ON THE CKYSTALLINE STKUCTURE OE LEAD. 
227 
where the difference is great, as it is between two crystals, a furrow^ which can 
be easily seen without any magnification is ])roduced. It may be accounted for 
by electrohhic differences between the two portions producing a more violent action 
where they meet. 
The large size of the crystals in this casting suggested that it would be possible to 
obtain a fairly large sized specimen of lead haying a uniform orientation throughout, 
so that the effects of strain upon such a specimen might be studied. To do this an 
attempt was made to obtain a casting in which indiyidual crystals went right 
through from top to bottom, as they could then be separated one from another with 
a minimum amount of cutting. It was found to be yery difficult to do this by direct 
casting, as crystallisation always seemed to take place from both top and bottom, 
thus producing a casting such as has been described aboye. The followizig method 
was therefore resorted to :— 
The lead was melted in a flat sheet-iron dish oyer a small gas furnace, an iron 
plate being placed between the flame and the dish to equalise the temperature. The 
gas was then turned low and the surface of the 
metal was allowed to solidify, one comer, howeyer, 
being kept molten by remoying the crust with a 
hot iron spike. Owing to the contraction of the 
lead upon solidifying the growth of the Amrious 
crystalline grains could be obseiwed, a slight 
difference of level being formed betAveen the solid 
and molten portions. The groAvth Avas, perhaps, 
best seen in those crystals Avliich formed Avith tAvo 
cubic axes parallel to the surface. Fig. 3 illus¬ 
trates diagrammatically the manner of this groAvth. 
One or more parallel arms, such as are seen in the 
figure, shot out from the place Avhere crystallisation 
started and gradually spread outAvards into the 
molten metal. At certain intervals along these 
came others at right angles to them, then a third 
set from these parallel to the first, and so on, 
numerous successive sub-branches groAving at riglit 
angles and gradually filling in the intervening 
spaces. The growth of an individual crystal pro¬ 
ceeded in this manner until arrested by meeting either the sides of the dish or 
another crystal. Tlie outlines of the branches remained even after the AAffiole casting 
Avas cool, forming the ridges Avdiich have been already mentioned. 
When a sufficiently strong crust had formed, the dish Avas lifted up and the still 
molten metal was poured out through the free corner. When cool, it Avas found that 
a certain portion round the edges of the dish had solidified to the full depth of the 
2 G 
