374 Sir Humphry Davy on the state of water 
source above water, would render it capable of receiving a 
very high degree of heat. Lime retains water in combina- 
tion at a heat above 250° Fahrenheit ; baryta retains it (even 
under ordinary pressures) at a strong red heat, and fuses 
with it. It is extremely likely that a liquid hydrate of silica 
would exist, under pressure, at high temperatures ; and like 
all liquid bodies in the atmosphere, would probably contain 
small quantities of atmospheric air ; and such a supposition only 
is necessary to account for the phenomena presented by the 
water in rock crystal and chalcedony. 
As, however, steam or aqueous vapour may be considered 
as having a share in these results, if it be supposed included 
in the cavity, no exact conclusions can be drawn from the 
apparent degree of contraction of the water ; particularly as 
the late ingenious researches of Mr. Perkins show, that water 
is much more compressible than was formerly imagined ; and 
the volume of water, however high its temperature, must be 
influenced by the pressure to which it is exposed ; so that a 
certain compressing weight may not only impede, but alto- 
gether counteract the expansive force of heat. 
Many speculations might be indulged in on this subject, but 
I shall not at present enter upon them ; and I shall conclude 
by observing, that a fact, which has been considered by the 
Neptunists, above all others as hostile to the idea of the igne- 
ous origin of crystalline rocks, namely, the existence of water 
in them, seems to afford a decisive argument in favour of the 
opinion it has been brought forward to oppose. 
June 1, 1822, 
