PRESIDENT’S ADDRESS. 39 
Whether it is likely to be useful or not, it is a most won- 
derful thing to find one metal travelling through another 
solid metal, just as gases diffuse through gases, and liquids 
through liquids—the motion is slower, it is true, but it 
exists, and it is none the less marvellous. The wonder is 
that the apparently rigid, solid, metallic molecules undergo a 
motion of mechanical translation at all, 7.e., as distinguished 
from a vibratory motion. 
We are all well acquainted with the reverse of the above, 
for when two metals are melted together and allowed 
to cool slowly they often tend to separate. This is much 
more easily accounted for, simce both metals have been in a 
fluid and mobile condition and are free to rearrange them- 
selves, i.e., the attraction between the molecules of a given 
metal is greater than that between the molecules of certain 
dissimilar metals. 
ACETYLENE. 
Two or three years ago it was only prepared in quite 
small quantities for experimental purposes, and was only 
known to chemists. It occurs in small quantity in coal gas, 
and is one of the most powerful illuminants present in it. 
Now, however, thanks to cheap electricity, a compound 
of calcium and carbon, known as calcium carbide, is largely 
manufactured. On the addition of water to this material 
decomposition takes place, and acetylene is evolved. 
It is true that acetylene is lable to explode, but only 
when it is stored under pressure. So long as the pressure 
is not much above that of the atmosphere, it appears to be 
safe enough; it must not be subjected to either rapid 
expansion or rapid condensation. In the liquid form it is 
not so safe; but, if properly handled, it should be a safe 
and an extremely useful illuminant for country and other 
places where coal gas or electricity 1s not obtainable. 
Acetylene is also of great interest from a purely scientific 
point of view, as being the first and simplest synthesised 
compounds of carbon and hydrogen. From it, as a starting- 
point, other substances like alcohol, as well as complex 
