110 MESSES. WALTER ROSENHAIN AND P. A. TUCKER. 



tend to confirm the view that the transformation connected with this heat-evolution 

 is liable to be suppressed, or at all events to be retarded to a much lower temperature, 

 in the absence of an initiating cause. 



The data derived from the cooling-curves as just described appear to admit of a 

 variety of interpretations, but their comprehension is so much assisted by the 

 microscopic data that it will be preferable to deal with their interpretation in that 

 connection. 



The Micro- Structure of the Alloys. 



The general micro-structure of the alloys is represented in figs. 14 to 22 inclusive 

 (Plates 5-6*). The alloys near the lead end of the series will be discussed below. 

 Fig. 14 shows the structure of the alloy with 15 per cent, of tin when rendered 

 approximately homogeneous by four weeks' exposure to 175 C. (magnification x 100 

 vertical illumination). The dark ground-mass represents the solid solution of tin in 

 lead ; the bright dots correspond to the residue of free tin remaining unabsorbed 

 from the eutectic which was formed during the initial relatively rapid solidification 

 of the alloy. On more prolonged heating to 175 C., followed by rapid cooling, this 

 alloy becomes entirely homogeneous. The next figure (No. 15) shows the alloy with 

 20 per cent, of tin, in which no amount of prolonged heating appears to be capable 

 of entirely removing the free or eutectic tin. This photo-micrograph has been taken 

 under a higher magnification in order to show the remains of the eutectic structure in 

 the light areas which represent the free tin. Owing to the prolonged heating which 

 this specimen has undergone, the lead constituent of the eutectic has almost entirely 

 coalesced with the ground-mass, leaving the residual free tin in segregated masses 

 containing only a few small globules of lead. The appearance of these segregated 

 masses of eutectic tin is very characteristic, and can be readily used for the identifica- 

 tion of free tin which is derived from eutectic as compared with free tin subsequently 

 separated from the solid solution ; apart from the difference in size of the masses, the 

 secondary tin separated from the solid solution never shows the black dots and 

 patches found in the primary or eutectic tin. 



Fig. 16 shows the alloy with 45 per cent, of tin (magnification 200 diameters). 

 This example is given as typical of the structure of the alloys lying between the 

 eutectic alloy and a tin content of 20 per cent. In all these alloys the primary 

 crystals of lead appear as dendritic crystallites of the type shown in this figure, 

 appearing embedded in typically laminated eutectic. A special feature of these alloys 

 is the fact that the lead crystallites are surrounded by zones ot pure tin, i.e., of 

 eutectic whose lead constituent has coalesced with the lead of the crystallite. This 

 feature is of considerable importance, because it explains the manner in which the 

 sheaths of pure tin isolate the free lead of the crystallites from the lead constituent 



of the eutectic. 



* See Description of Plates, p. 122. 



