dh SECTIONAL ADDRESSES. 
the rate was 1/10000 of that in molten lead. The same method has been 
used to determine the rate of diffusion of radioactive elements through 
gold, silver, and platinum. 
The matter is, however, by no means simple. On the one hand, 
inter-diffusion at the junction of two metals proceeds in both directions, 
although sometimes at very unequal rates. Dr. J. W. Jenkin, working 
in my laboratory, has been able to show that at 1000° copper diffuses 
into solid nickel about twenty times as fast as nickel into copper, the 
observed diffusion curve being the sum of two curves of similar type. A 
further complication arises from the fact that ordinary laboratory 
specimens of metal consist of an aggregate of crystalline individuals, the 
axes of which are directed at random, so that the whole mass is con- 
sidered for ordinary purposes as though it were isotropic. It is unlikely 
that foreign atoms can travel with equal ease in all directions in a crystal, 
and the observed diffusion will be an average value. Now that single 
crystals of many metals are available it is natural that experiments on 
diffusion in solids should have been made with them, and the results are 
rather surprising. Geiss and van Liempt found that neither molybdenum 
nor iron diffused into wires consisting of single crystals of tungsten, even 
when the temperature was near to the melting point of the more fusible 
metal, whilst mixed powders of the two metals became completely 
homogeneous on being heated for a few hours at the same temperature. 
In a similar way, Hevesy has found that his radioactive isotopes do not 
diffuse appreciably through a sheet consisting of a single crystal of lead. 
An explanation has been offered, based on the assumed existence of a layer 
of amorphous material between the crystalline grains. It is supposed 
that diffusion through the mass of the crystal does not occur, and in favour 
of this view Hevesy notes the fact that in different specimens of lead, 
varying in the size of their crystal grains, diffusion was much slower in 
that which had the largest grains, and therefore the smallest propor- 
tion of intercrystalline substance. On the other hand, microscopical 
observation of such pairs of diffusing metals as copper and nickel prove 
that the advance of the diffusing metal, as shown by the change of colour, 
proceeds through the whole mass, and not merely along the boundaries. 
Indeed, it is hard to see how any mass of metal could become homogeneous 
if diffusion were confined to inter-granular boundaries, as it is certain 
that the position of those boundaries may remain unchanged throughout 
the whole of the experiment. Hevesy finds that polonium, which is not 
isotopic with lead, diffuses through lead foil or through a single crystal of 
lead at the same rate, and suggests, as he has done regarding diffusion in 
solids in general, that the process is one of loosening of the space lattice, 
the invading atoms travelling through the progressively loosened patches. 
It remains to be seen whether the X-rays afford any support for this view. 
On the other hand, it may be suggested that much will depend on the 
particular crystal face selected for an experiment, as it is certain that if 
true diffusion through a crystal be possible—and I fail to see how such an 
assumption can be dispensed with—it must be much easier in the direction 
of certain crystalline planes than across them. This point calls for a 
systematic examination. 
When a liquid mixture of two substances which are miscible in the 
solid as well as in the molten condition, such as an alloy of copper and 
