172 Mr. Leslie on Sounds excited in Hydrogen Gas. [Sert. 
ARTICLE IV. 
On Sounds excited in Hydrogen Gas, By John Leslie, Esq’ 
FRSE. &e. &e.* 
Ir is well known that the intensity of sound is diminished by 
the rarefaction of. the medium in which it is produced. We 
might, therefore, expect the sound excited in hydrogen gas to 
be feebler than what is, in like circumstances, produced in atmo- 
spheric air. But the difference is actually much greater. 
A small piece of clock-work, by which a bell is struck every 
half minute, being placed within the receiver of an air-pump, a 
successive rarefaction was produced; and after the air had been 
rarefied 100 times, hydrogen gas was introduced. But the sound, 
so far from being augmented, was at least as feeble as in atmo- 
spheric air of that extreme rarity, and decidedly much feebler 
than when formed in air of its own density, or rarefied 10 times. 
The most remarkable fact is, that the admixture of hydrogen 
gas with atmospheric air has a predominant influence in blunting 
or stifling sound. If one half of the volume of atmospheric air 
be extracted, and hydrogen gas be admitted to fill the vacant 
space, the sound will now become scarcely audible. 
These facts, I think, depend partly upon the tenuity of hydro- 
gen gas, and partly upon the rapidity with which the pulsations 
of sound are conveyed through this very elastic medium. The 
celerity of the transmission of sound through common air is the 
same in every degree of rarefaction ; but in hydrogen gas, it is 
more than three times swifter. The bell, therefore, strikes a 
medium which is at once thin and fugacious ; fewer particles are 
struck, and these sooner escape from the action of the stroke. 
To produce undulations similar to what are excited in atmo- 
spheric air, or to cause equal reciprocations in the tide of sound, 
it would require the impulse to be as the square of the celerity, 
or 10 times greater than on common air. If this view of the 
matter be just, I should expect the intensity of the sound to be 
diminished 100 times, or in the compound ratio of its tenuity 
and of the square of the velocity with which it conveys the vibra- 
tory impressions. 
When hydrogen gas is mixed with common air, it probably 
does not intimately combine, but dissipates the pulsatory impres- 
sions before the sound is vigorously formed. 
It would be desirable to prosecute such observations with 
different gas, and at various degrees of rarefaction. But I have 
not yet found time, and merely throw out these hints for subse- 
quent examination and research. JoHN LEsLiE. 
* From the Transactions of the Cambridge Philosophical Society, 
