204 MESSRS. C. T. HEYCOCK AND F. H. NEVILLE 



The Curve. 



The curves record the freezing points observed during the experiments. The 

 mean composition of the alloy in the crucible is represented in atomic percentages by 

 the figures above each curve, and temperatures are measured vertically in degrees 

 Centigrade, and recorded by the figures below the curves. For the sake of conciseness 

 we shall use the term atoms instead of atomic percentages. A statement of the 

 number of atoms present completely determines the composition of the alloy, it 

 being understood that the number of atoms of gold is obtained by subtracting the 

 number of atoms of aluminium from 100. 



Let us consider the case when the alloy in the crucible has, taken as a whole, the 

 composition AuAl, that is to say, contains 50 atoms of aluminium. The observed 

 points on a vertical line drawn through 50 give a record of the way in which the 

 alloy cools and solidifies. Above 850 it is wholly liquid, but when the crucible has 

 cooled to this temperature there is an evolution of heat causing the thermometer to 

 remain stationary for a short time, and we observe the upper freezing point. The 

 cooling soon recommences, but more slowly than before, and when the temperature has 

 fallen to 625 there is another halt. After some time, however, the temperature 

 again begins to fall, and soon falls rapidly until about 568 is reached, when an 

 extremely steady temperature is indicated, and the alloy, if it has not done so before, 

 sets to a solid mass. These three points are the freezing points of the alloy. They 

 may, of course, be due to any exothermic changes going on in the crucible, such as 

 the breaking up of the alloy into conjugate liquids or to allotropic changes ; but the 

 microscope has satisfied us that in the case of gold-aluminium we have only to do with 

 the separation of solids, in fact, that at each of the three observed points a new 

 substance begins to solidify. 



It will be seen that multiple freezing points, although frequent, are not universal ; 

 for example, the alloys Au 2 Al and AuAl 2 have only one freezing point. 



It is important to bear in mind that the diagrams of Curves 2 and 5 give a record of 

 all the freezing points observed, and contain more information than an ordinary 

 equilibrium curve. The first, or upper, freezing points of each alloy do indeed 

 constitute an equilibrium curve, but our lines of second and third freezing points 

 do not, strictly speaking, belong to the equilibrium curve. For example, the second 

 freezing point which we mentioned as occurring at 625 does not represent a state 

 in which a liquid having the composition Au^Al^ is in equilibrium with solid. For in 

 the solidification of this alloy the liquid part of the matter in our crucible has, ever 

 since the first freezing point at 850, been getting richer and richer in gold, until 

 when the freezing point at 625 is reached the matter still liquid is of the composition 

 Au M Al44 or thereabouts. In fact all the freezing points on the horizontal line through 

 G are points of equilibrium between this same liquid and a certain solid. In an equili- 

 brium curve as usually drawn all these freezing points would be represented by the 

 point G. The other lines of second and third freezing points have a similar meaning. 



