1 1 8 MESSRS. WALTER ROSENHAIN AND P. A. TUCKER. 



more accurate, since even very delicate pyrometric observations fail to detect any 

 difference in freezing-point between alloys lying between 62'5 and 63'5 per cent, of 

 tin ; GUTHRIE'S method of separation, by liquation, was necessarily subject to the 

 same inaccuracies. 



From the data as to the chemical composition, constitution and density of the 

 eutectic alloy and its two component phases as arrived at in the earlier parts of this 

 paper, it is possible to calculate the volume composition of the eutectic alloy. The 

 eutectic has been found to consist of 55'87 parts of tin and 44'13 parts of /8 per cent., 

 the densities of tin and of the fl body being 7'30 and 10'38 respectively, while the 

 density of the eutectic is 8'40. The volumes of free tin and of the /3 body present in 

 LOO grammes of the eutectic alloy are therefore equal to 55'87/7'30 and 44'13/10'38 

 respectively ; this gives the volume ratio of the two constituents as 7 '653 to 4'252 or 

 1. '80 to 1. An attempt was made to verify this volume relationship by means of 

 pliinimetric measurements of the relative areas of the light and dark constituents 

 of the eutectic as represented in some of the best defined of the photo-micrographs 

 reproduced in figs. 34, 35 and 30, but the results varied too widely to allow of any 

 satisfactory deduction. Consideration of the fact that the structure of the eutectic as 

 shown in these photographs frequently assumes the form of plates or rods of one 

 constituent embedded in the other at once serves to explain these variations ; in the 

 case of a bunch of parallel needles of the /3 body lying in a matrix of tin, the relative 

 areas of the cross-sections as seen in a micrograph would depend upon whether the 

 plane of the section ran parallel to or at right angles to the length of the needles. 

 These variations are, of course, further accentuated by the fact that the eutectic 

 structure is always somewhat minute, so that the areas measured at any one time are 

 confined to a very minute portion of the actual alloy. 



Examples of the typical micro-structure of the lead-tin eutectic alloy are given in 

 figs. 31 to 36 inclusive (Plate 8), all of which have been taken from samples of 

 pure eutectic prepared in the manner described above. Fig. 31 (magnification 600 

 diameters) shows an example of regular lamination, the layers of the two constituents 

 lying approximately parallel ; the photograph, however, shows the junction of two 

 regions or "grains" in the eutectic, the general direction of these parallel bands being 

 decidedly different in the adjacent " grains," while more or less constant throughout 

 each region or " grain." In figs. 32 and 33 (magnification 300 diameters) a similar 

 feature is shown over a larger area, fig. 33 showing the junction of three areas of 

 different and more or less regular orientation. In figs. 31 and 32 it should be noticed 

 that the regular pattern of the eutectic remains undisturbed up to the very edges 

 of each region (the black dots in fig. 3 1 are small holes in the metal, not areas of /3). 

 As close observation has shown, this is a severe test of the accuracy with which the 

 true eutectic composition has been attained in the synthesis of the alloy, the first 

 effect of a slight addition of either constituent being to produce a slight coarsening 

 of the structure combined with a slight relative increase in the amount of the excess 



