74 
Sir Humphry Davy on the 
ther they be specific affections of a subtile imponderable fluid, 
or peculiar properties of matter) as primary and invariable 
electrical phenomena. 
I have mentioned in the last page the suspicion, that melted 
tin may contain air. I shall conclude this paper by stating the 
grounds of this suspicion, and noticing a circumstance which 
appears to be of considerable importance, both in relation to 
the construction of barometers and thermometers, and to the 
analysis of gaseous bodies. Recently distilled mercury that 
has been afterwards boiled and cooled in the atmosphere, and 
which presents a perfectly smooth surface in a barometer 
tube, emits air when strongly heated in vacuo, and that in 
quantities sufficient to cover the whole interior of the tube 
with globules ; and on keeping the stop-cock of one of the 
tubes used in the experiments on the mercurial vacuum open 
for some hours, it was found that the lower stratum of mer- 
cury had imbibed air, for when heated in vacuo, it emitted 
it distinctly from a space of a quarter of an inch of the co- 
lumn ; smaller quantities were disengaged from the next part 
of the column ; and its production ceased at about an inch 
high in the tube. There is great reason to believe, that this 
air exists in mercury in the same invisible state as in water, 
that is distributed through its pores ; and the fact shows the 
necessity of long boiling the mercury in barometer and ther- 
mometer tubes, and the propriety of exposing as small a sur- 
face of the mercury as possible to the air. It may explain, 
likewise, the difference of the heights of the mercury in diffe- 
rent barometers ; and seems to indicate the propriety of re- 
boiling the mercury in these instruments after a certain lapse 
of time. 
