448 
SECOND REPORT—1832. 
membrane burst outwards. On the contrary, when the vessel 
was filled with hydrogen gas, and placed in another vessel con¬ 
taining common air, the hydrogen escaped from the inner ves¬ 
sel, till by the increased pressure in the outer the membrane 
burst inwards. 
This suggested to him the construction of an instrument by 
which the relative velocity and force , with which gases pass 
through such a membrane may be ascertained. He bent a 
long tube into the form of a siphon, widened the end of the 
shorter arm like the mouth of a funnel, and covered it with the 
membrane of caoutchouc. Mercury was then poured in, so as 
to be of equal height in both arms; and the shorter, filled with 
common air and closed with the caoutchouc, was introduced into 
a receiver containing the gas to be experimented on. Under 
these circumstances he found ammoniacal gas to pass through 
the membrane more quickly than any other, and equal quanti¬ 
ties of different gases in the following times: 
m 
1 
Si 
54 
6J 
h m 
Olefiant gas.0 28 
Hydrogen. 0 37^ 
Oxygen.1 13 
Carbonic oxide . ... 2 40 
Azote. 3 15 
h 
Ammoniacal gas .... 0 
Sulphuretted hydrogen 0 
Cyanogen . 0 
Carbonic acid.0 
Protoxide of azote . . 0 
Arseniuretted hydrogen 0 27\ 
The gases continued to enter without sensible diminution of 
velocity, till the mercury in the longer arm rose to the height 
of sixty inches, when the membrane gave way. 
In all these experiments it had been observed that a mutual 
interchange of the gases took place to a greater or less degree, 
but it had not been made out that this interchange was regu¬ 
lated by any fixed law. Mr. Graham first pointed out the 
extent of this mutual diffusion, and returning* to the subject, 
he has shown,— 
1°, That the tendency of two different gases, separated by a 
porous diaphragm of any kind, is to mutual equable diffusion: 
2° s That this mutual diffusion is not necessarily in equal 
volumes, being inversely proportional to the square root of the 
density of each gas. 
Thus, when a bladder of common air, or, as in Mr. Mitchell’s 
experiments, a tube covered with bladder, is introduced into 
an atmosphere of hydrogen gas, the tendency of the two gases 
is to diffuse themselves through each other equably, till the 
ratio between their volumes wdthin the bladder, and in the re- 
* Edinb. Transactions, xii. p. 222. 
