' CHEMISTEY OF SOLIDS — DESCH 239 



way through the solid, must be possible. Experiment shows that 

 diffusion in solids, whilst naturally a slow process in comparison 

 with diffusion in liquids, proceeds at quite measurable rates. 



The classical example of such measurements, and for many years 

 the only one, is the study of the diffusion of gold in solid lead, 

 undertaken by Roberts-Austen in 1896. A much simpler example 

 is that of silv^er and gold, two metals which resemble one another 

 closely in chemical character and in atomic volume, so that diffusion 

 causes less change of properties than in any pair of less closely 

 similar metals. The experimental results prove, as might have been 

 anticipated, that diffusion is a much slower process when there is 

 so little difference in chemical character. When the two kinds of 

 atoms are closely alike, the tendency to diffuse must be small, but 

 it is certainly not zero. By making use of an ingenious device, 

 Hevesy has been able to determine the coefficient of self-diffusion 

 of liquid and solid lead. Two isotopes should not differ appreciably 

 in their rates of diffusion, so that when the radioactive isotope 

 thorium B is allowed to diffuse in ordinary lead, the experiment is 

 equivalent to selecting a certain number of lead atoms and attaching 

 labels to them by which they may be identified in the course of 

 their journey. In this way he found that the diffusion in liquid 

 lead near to the melting point was of the order of that of salt in 

 water, but that in the solid state it was very small. Further experi- 

 ments, using a thin foil, proved that at 2° below the melting point 

 the rate was 1/10,000 of that in molten lead. 



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 nickel or a fused mass of albite and anorthite, begins to solidify, 

 the composition of the crystals has to adjust itself continuously in 

 order to maintain equilibrium with the changing liquid phase, as 

 was shown by Roozeboom in his classical work on solid solutions. 

 Such an adjustment is onl}^ possible by means of diffusion, and 

 when cooling is sufficiently slow, the adjustment does in fact keep 

 pace with the change in the liquid, but with more rapid cooling the 

 interior of each crystal differs in composition from its outer layers, 

 there being a concentration gradient from the center to the boundary. 

 This condition produces the " cored " crystals which are familiar 

 to every metallurgist, and the " zoned " crystals of the mineralogist. 

 In most alloys this want of homogeneity disappears after a suffi- 

 ciently long period of heating at some temperature below that at 

 which the first drops of liquid are formed, but alloys of bismuth 

 and antimony fail to become uniform even after weeks of annealing, 

 whilst the felspars and similar minerals have never been persuaded 

 to lose their zoned structure by any methods known in the laboratory. 



