102 SCIENCE PROGRESS 



If an impurity be present as a distinct constituent, its 

 detection by means of the microscope is not difficult. The 

 case of copper may be taken as an example. Highly purified 

 copper, such as is used for electrical purposes, exhibits the 

 typical structure of a pure metal. If, as is usually the case, 

 it has been rolled and subsequently annealed, the crystals 

 are polygonal with almost straight boundaries and show 

 repeated twinning. There is perfect contact between neighbour- 

 ing crystals. A small quantity of iron, nickel or arsenic does 

 not alter this structure appreciably but a very different effect 

 is produced by sulphur or oxygen. The sulphide or oxide is 

 visible in a polished section in the form of minute globules, 

 which have a characteristic blue colour by reflected light and 

 are therefore readily seen against the red background without 

 the application of any etching-agent. The examination is most 

 easily performed in the case of the cast metal. The fusible 

 eutectic, which is the last portion of the metal to solidify, is 

 then a mixture of copper with either cuprous oxide (Cu 2 0) or 

 cuprous sulphide (Cu 2 S) and occupies spaces between the 

 crystals. The eutectic, when present in any considerable 

 quantity, takes the form of globules or elongated rods of 

 the oxide or sulphide, the intervals between these being filled 

 with copper. In the micro-section, therefore, a dotted pattern 

 is seen between the crystals. As the proportion of impurity 

 becomes less, the eutectic occupies a smaller area and is at 

 last only recognisable as a narrow, discontinuous layer of 

 globules at the boundaries. 



It sometimes happens that the eutectic alloy of a series 

 contains so little of the less fusible metal as to be practically 

 indistinguishable from the second metal. This is the case, for 

 example, in alloys of copper and bismuth. The eutectic point 

 lies so near to the bismuth end of the series that no structure 

 whatever can be detected in the most fusible portion of the 

 alloy, which has the properties of bismuth almost entirely free 

 from copper. Hence, an examination of copper contaminated 

 with bismuth but free from oxygen reveals crystals of copper, 

 usually much reduced in size, separated by a thin film of bis- 

 muth, as in fig. 8. It is evident that the presence of such a 

 highly brittle impurity, forming almost continuous layers be- 

 tween the crystals of the copper, must be a source of great 

 mechanical weakness ; in point of fact, the specimen represented 



