66 
Sir Humphry Davy on the 
greater accuracy, the exhaustion was made after the tube 
and apparatus had been filled with hydrogene.* 
Operating in this way, it was easy to procure a vacuum 
either of a large or small size, for the rarefied air or gas could 
be made to balance a column of fluid metal of any length, 
from 20 inches to the 20th of an inch, and by using only a 
small quantity of metal, it could be more easily purged of air. 
I shall first mention the results I obtained with quicksilver. 
I found that by using recently distilled quicksilver in the 
tubes, and boiling it in vacuo six or seven times from the top 
to the bottom, and from the bottom to the top, making it 
vibrate repeatedly by striking it with a small piece of wood, 
a column was obtained in the tube free from the smallest 
particle of air; but a phenomenon occurred, in discovering 
the cause of which I had a great deal of trouble. When I 
used a short tube of four or five inches long only, I found, 
that after continued boiling and much agitation of the mer- 
cury, though there was no appearance of elastic matter, when 
the mercury adhered strongly in the upper part of the tube, 
yet that, after electrization, or even on suffering the mercury 
to pass slowly back into the closed part, a minute globular 
space sometimes appeared : I thought at first that this was 
air, which, though so highly rarefied, as it must have been by 
the exhaustion, adhered to the mercury ; and I endeavoured 
by long boiling the mercury in an exhausted double syphon, 
and making the vacuum in one of the curves, to prevent en- 
tirely the presence of air : but the phenomenon always 
occurred when there was no strong adhesion of the mercury 
* The figure will best explain the form of the apparatus. 
