508 THE RARER METALS AND THEIR ALLOYS 



It is only necessary for our purpose to use a portion of the long scale, 

 which may be traced across the end of the room by the spot of light 

 from the galvanometer, but we must make that portion of the scale 

 movable. Let me try to trace before you the curve of the freezing of 

 pure gold. It will be necessary to mark the position occupied by the 

 movable spot of light at regular intervals of time during which the 

 gold is near 1,015° C.,that is, while the metal is becoming solid. Every 

 time a metronome beats a second, the white screen A (fig. 1, PI. XXV), 

 a sheet of paper, will be raised a definite number of inches by the gear- 

 ing and handle B, and the position successively occupied by the spot of 

 light C will be marked by hand. 



You see that the time-temperature curve, x y, so traced is not con- 

 tinuous. The freezing point of the metal is very clearly marked by the 

 vertical portion. If the gold is very pure the angles, are sharp; if it is 

 impure they are rounded. If the metal had fallen below its freezing 

 point without actually becoming solid, that is, if supervision, or sur- 

 fusion, had occurred, then there would be, as is often the case, a dip 

 where the freezing begins, and then the temperature curve rises 

 suddenly. 



If the metal is alloyed with large quantities of other metals, then 

 there may be several of these freezing points, as successive groups of 

 alloys fall out of solution. The rough diagrammatic method is not suf- 

 ficiently delicate to enable me to trace the subordinate points, but they 

 are of a vital importance to the strength of the metal or alloy, and pho- 

 tography enables us to detect them readily. 



Take the case of the tin-copper series; you will see that as a mass of 

 tin-copper alloy cools, there are at least two distinct freezing points. 

 At the upper one the main mass of the fluid alloy became solid; at 

 the lower, some definite group of tin and copper atoms fall out, the 

 position of the lower point depending upon the composition of the mass. 



Xow turn to more complex curves taken on one j>late by making the 

 sensitized photographic plate seize the critical part of the curve, the 

 range of the swing of the mirror from hot to cold being some GO feet. 

 The upper curve (fig. 2, PI. XXV) gives the freezing point of bismuth, 

 and you see that surfusion, a, is clearly marked, the temperature at 

 which bismuth freezes being 268° 0. The lower curve, marked "tiu," 

 represents the freezing point of that metal, which we know is 231° C, and 

 in it surfusion, &, is also clearly marked. The curve marked standard 

 gold contains a subordinate point, c, which you will observe is lower 

 than the freezing point of tin, and it is caused by the solidification of a 

 small portion of bismuth, which alloyed itself with some gold atoms, and 

 remained fluid below the freezing point not only of bismuth itself but 

 of tin. Xow gold with a low freezing point in it like this is found to be 

 very brittle, and we are in a fair way to answer the question why 0.2 

 per cent of zirconium doubles the strength of gold, while 0.2 per cent of 

 thallium, another rare metal, halves the strength. In the case of the 



