4?. 3 Dr. priest l-ey’s Experiments relating to Phlogifton , 
Another preemption in favour of the generation of our 
atmofphere from water was, that the purity of the air that I 
produced from it is fo very nearly the fame with that of the 
atmofphere. And the degree of heat requifite to produce it is 
no greater than may be given by the rays of the fun in certain 
circumftances. Subterraneous fires, however, would be abun- 
dantly fufficient for the purpofe, as it appears to be fufficient 
for the converfion of water into refpirable air, that it come 
into contact with clay, and perhaps many other earthy fub- 
ftances in the form of vapour. I muff, however, obferve, 
that when I threw the focus of a burning lens upon a quantity 
of moift clay, either in vacuo, or in common air, I got no air 
from it. 
I made this experiment both with the clay expofed in an 
open difh, and alio confined in a fhort earthen tube. Had I 
then proceeded to repeat this laft procefs with a communica- 
tion between the infide of the earthen tube and the external 
air, as I then propofed to do, but was prevented, 1 fhould much 
fooner have difeovered what I did afterwards, viz, that there 
was no real converfion of water into air in this procefs. In 
favour of which, however, it may not be amils to obferve, 
that the great difficulty Mr. de luc and others have found in 
expelling all air from water, is bell accounted for on the fup- 
pofition of the generation of air from water, though in other 
circumftances than thofe that I have obferved. I have the 
pleafure to add, that Mr. de luc himfelf concurs wfith me in 
this opinion. 
The difficulty that ftrikes many perfons the moft forcibly, is 
the want of analogy between the converfion of water into air 
with any other known fafils in philofophy or in nature. But 
admitting that this converfion is effected by the intimate union 
i of 
