152 T)r. Priestley’s Experiments and Obflervations on 
experiments in which the air-pump was ufed, this objection 
cannot well be made to thofe in which that instrument was 
not' ufed ; and in them the (lowly condenfable vapour above 
mentioned feems to be an evident fymptom that the produce 
was not mere water. But it is a fatisfactory anfwer to this 
objection, from the prefence of phlogifticated air in the tube, 
that this kind of air is not decompofed, or at all affeCted, by 
this procefs, as will be found by mixing any quantity of it 
with the two other kinds of air. 
That a conliderable quantity of water enters into the com- 
pofition of dephlogiflicated air, will not be thought impro- 
bable, when it is confid'ered that, in my former experiments, 
this appeared to be the cafe with refpeCt to inflammable air. For 
without water this air cannot be procured. I can alfo now fay, 
that the fame is the cafe with refpeCl to fixed air . It is not 
therefore improbable, that the (lime may be true of every 
other kind of air, (nice water is ufed in the production of 
them all. 
Terra ponderofa aerata (a fubflance of which Dr. Withering 
has given us an excellent analyfis) gives no fixed air by mere heat. 
But I find, that when fleam is fent over it, in a red heat, in 
an earthen tube, fixed air is produced with the greateft rapi- 
dity, and in the fame quantity as when it is diflolved in fpirit 
of fait : and, making the experiment with the greateft care, I 
find, that fixed air confifts of about half its weight of water. 
From two ounces of the terra ponderofa I got, by means of 
fleam, 190 ounce meafures of fixed air, fo pure that at firfl 
150 ounce meafures of it w’ere reduced by agitation in water to 
3I, and of the laft produce, 30 ounce meafures were reduced 
to one. Examining the refiduum of the firfl portion by means 
of nitrous air, I found it to be of the flandard of 1.5. 
After 
