114 1W. -I. A. Kwi: 



temperatures. Our attention was directed to this ly noticing th.-u u 

 piece of plumber's sheet laid, when etched with dilute nitric acid, 

 exhibits a strikingly crystalline structure, with large crystals. The 

 character of this appearance led us to the view that a slow process of* 

 anm-aling or recrystallisation was at work in such lead at ordinary 

 atmospheric temperatures, and we have satisfied ourselves that this is 

 the case. The method of investigation consisted in taking a series of 

 micro-photographs, at low magnifications, of certain marked areas in the 

 surface of a specimen, in order to watch the change which went on 

 through lapse of time, or after application of some thermal treatment. 

 It was necessary, for the reasons given above, to re-etch the surface 

 l)efore each photograph was taken. 



We have observed that when a piece of cast lead is severely strained 

 by compression, the originally large crystals, after being considerably 

 flattened, are driven into and through one another, so that the etched 

 surface of a strained specimen presents a fine grain, whose crystalline 

 nature only becomes apparent under considerable magnification (80 to 

 100 diameters). A piece of lead severely strained in this way, and 

 kept for nearly six months in an ordinary room without any special 

 thermal treatment, was found to be undergoing continuous change 

 during that time. A series of photographs of this specimen, taken at 

 intervals during the six months, show that a great number of the small 

 crystals have grown larger at the expense of their neighbours. In 

 similar specimens which have been kept at 200 C., the growth has 

 been much more rapid and more pronounced. The rate of growth 

 is a function of time and temperature, but some specimens show much 

 more rapid changes than others under similar conditions of tempera- 

 ture ; in some cases five minutes' exposure to a temperature of 200 C. 

 is sufficient to alter the crystalline pattern completely. Experiment* 

 have also been made at 100 C. and 150 D C., leading to the general 

 result that crystalline growth will occur at any temperature from that 

 of an ordinary room, i,e., 15 C. or 20 C. up to the melting point of lead, 

 and that in general the higher the temperature the more rapid is the 

 initial rate of change. No numerical data can be given, as the crystals 

 are quite irregular, both in size and shape. So far as our observations 

 go, they lead to the result that when such crystalline growth has con- 

 tinued for some time at a given temperature, the structure becomes 

 more or less stable, so far as that temperature is concerned, but expo- 

 sure to a higher temperature may cause further growth to occur. 



A comparison of micro-photographs of the same specimen at various 

 stages reveals the fact that the growth of an individual crystal occurs, 

 not in uniform layers all round it, but by the formation of arms and 

 branches that invade the neighbouring crystals, the intervening 

 portions sometimes changing at a later stage. This action is analogoi 

 to the formation of skeleton crystals in a metal during solidificatic 



