68 Sir Humphry Davy on the 
the vapour of mercury; and at o° Fahrenheit it was very 
feeble indeed. 
A very beautiful phenomenon occurred in boiling the 
mercury in the exhausted tube, which showed the great bril- 
liancy of the electrical light in pure dense vapour of mer- 
cury. In the formation and condensation of the globules of 
mercurial vapour, the electricity produced by the friction of 
the mercury against the glass, was discharged through the 
vapour with sparks so bright as to be visible in day light. 
In all cases when the minutest quantity of rare air was 
introduced into the mercurial vacuum, the colour of the light 
produced by the passage of the electricity changed from 
green to sea green ; and, by increasing the quantity, to blue 
and purple ; and when the temperature was low, the vacuum 
became a much better conductor. 
I tried to get rid of a portion of the mercurial vapour, by 
using a difficultly fusible amalgam of mercury and tin, which 
was made to crystallize by cooling in the tube ; but the re- 
sults were precisely the same as when pure mercury was used. 
I tried to make a vacuum above the fusible alloy of bis- 
muth ; but I found it so liable to oxidate and dirt the tube, 
that I soon renounced farther attempts of this kind. 
On a vacuum above fused tin I made a number of experi- 
ments ; and by using freshly cut pieces of grain tin, and 
fusing them in a tube made void after being filled with hydro- 
gene, and by long continued heat and agitation, I had a 
column of fused tin which appeared entirely free from gas : 
yet the vacuum made above this, exhibited the same pheno- 
mena as the mercurial vacuum. At temperatures below o°, 
the light was yellow, and of the palest phosphorescent kind, 
