president's address SECTION H. 625 



but may come to rest in some intermediate position where they 

 are in equilibrium of perhaps a not very stable kind. We may thus 

 have groups of molecules in a condition of very feeble stability, ready 

 to tumble into positions of greater stability at a mere touch. The 

 time effects proceed until all such groups have settled down into 

 their most stable configuration. 



The use of the microscope has thrown very important light upon 

 the process and nature of crystal growth in metals, a knowledge of 

 which is of the greatest practical importance. This process goes 

 on not merely during the solidification of a metal from the liquid 

 state, but commonly while the metal is still in a solid state, provided 

 it has been heated up to a certain ci-itical temperature. Under 

 certain conditions crystallization goes on at very moderate tempera- 

 tures. Thus after cast lead* has been severely strained by com- 

 pression the large crystals of which it was originally built get broken 

 up. They may be driven into and through one another, until at 

 last the component grains of the overstrained lead are all quite 

 small. Left at rest at ordinary temperatures, such lead is found 

 to undergo a slow process of recrystallization, certain crystals grow- 

 mg larger at the expense of their neighbours. If the lead is exposed 

 t-j higher temperatures the rate of growth of the ciystals is still 

 more rapid and pronounced. When such crystalline growth has con- 

 tinued for some time at a certain temperature, the stracture seems 

 to become stable at that temperature and ciystalline growth ceases, 

 but exposure to a higher temperature may cause further growth. 

 It, is curious that such ciystal growth at ordinar}^ temperatures seems 

 to occur only in specimens that have been subjected to severe plastic 

 strain. By casting in a chill mould specimens may be obtained 

 having just as minute crystals as the severely-strained specimens, 

 but the structiu-e is stable and tno growth takes place. iZinc, 

 cadmium, and tin behave in a similar way. The process of anneal- 

 ing consists essentially in the raising of the temperature of the 

 metal to one sufficiently high to allow recrystallization to take place, 

 but this arrangement goes on while the metal is still in the solid 

 state. Stead and Richards have made a most important application 

 of the principles involved to the restoration of dangerously crystalline 

 steel by heat treatment.! They have shown that iron and steel of 

 a coarsely crystalline and brittle character may by simple heating 

 up to a temperature of about 870 degrees C. be restored to very 

 excellent qualities, the original coarse structure being thus destroyed 

 and replaced by one of much greater fineness. Such coarsely crystal- 

 line steel is most commonly produced by long-continued heating at high 

 temperatures, but with some steels it may be produced by annealing 

 for a long period at too low a temperature in a slightly-oxidising 

 atmosphere. If the steel has been heated until it is practically burnt, 

 then an evolution of gas takes place in the interior which separates 

 the crystals from each other, making the whole mass more or less 

 discontinuous. In such a case they found that no heat treatment 

 was successful in restoring the condition of the metal, but in all 

 other cases the heat treatment produced a much finer structure, the 



* Ewing and Rosenhaim, Proc. R.S., Vol. 67, 1901. 

 t .Journal Iron and Steel Inst., No. II., 1903. 



