228 Dr. T. Kirke Eose. On certain [Feb. 9, 



from the alloys containing between 50 and 60 per cent, of silver, and 

 this is borne out by the micro-structure. The alloys when rubbed 

 with potassium cyanide solution are seen to consist of white oblong 

 crystals set in a red-coloured matrix (see fig. 5). As the percentage o 

 silver diminishes the area covered by the crystals is reduced, and th 

 amount of matrix increases. The white crystals evidently consist of 

 the compound Ag 3 Cd 2 , and the red matrix of AgCd. 



The alloys containing between 50 and 40 per cent, of silver are 

 remarkable for resisting the attack of the ordinary etching reagents. 

 Nitric acid has little effect on them, but they are slowly dissolved by 

 hot acid of a specific gravity of 1-3 without their structure being 

 revealed. A similar lack of success attended the use of sulphuric and 

 hydrochloric acids, of soluble sulphides, of alkaline sulphides with the 

 aid of a galvanic battery, and of heat with free exposure to the air. 

 The alloys are not affected by ammonium sulphide under conditions in 

 which silver-copper alloys or pure silver are instantly tarnished. The 

 structure was developed by a polish attack, which consisted in rubbing 

 the polished specimens on parchment with fine alumina moistened with 

 a solution of 0*5 per cent, of cyanide of potassium. The action ia 

 partly abrasive, the red colour being strongly developed in some of the 

 crystals. In the 50-per-cent. alloy, slowly cooled specimens are seen to 

 consist of large crystals of 2 3 mm. in diameter. There is no 

 cementing material, and the crystals are distinguishable from one 

 another under low powers only by their shades of colour. Under high 

 powers (above 1000 diameters) the most strongly coloured crystals are 

 seen to be covered with a number of minute irregular pits and short 

 trenches, which are bright red, the space between being colourless. 

 The difference between the crystals seems to be only that of the 

 orientation of the particles forming them, so that the surface of some 

 crystals is more readily abraded than that of others. The structure, 

 however, is not readily revealed without the aid of the cyanide 

 solution. The abrasions form an irregular network, which gives some 

 of the crystals a minutely cellular appearance, visible at a magnification 

 of 50 diameters. In quickly cooled cast specimens the crystals are 

 much smaller, few being more than 0'3 mm. in diameter, and traces of 

 a hard white cement are discernible between the crystals. 



The same alloys when re-heated to 350 for 6 hours and chilled 

 show an even more minute structure, the crystals being only about 

 0-02 mm. in diameter (see fig. 6). A few hard white projecting grains, 

 probably existing in the alloy after casting and before reheating, are 

 visible under a magnification of 1800 diameters on a slightly pink 

 ground consisting of a solid solution of AgCd and Ag 2 Cd 3 . 



When the 50-per-cent. alloy is heated for 6 hours above 420 and 

 chilled, the separation of the two constituents is far more complete. 

 The alloy is now made up of large bright red hexagonal crystals about 



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