Natural Philosophy^ — Electricity. 181 
which it Is produced, it might have been expected that the 
sound in hydrogen gas would be feebler than when produced in 
atmospheric air in similar circumstances. Mr Leslie, however, 
has found the difference to be actually much greater. Having 
placed within the Teceiver of an air-pump a small piece of clock- 
work, by which a bell was struck every half minute the air was 
rarified, and after the reacation had been carried the length 
of 100 times, hydrogen-gas was introduced. The sound, how- 
ever, so far from being augmented, was at least as feeble as in 
atmospheric air of that extreme rarity, and decidedly much 
feebler than w^hen formed in air of its own density, or rarified 
ten times. Mr Leslie likewise observed the very curious fact, 
that the mixture of hydrogen gas with atmospheric air, has a 
predominant influence in blunting or stifling sound. When one 
half of the volume of atmospheric air is extracted, and hydro- 
gen gas admitted to fill up the vacant space, the sound will now 
become scarcely audible ; an effect which he ascribes to a want 
of intimate combination between the two gases, which causes 
the pulsatory impressions to be dissipated before the sound is 
originally formed. Mem. Cambridge Phil. Soc.,, vol. i. p. 267. 
ELECTRICITY. 
W. On the ordinary and extraordinary Electricity of Mine- 
rals. — The effect of heat in developing electricity in certain 
minerals, has been long known. M. Haiiy, however, has dis- 
covered, that the electricity which he had formerly observed, in 
minerals, instead of vanishing abruptly at the ordinary tem- 
perj^ture at which it seemed to disappear, had only reached, the 
node through which it passed to an opposite state, by a farther 
reduction of temperature. With oxide qf zinc Sind tourmaline 
he invariably found, that the opposite electricity could be de- 
veloped by cold, so that the pole which possessed vitreous elec- 
tricity when it was hot, developed resinous electricity when it 
was cold. M. Haiiy calls the electricity produced by beat 
ordinary, and that which is produced by cold extraordinary , — 
tei’ms borrowed from the phenomena of double refraction, but 
quite inapplicable to the present case. 
METEOROLOGY. 
21. Formation of Clouds. — The following observations on the 
formation of clouds, we owe to W. C. Trevelyan, Fsq. who 
