547 
Our subject may be said to bave its bistorical beginning 
in tbe observation by Priestley, tbat wben gases were passed 
through red-hot earthenware tubes, they penetrated and 
escaped through the walls of those tubes, whilst the gases of 
the fire passed inwards. The permeability of red-hot metal 
tubes to gases, is a fact much more recently observed. 
In 1825, Dobereiner observed that hydrogen kept in a 
cracked glass receiver over water escaped by degrees into 
the surrounding air, whilst the water rose from two to three 
inches within the receiver. 
About the year 1833, Mr. Graham, the present Master of 
the Mint, investigated and determined the laws governing 
the intermixture of gases. This process of intermixture is 
termed diffusion; and as the cohesion of the particles of a 
gas is very small, diffusion between different gases takes place 
readily. Thus, if atmospheric air and coal-gas (fire-damp) be 
brought into contact, they readily mingle, and during this 
admixture the particles of each are travelling at a uniform 
speed peculiar to that gas. This speed remains the same 
whether the gas diffuses into a vacuum or into a vessel filled 
with another gas. The specific gravity of chlorine is thirty- 
six times as great as that of hydrogen, and yet, when these 
gases have once been mixed, they will never separate again. 
More than this, when a bottle filled with chlorine is connected 
by a long narrow tube with another containing hydrogen, and 
the latter placed uppermost, a short time suffices for the 
passage of a part of the heavy gas upwards and of the light 
gas downwards, until both bottles are filled with a similar 
mixture. 
The diffusion of gases is shown very satisfactorily by the 
following experiment : A long glass tube, of the diameter of 
an inch, has one end closed by a plate of plaster of Paris ; it 
is filled with coal-gas, and supported in a vessel of water, 
when the water begins to rise at once, and in a few minutes 
