1Q 



REPORT — 1901. 



must have had its origin at lower temperatures, we have attempted to 

 obtain a permanent record of the structure of the alloys atditf^rent stages 

 of temperature by cooling them slowly from a molten state to selected 

 temperatures, and then chilling them. When an alloy had solidified 

 before the moment of chilling, the subsequent changes in structure are 

 generally very minute, often sub-microscopic, even if they take place at 

 all. It may be doubted whether the chilling does absolutely prevent the 

 later changes, but it enables us to distinguish the large scale structures 

 already existing before the chill from the necessarily much more minute 



structure formed during and after the chilling. We are thus by chilling, 

 polishing, and etching able to form very trustworthy conclusions as to 

 the structure of an alloy immediately before it was chilled. 



Numerous experiments of this kind show that an alloy chilled in the 

 region of temperature between the solidus and liquidus contains large 

 primary combs which, from their size, must have been formed before the 

 chilling ] and that between them one often sees a crop of minute 

 primaries similar to the large ones, but formed during the chilling. 

 When the polished surface of a section of alloy is heated in the air the 

 combs oxidise more rapidly than the mother substance in which they are 

 imbedded. They are also softer than the ground, for by prolonged 

 polishing they are eaten out into a pattern. These peculiarities, as well 



