120 MESSRS. WALTER ROSENHAIN AND P. A. TUCKER. 



of these alloys is, however, rendered very difficult by the presence of the second 

 constituent. In the present case the ft body is so much softer than the tin that on 

 deformation it is immediately driven out at the surface, thus obscuring the deforma- 

 tion phenomena of the tin matrix which constitutes the greater bulk of the alloy. 



A study of the micro-structure of the lead-tin eutectic shows as would be 

 anticipated from the volume-relation of the two substances that in most sections the 

 yS body can be seen definitely embedded in and surrounded by the tin, which has 

 therefore been called the matrix. Whether the two bodies actually crystallise 

 simultaneously or whether there is a slight interval of time or of temperature 

 between their solidification has yet to be ascertained, but it would appear probable 

 that at each point of the eutectic where solidification was beginning the crystallisa- 

 tion of the predominant constituent determined the arrangement of the whole mass ; 

 on that view, in the present case, the tin crystallised out in the form of radiating 

 dendrites whose arms met and more or less interpenetrated those of adjacent 

 dendrites, the lead liquid or solid being forced into the interstices, whose shape 

 would vary according to the form of each particular dendrite. Whether this picture 

 of the solidification process be accurate or not, the existence of clearly marked 

 spherulitic structure throughout the individual grains of the eutectic proves quite 

 definitely that the solidification of each grain must have proceeded from a centre, and 

 that therefore one or both of the constituents are arranged in some definite crystalline 

 manner throughout each grain ; the alternative view that each separate particle 

 of eacli constituent might be a minute independent crystal, having 110 systematic 

 connection with its neighbours, being thus entirely discredited. 



In the hope of throwing further light on the nature and structure of eutectic alloys, 

 the micro-structure of lead-tin alloys of eutectic composition was investigated under 

 other conditions. In one experiment the specimen of alloy, which originally showed 

 the laminated structure usually associated with the term " eutectic," was exposed to 

 prolonged heating at a temperature of ] 75 C. (just below the melting-point of the 

 eutectic). On subsequent examination it was found that the /8 body of the alloy had 

 segregated into relatively large masses in the manner shown in fig. 38. In all other 

 respects the alloy had preserved its true eutectic character, thus serving to show that 

 the ordinary "typical" eutectic structure is a consequence only of the manner in 

 which these alloys usually solidify from fusion, and is not an inherent property of 

 alloys of that particular composition. 



In connection with this line of enquiry it occurred to the author that a solid alloy 

 of lead and tin of the eutectic composition might be produced by the method of 

 SPRING,* i.e., by strongly compressing clean fresh filings of the two metals by means 

 of an hydraulic press. A small compressing apparatus was therefore prepared (by the 

 kind co-operation of Dr. T. E, STANTON), and powdered lead and tin, mixed in the 

 correct proportion of the eutectic composition, were compressed in it by the aid of a 



* SPRING, 'Bull, de 1'Acad. Roy. de Belgique,' (2), vol. xlv. (1878), No. 6. 



