7 
THE constitution" OF THE COPPER-TIN SERIES OF ALLOYS. 
Let the figure be a portion of the concentration-temperature diagram of binary 
mixtures containing the two components A and B, the concentration being measured 
horizontally and the temperature vertically. The vertical line on the extreme left 
corresponds to pure A, and that on the extreme right to pure B, every other vertical 
line corresponding to a particular mixture. Now let the curve LL be a portion of 
the liquidus or freezing-point curve, and let us draw a vertical straight line, cutting 
LL in the point n. Let us consider the particular mixture represented by this 
vertical straight line, and think of it as cooling. When the temperature has fixllen 
to the point n, the liquid mixture will l:)e saturated, and if maintained slightly below 
this temperature it will, if siirfusion he avoided, deposit a small quantity ol a 
crystalline solid. This solid phase may he either the pure substance A, or a pure 
compound of A and B, or it may be, and this is the case we wish specially to consider, 
an isomorphous crystalline mixture of A and B or of compounds of the two. 
(The German word for such a phase, “ Mischkrystalle,” naturally translates itself as “ mixed crystals,” 
but this phrase is liable to be misunderstood, in so far as it may be taken to stand for a complex of two 
or more different kinds of crystals instead of for crystals all alike and each a homogeneous mixture of 
two components, the latter being the real meaning. We have, therefore, reluctantly decided to abandon 
the term and use instead the term “ solid solution.” This term implies that the two componeiits are 
capable of being as uniformly mixed as in a licj[uid solution, and also implies the possibility of continuous 
variation in the percentage composition of the mixture. It does not imply the necessity of crystalline 
structure, and therein the phrase has a wider scope than the otherwise identical term “isomorphous 
mixture ” or isomorphous crystal. But, so far as we know, all the solid solutions met with in alloys are 
crystalline, and we shall, therefore, use the three terms as synonymous. The idea does not exclude the 
possibility that the same crystal may, as is often the case, be made up of layers of different composition 
but, contrary to the views of some writers, we are disposed to think that such a structure is not essential, 
but is the result of imperfect equilibrium adjustments during the formation of the crystal; this, we 
believe, is the view of Professor Roozeboom.) 
Whether the solid phase which crystallises from the saturated liquid n be pure 
component, compound, or solid solution, it must, if left long enough in contact with 
the liquid at the constant temperature n, become homogeneous and of quite definite 
composition, for the phase rule forbids the possibility of two difierent solid phases 
being both in equilibrium with the same liquid except at points where two branches 
of the liquidus meet. If, therefore, the crystals were extracted and analysed, their 
composition and temperature could he represented by a point o on the diagram, the 
line no being horizontal. We know, moreover, that the solid jiliase will be lichei in 
the component A than the liquid with which it is in equilibrium; in other words, that 
0 will lie to the left of n. It is evident that, conjugate to every point on the liquidus, 
there is one and only one point analogous to o. The curve through all these points 
is the solidus as defined by P^oozeboom. In other words, if a horizontal line be drawn 
cutting the liquidus and the solidus, then the two intersections give the compositions 
of the liquid and solid phases that can exist in equilibrium at the temperature of the 
