4io Dr. Priestley’s Experiments relating to PhhgiJioH , 
produced from that water which has entered the pores, the reft: 
is fometimes vilibly making its efcape in the form of a copious 
finoke on the outfide. It was evidently impofiible, however, 
and contrary to all the laws of hydroftatics, that air fhould 
enter by the fame pores by which the water or fteam was efcap- 
ing, and at the fame time that its endeavour to force its way 
out of the retort was fuch that it overcame a considerable re- 
finance from the column of water, at the mouth of my reci- 
pients. Air might have efcaped through any unobferved pore9 
in the retort, but none could have entered that way *. and if 
there was the leaft fenfible crack in any part of the retort, I 
was never able to collect any air at all. 
But the following experiments may, perhaps, fhew that it 
is fufficient for the produftion of air that (team come into con- 
tact with clay lufficiently heated. Between a copper ftill and 
the glafs tube communicating with my recipient for air, I in- 
troduced the Item of a tobacco-pipe ; and by means of a fmall 
furnace, I kept about three inches of the middle part of it 
moderately red-hot. In this ftate, making the water boil, I 
uniformly received air, though mixed with fteam, at the rate 
of five ounce meafures in twelve minutes for more than an 
hour ; but when I let the pipe cool, nothing but fteam was de- 
livered by it without any air at all. There was no fixed air in 
this produce, and it was all fuch as a candle would hardly have 
burned it. It might, I thought, have been better and alfo 
more in quantity if I had not ufed the ftem of a foul pipe. But 
when I ufed a clean pipe in the fame manner, I did not find 
the air much, if at all improved. Sufpe&ing this to arife from 
the near contadl of the fuel, I inclofed the tobacco-pipe in an 
earthen tube, and then I had air as good as I had generally 
got 
