226 



Dr. T. Kirke Kose. On certain 



[Feb. 9, 



compound AgCd, and a third corresponding to the freezing point 

 either of the compound AgCd 3 or of a eutectic alloy consisting chiefly 

 of this compound. 



(5) Between the compounds AgCd and Ag 2 Cd 3 . A curve concave 



upwards. 



(6) Between the compounds Ag 2 Cd 3 and AgCd 3 . 



(7) Between AgCd 3 and cadmium. A curve with a distinct minimum 

 at about 1-2 percent, of silver. There are two horizontal branches, 

 one due to the solidification of a eutectic of this composition, and the 

 other to the freezing of the compound AgCd 3 . 



Micro-structure of the Alloys. 



The alloys containing upwards of 80 per cent of silver all present 

 similar characteristics. Their structure is well developed on polished 

 surfaces by dipping them for a few seconds in hot nitric acid of a specific 

 gravity 1'25. Slowly cooled specimens, in which the solidification 

 occupies about 5 minutes and the subsequent cooling to a low red heat 

 about half an hour, are seen to consist of crystallites of a silver-rich 

 body set in a darker coloured matrix which has been more deeply 

 etched by the acid than the crystallites (see fig. 3, Plate 10). The 

 limiting case of pure silver shows no matrix and the crystallites are 

 large, the crystal grains in small specimens being from 58 mm. in 

 diameter. Similarly no matrix was observed in the alloy containing 

 97 - 3 per cent, of silver but it was detected in the 95-per-cent. alloy, and 

 showed some increase as the silver diminished to 80 per cent. At the 

 same time the size of the crystallites diminished to a diameter of 

 about 1 mm. When these alloys were re-heated for from 1 4 hours 

 at temperatures intermediate between the initial freezing point and the 

 " eutectic " point referred to above, the crystal grains broke up and a fine 

 network of the dark-coloured matrix was formed. When reheated at 

 any temperature above a red heat but below the final solidification 

 point, the matrix disappeared and crystals were formed occupying the 

 whole area of the field. The higher the temperature (below this point) 

 and the longer the time during which it was maintained, the larger the 

 crystals grew and the straighter and more regular became their 

 bounding faces. All the specimens were chilled after re-heating. 



It appears, therefore, that at all temperatures below the solidus curve, 

 these alloys are homogeneous, consisting of a single solid solution* 

 doubtless consisting of varying proportions of silver, and the compound 

 Ag 4 Cd the existence of \yhich has been referred to by Heycock and 

 Neville.* These two substances, silver and Ag 4 Cd, must be regarded as 

 isomorphous. It follows that the more slowly these alloys are cooled,. 



* "The Freezing-point of Triple Alloys," 'Chem. Soc. Trans.,' vol. 65 (1894), 

 p. 65. 



