t ■«* ] 
XI. On the Decompojltion of Fixed dir. 
By Smithfon Tennant, Efq. F. R. S. 
Read March 31, 1791* 
A S fixed air is produced by the combuftion of charcoal, it 
has long been thought highly probable that vital air and 
charcoal are its conftituent ingredients. This opinion is con- 
firmed by the experiments of M. Lavoisier, from which he 
difcovered that the weight of the fixed air which is formed 
during the combuftion is nearly equal to that of the vital air 
and charcoal confumed in the procefs; and that the fmall dif- 
ference of weight may, with great reafon, be attributed to the 
production of water arifing from inflammable air contained in 
the charcoal. The compofition of fixed air therefore feems to 
be determined, by uniting its conftituent parts, with as much 
certainty as by that mode of proof alone it is poffible to ob- 
tain. But as vital air has a ftronger attraftion for charcoal 
than for any other known fubftance, the decompofition of 
fixed air has not hitherto been attempted. By means, how- 
ever, of the united force of two attractions I have been able 
to decompofe fixed air, and thus to determine its conftituent 
parts in confequence of their reparation. 
It has long been known, that when pholphoric acid is com- 
bined with calcareous earth, it cannot be decompofed by dif- 
tillation with charcoal ; for though vital air is more ftrongly 
attraded 
