of Aimospherical Air, Se. 153 
ponderosa aérata by mere heat;* but procuring large 
quantities from it by means of steam, at the expence 
of some water (about one half of the weight of the 
gas), he thinks it impossible to conclude otherwise, 
than that the water has combined with the carbonic 
acid gas and makes one half of its weight. M. 
‘Berthollet has already noticed the insufficiency -of 
this experiment.t Dr. Priestley should certainly 
have shewn the presence of the water in the fixed 
air; or, at least, that it was not combined with, or im- 
bibed by, the barytes, or otherwise lost. In con- 
firmation of the above, he relates the following ex- 
periment. Forty-eight grains of terra ponderosa 
aérata were dissolved in spirit of salt, and yielded 
7.2 grains of fixed air. The solution was then eva- 
porated to dryness, and the salt exposed to a red 
heat, in which it lost 4 grains: consequently the 
weight of the air and of the residuum exceeded the 
original weight of the aérated barytes by 3.2 grains; 
and, as he believes that all the muriatic acid was ex- 
pelled from the earth, he concludes that this ex- 
. * Since this paper was written, I have been informed, that 
Dr. Hopeof Edinburgh has succeeded in freeing the aérated 
barytes completely from its carbonic acid, by mere heat, 
without the aid of water. This perfectly pure barytes has 
the remarkable property of being soluble in a very small 
quantity of water. 
+ Dr, Priestley did not even weigh the residuum of the 
barytes. ; 
VOL. V. U 
