218 MESSRS. C. T. HEYCOCK AND F. H. NEVILLE 



give a photograph (fig. 3) of a patch of this etched eutectic taken between the bars of 

 pattern so that no primary crystals are in the field. The somewhat cellular character 

 of the eutectic strongly suggests three stages in the freezing, first, the formation of 

 the large primary crystals of gold, then, probably after surfusion, the solidification of 

 most of the second substance, and, finally, that of the residual gold in the interstitial 

 spaces to form what becomes the minute raised pattern of the eutectic. Various other 

 sections have suggested the same three stages. In the case of the slowly -cooled alloy 

 with 18'1 atoms, the primary crystals or blobs, which are presumably gold, show a 

 marked pattern after etching. Each blob, instead of being uniformly attacked by the 

 etching agent, is eaten away into deep grooves running in various directions, and 

 having no relation to the lines of polishing.* This, as we have said, may mean that 

 the blob is not a crystal, but a mass of crystals ; a longer etching reduces a blob to a 

 cellular appearance very much like the drawing of the 19 '8 alloy, t 



With 19 '4 and 19 '8 atoms of aluminium we see a complete change in the pattern of 

 the sections. Instead of the isolated primary crystals of gold, fairly uniformly 

 scattered through a finely grained eutectic that constitute the pattern at 16 '9 and 

 18'1 atoms, we now have, after bromine etching and with normal light, a dark brown 

 or black surface finely reticulated by a system of slender golden lines that divide it up 

 into cells (fig. 5). Thus the appearance is that of a nearly pure body, the material 

 inside the cells presumably being that of the primary crystallisation, and the slender 

 golden boundaries being the mother-substance that solidified last in order of time. 

 The two alloys are very similar in appearance, but there is more of the golden network 

 in the 19'4 atom alloy. The curve gives for this alloy two freezing points, at the 

 upper of which primary crystals of gold should have formed, but the section polished 

 was cut from a portion of the extract that had run out of the Jena pipette after 

 removal from the crucible, hence it was the most fusible part, and probably had a 

 composition nearer to B than the point recorded on the curve. Oblique illumination 

 brings out the golden boundaries of the polygons much better than normal light, and 



* In this and other cases where there seemed a danger of the scratches causing a false pattern, care 

 was taken to polish by motion parallel to one direction, so that scratches might be easily recognised by 

 their parallelism to this direction. 



t Jantiary 30, 1900. As the transition from the alloy with 18'1 atoms of figs. 2 and 3 to the 19'8 

 atom alloy of figs. 5 and 6 was a considerable one, we have lately made an intermediate alloy (fig. 4) 

 containing 19 atoms of aluminium. This alloy was made by melting together in a sealed and vacuous 

 tube of Jena glass appropriate amounts of the 18 - 1 atom alloy and of aluminium. This alloy, when 

 slightly etched, showed a small number of very slender rows of dots of gold on a brown ground; it was 

 evident that the primary crystallisation was much less in amount than that of the 18'1 atom alloy of fig. 2. 

 The surface of the 19 atom alloy was then deeply etched by immersion for twenty -four hours in bromine 

 water. The resulting surface, photographed by oblique illumination (fig. 4), ahows very well the slender 

 but very regular crystal skeletons of gold that exist in the alloy. Owing to the small amount of the 

 primary crystallisation (due to nearness to the eutectic point) these skeletons have not filled up with 

 gold, and so lost their crystalline form. Fig. 4 gives us much the same insight into the structure of the 

 solid alloy as that afforded by fig. 30, the Rontgen ray photograph. 



