1822.] Philosophical Transactions for 1822, Part 1. 377 
light was not producible in a perfect torricellian vacuum; and 
the latter that such a vacuum likewise prevented the charging 
of coated glass; but it being well known that very rare 
vapour of mercury exists in the most perfect vacuum of that 
nature that can be made, Sir H. could not help doubting the 
perfect accuracy of these results, and “ resolved not only to exa- 
mine them experimentally, but likewise, by using a compara- 
tively fixed metal in fusion for making the vacuum, to exclude, 
as far as was possible, the presence of any volatile matter.” 
The apparatus that he employed consisted of a curved glass 
tube A D, with one leg A closed, and longer than the other. In 
this closed leg, a wire of platinum B was hermetically cemented, 
for the purpose of transmitting the electricity ; or to ascertain 
the power of the vacuum to receive a charge, a small cylinder of 
tin or platinum foil E was placed as a cap on tubes not havin 
the wire B. The open end D, when the closed leg had been filled 
with mercury or fused tin, the surface of which stood at C, was 
exhausted through the stop-cock F connected by the moveable 
tube G with an excellent air-pump; “ and in some cases to 
ensure greater accuracy, the exhaustion was made after the tube 
and apparatus had been filled with hydrogen.” 
Operating in this way, it was easy to procure a vacuum 
either of a large or small size: and “ 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 vapour of mercury was sometimes 
produced, filling a minute globular space, to discover the cause 
of which gave the author a great deal of trouble. 
“ He found that in all cases when the mercurial vacuum was 
perfect, it was permeable to electricity, and was rendered lumi- 
nous by either the common spark, or the shock from a Leyden 
jar, and the coated glass surrounding it became charged ; but 
the degree of intensity of these phenomena depended upon the 
temperature. When the tube was very hot, the electric light 
appeared in the vapour of a bright-green colour, and of great 
density ; as the temperature diminished, it lost its vividness ; and 
when it was artificially cooled to 20° below zero of Fahr. it was 
so faint as to require considerable darkness to be perceptible.” 
The change communicated to the metallic foil was likewise 
higher, the higher the temperature, which, like the other pheno- 
menon, must depend upon the different density of the mercurial 
vapour. 
“A very beautiful phenomenon occurred in boiling the mer- 
cury in the exhausted tube ; which showed the great brilliancy 
of the electrical light in pure dense vapour of mercury. In the 
formation and condensatiqn of the globules of mercurial vapour, 
