Although it is impossible here to discuss fully the experimental results and the oftentimes very complicated relationships which the study of the alloys has brought to light, a brief reference to these bodies will be advisable on account both of the scientific interest and of the industrial importance attaching to them.[303] We have already seen that there are three chief types of freezing-point curves in systems of two components, viz. those obtained when (1) the pure components crystallize out from the molten mass; (2) the components form one or more compounds; (3) the components form mixed crystals. In the case of the metals, representatives of these three classes are also found. 1. _The components separate out in the pure state._ In this case the freezing-point curve is of the simple type, Fig. 63, I. Such curves have been obtained in the case of a number of pairs of metals, _e.g._ zinc--cadmium, zinc--aluminium, copper--silver (Heycock and Neville), tin--zinc, bismuth--lead (Gautier), and in other cases. From molten mixtures represented by one branch of the freezing-point curve one of the metals will be deposited; while from mixtures represented by the other branch, the other metal will separate out. At the eutectic point the molten mass will solidify to a _heterogeneous mixture_ of the two metals, forming what is known as the _eutectic alloy_. Such an alloy, therefore, will melt at a definite temperature lower than the melting point of either of the pure metals. {222} In the following table are given the temperature and the composition of the liquid at the eutectic point, for three pairs of metals:-- ------------------------------------------------------------------- | Temperature. | Composition of liquid. ------------------------------------------------------------------- Zinc--cadmium | 264.5° | 73.5 atoms per cent. of cadmium. Zinc--aluminium | 380.5° | 11 " " aluminium. Copper--silver | 778° | 40 " " copper. ------------------------------------------------------------------- The melting points of the pure metals are, zinc, 419°; cadmium, 322°; silver, 960°; copper, 1081°; aluminium, 650°. 2. _The two metals can form one or more compounds._ In this case there will be obtained not only the freezing-point curves of the pure metals, but each compound formed will have its own freezing-point curve, exhibiting a point of maximum temperature, and ending on either side in an eutectic point. The simplest curve of this type will be obtained when only one compound is formed, as is the case with mercury and thallium.[304] This curve is represented in Fig. 74, where the summit of the intermediate curve corresponds with a composition TlHg_{2}. Similar curves are also given by nickel and tin, by aluminium and silver, and by other metals, the formation of definite compounds between these pairs of metals being thereby indicated.[305]