipitate. The fixed air in all these experi- 

 ments was mixed with phlogisticated and dephlogisticated air. Mr. Kirwan 

 found, that the simple calx of mercury with iron filings and water produced fixed 

 air. The same author also observed, that iron calcined with nitrous acid gave 

 out, on being heated, fixed air ; and he found the production of this air renewed 

 on the addition of water. Dr. Priestley obtained fixed air from iron converted 

 into rust by exposure to nitrous air. In all these experiments the 3 elementary 

 airs are present, and, being expelled by heat from the metals with which they 

 were combined, unite with each other, and form fixed air. It is not material to 

 the present argument, whether the light inflammable air be supposed to be fur- 

 nished from water, or from the regulus of a metal : it is enough for our purpose, 

 that none of the substances employed in these experiments, contain heavy in- 

 flammable air or charcoal, in sufficient quantity to account for the fixed air pro- 

 duced, as Dr. Priestley has justly observed. 



The growth of plants affords a strong proof of the formation of charcoal 

 from the substances which have been assigned. If we may believe experiments, 

 water and air alone are necessary to this natural process ; yet vegetation is the 

 great source of charcoal or heavy inflammable air. This inquiry is still in its in- 

 fancy ; but from the best experiments that have been made it should seem, that 

 plants grow best in phlogisticated air ; that they take in phlogisticated air, and 

 give out dephlogisticated air. These phenomena cannot be accounted for but 

 by supposing, that water is decomposed by growing plants ; that part of its de- 

 phlogisticated air is discharged into the atmosphere ; and that the other con- 



