358 PROFESSOR J. A. EWINO AND MR. W. ROSEXHAIN 



thing that is shown by etching. The cast surface generally shows clearly, without 

 any etching, the boundaries between the crystalline grains. Good examples of this 

 are found in cadmium, lead, tin, and zinc. In some instances the boundaries be- 

 tween the grains are emphasised in a remarkable way by the accumulation there 

 of air or of gas given off by the metal during solidification. The boundaries then 

 appear as more or less deep and wide channels. As the growth of the crystalline 

 grains proceeds most of the air or gas which has been entrapped between the glass 

 and the metal, and most of the gas given out from solution in the metal itself, 

 tends to accumulate at the boundaries, which are the last parts of the surface to 

 undergo solidification. A network of channels consequently appears on the surface 

 next the glass ; in general these channels coincide with boundaries, but occasionally 

 a channel forms a cul-de-sac or terminates in a large cavity which is obviously a 

 bubble or blow-hole. Fig. 6 shows the appearance of a surface of cadmium cast 

 against glass under conditions of temperature favourable to the formation of these 

 channels. By etching or straining the specimen it is easy to prove that each 

 channel (except when it is a cul-de-sac formed by excess of gas) coincides in 

 general with a real boundary between the crystalline grains. The true boundary 

 is merely the trace of a surface on the plane of casting, but it may be broadened out 

 in this way, by the presence of gas, into a shallow channel of considerable width. It 

 is only in certain conditions of temperature, on the part of the metal and of the body 

 against which it is cast, that any marked development of such channels is obtained. 

 Under most conditions, however, it is easy to distinguish the intergranular boundaries 

 in the cast surface, either by the presence of some gas there or by slight differences 

 of level between one grain and the next. 



Occasionally the faces of some of the grains in the surface cast against glass are 

 covered with a number of very minute pits, whose true character appears only under 

 a magnification of about 1000 diameters. They are then seen to be pits of definite 

 geometrical form both as to outline on the surface and as to inner facets. Figs. 7, 8, 

 and 9 are photographs of these pits in glass-cast cadmium. It will be seen that they 

 are systems of geometrical figures which remain similar and similarly sitiiated over 

 the entire surface of a single grain, but change in character from one grain to the 

 next. These pits appear to be excessively small bubbles or blow-holes which have 

 taken a geometrical shape, forming, in fact, negative crystals. During the crystal- 

 lisation the crystalline elements have built themselves around the gas bubbles in 

 regular orientation, finally leaving the pits as we see them. In support of this view 

 the appearance seen on fig. 8 may be cited ; the pits are seen to be surrounded by a 

 halo or circular patch of bright surface -due apparently to the absorption by these 

 larger geometrical bubbles of the numerous tiny bubbles that dot the surface 

 elsewhere. 



The photographs show these "air-pits" in cadmium cast against glass under a 

 magnification in most cases of 1000 diameters, and in one case (fig. 9) of 4200 



