202 SCIENCE PROGRESS 



practice. The "season-cracks" which develop in brass are due 

 to differences of stress existing in the inner and outer layers of 

 worked brass objects and are the outcome of the process of 

 spontaneous recrystallisation. Still more remarkable examples 

 are seen in objects of brass or German silver which have been 

 subjected to very severe cold-working in the shape of " spinning " 

 or pressing between dies. In thin articles such as brass lamp- 

 reservoirs numerous cracks are apt to develop which gradually 

 involve complete disintegration of the metal. This change pro- 

 ceeds more quickly in a warm than in a cold atmosphere ; it 

 has been described by Prof. Cohen 1 as " strain-disease," owing 

 to a remarkable similarity to the now well-known "tin plague" 

 which occurs in cold countries. The tin plague is due to the 

 change of ordinary white tin, which is unstable below i8°, into 

 grey tin and is propagated by contact with articles of grey tin. 

 So also the recrystallisation of severely strained metal is acceler- 

 ated by contact with the stable crystalline modification. In 

 some of the experiments a design was etched on a sheet of 

 metal in order to expose the crystalline structure by removing 

 the superficial fluxed layer and the clean surface was then 

 placed in close contact with another sheet of the same metal in 

 a cold-worked condition ; in the course of one or two days, at a 

 temperature of ioo° or upwards, the design was found to have 

 been transferred to the second sheet, the unstable modification 

 on the surface having reverted to the stable crystalline form. 



The view was and frequently still is held by engineers and 

 others that a metal in use, especially when the load which it 

 carries varies in direction or intensity, tends to become more 

 coarsely crystalline ; in fact, failures of structures under stress 

 are very commonly attributed to crystallisation of the metal. 

 Growth of crystals takes place readily at high temperatures, to 

 such an extent that iron bars forming part of a furnace exposed 

 during several years to a temperature favourable to crystallisa- 

 tion have sometimes been found, when the furnace has been 

 dismantled, to consist of only two or three large crystals. At 

 somewhat lower temperatures vibration has been found to 

 favour this process by facilitating the rearrangement of the solid 

 particles when the metal was initially in a condition not that 

 of equilibrium but there is no evidence that a thermally stable 



1 E. Cohen and K. Inouiye, Zeitsch. physikal. Chem. 1910, 71, 301. 





