290 Dr. Priestley’s Experiments 
acid, and though, on withdrawing the fire, that vapour difap- 
peared, it inftantly re-appeared on renewing. the heat. When 
the tube was cool, I opened it under water, and a quantity of 
air rufhed out, though the acid had been made to boil vio- 
lently while it was doling, fo that there could not have been 
much air in the tube. This air, which muft therefore have 
been generated in the tube, was a little worfe than common 
air, being of the ftandard of 1.12 when the latter was 1.04. 
I repeated the experiment feveral times, and always with the 
fame refult. 
That this air fhould be worfe than common air, I cannot 
well explain. But in my former experiments it appeared that 
vitriolic acid air injures common air ; and that in proportion as 
pure air is expelled from this acid, the remainder becomes phlo- 
gifticated, or charged with vitriolic acid air, clearly appeared 
in the following experiment. 
Making a quantity of oil of vitriol boil in a glafs retort, and 
making the vapour pafs through a red-hot earthen tube, glazed 
infide and out, and filled with pieces of broken tubes, I col- 
lected the liquor that diftilled over, and found it to be the 
fame thing with water impregnated with vitriolic acid air. The 
fmell of it was exceedingly pungent, and it was evident, that 
more of this air had efcaped than could be retained by that 
quantity of water. The oil of vitriol ufed in this procefs was 
1 oz. 9 dw. i8gr. and the liquor collected was 6 dw. 1 2 gr. 
When I collected the air that was produced in this manner^ 
which I did not do at this time, it appeared to be very pure, 
about the ftandard of 0.3 with two equal meafures of nitrous 
air. 
At another time, expending 1 oz. 1 1 dw. 1 8 gr. of oil of 
vitriol, of the fpecific gravity of 1856 (that of water being 
1000), 
