38 PRESIDENT’S ADDRESS. 
the decrease in weight being about 1} tons, or on an 
ordinary train of twenty carriages a reduction of 30 tons. 
Tt has still to be seen whether aluminium will bear the 
wear and tear of railway work. It has not, perhaps, made 
such headway in boat-building as was anticipated, on 
account of its corrosion by sea water. 
DIFFUSION OF METALS. 
You know that if a cylinder, of a light gas like hydrogen, 
be placed with its open mouth against the mouth of a 
cylinder containing a heavy gas, like carbon dioxide, even 
if the heavy gas be in the lower cylinder, that after a short 
time it will be found that the heavy gas has ascended 
against the force of gravity into the upper cylinder, and 
that the light hydrogen has found its way down, and is 
equally intermingled with the heavy carbon dioxide—this is 
known as the diffusion of gases; the speed of diffusion of 
gases 1s a regular one, and it takes place at a rate inversely 
proportional to the square root of the density of the gases; 
so also with miscible liquids, if you place a lght liquid 
upon a heavy one, e.g., water upon a solution of copper 
sulphate, you will see after a time that the heavy blue 
liquid gradually rises against gravity, and spreads through 
the colourless water. So, too, Prof. Roberts-Austen has 
found, that if a cylinder of lead with a gold base be allowed 
to stand at a temperature of boiling water, that in the 
course of time a little of the heavy gold has diffused 
upwards through the lead. The gold diffuses still more 
rapidly at temperatures from 100° up to 150°, and, as might 
be expected, still more rapidly in melted lead at 550°; the 
diffusion can then be detected in a few days. Platinum 
does not diffuse so rapidly as gold; its molecules are there- 
fore probably more complex. 
I do not know that any practical application has yet 
been made of this fact, and some may accordingly ask what 
is the use of the experiment? But many of the most valu- 
able inventions of modern times have arisen from what 
many regard as useless knowledge. 
