116 MESSES. WALTER EOSENHAIN AND P. A. TUCKER 



constituent must be 10 '38. The density of the lead-rich constituent of the eutectic, 

 i.e., of the ft body in the meta-stable state, is thus 0'07 higher than that of the stable 

 a body with the same amount of tin. This result fully agrees with the view that the 

 a body has deposited some of its tin content in the free state, with an increase of 

 volume. 



The equilibrium diagram of the lead-tin alloys, as based upon the data and inferences 

 given above, is shown in fig. 30. The diagram consists in all of eight fields or regions 

 corresponding to different states of the alloys. In field No. 1, lying above the 

 liquidus AEB, the alloys are homogeneous liquids. In field No. 2, bounded by the 

 lines AE, EC, CA, the alloys consist of a mixture of liquid and crystals of the lead- 

 rich ft body ; it should, however, be noted that while the line EC and the position of 

 the point C have been determined, the line AC has not been fixed experimentally and 

 is therefore drawn as a dotted line, but its true position is not likely to depart widely 

 from that indicated. In field No. 3, bounded by AC, CF, FE, EPb, PbA, the alloys 

 consist of the homogeneous ft body, which is a solid solution of tin in lead which is 

 saturated at a temperature of 182 "5 C. with a tin content of a little more than 

 1G per cent. The point F, lying upon the line of the a./ ft transformation at the point 

 where that transformation first readies its highest temperature (at a concentration of 

 18 per cent, of tin), probably represents the limit of saturation of the ft body at 149 C., 

 and for that reason the limit of the region of pure ft has been drawn from C to F ; 

 this line also is to be regarded as tentatively drawn. The curve from F towards E 

 has been prolonged vertically downward, since no trace of the ft/a reaction can be 

 found in alloys containing less than 8 per cent, of tin. 



In region No. 4, bounded by CE, EG, GF, FC, the alloys consist of saturated ft 

 plus eutectic or in terms of phases of saturated ft and tin. In region No. 5, 

 bounded by FG, GK, KE, EF, the alloys consist of the a body (a saturated solution 

 of tin in lead saturated at a lower concentration of tin than 18 per cent.) plus eutectic 

 plus secondary tin, the eutectic itself consisting of tin plus the body in a meta-stable 

 condition. In terms of phases this region would contain the a body plus tin if 

 complete equilibrium were attained, but in all cases so far studied meta-stable ft was 

 present. Region No. 6, bounded by BE, ED, DB, contains alloys consisting of liquid 

 plus crystals of pure or very nearly pure tin. Region No. 7, bounded by ED, DH, HG, 

 GE, comprises alloys consisting of tin plus eutectic or in terms of phases of tin plus 

 the ft body in a stable state. Finally, in region No. 8, bounded by GH, HSn, SnK, 

 KG, the alloys contain tin plus eutectic or in terms of phases tin plus meta-stable 

 ft body. 



The Eutectic Alloy of Lead and Tin. 



The percentage composition of the eutectic alloy has been determined with special 

 care. The method adopted consisted in preparing an alloy of a composition known to 

 be approximately that of the eutectic and preparing a micro-section. On examining 



