RECENT PROGRESS IN THE STUDY OF ALLOYS. 95 



depend on the expansions or contractions that take place 

 during the final solidification of the lower eutectic alloy. 

 Behrens has found by microscopical examination that this 

 most fusible alloy can sometimes be seen to have contracted 

 in freezing, so that minute spaces are left between the less 

 fusible crystals that form the bulk of the substance. Here 

 we have a source of great weakness in a material that, to 

 other than a microscopical examination, would appear homo- 

 geneous. Lead-gold and bismuth-gold alloys are probably 

 examples of the same phenomenon. A very little lead 

 makes gold brittle, because of the great contraction of the 

 lead during the final stage of freezing. Perhaps the brittle- 

 ness of a gold-bismuth alloy may be due to the considerable 

 expansion which is known to take place during this final 

 stage, a state of strain being thereby produced in the sub- 

 stance. 



F. H. Neville. 

 ( To be continued.) 



