Two or three other theories have been more or less strongly advocated but these also suffer from lack of evidence. The one which perhaps ranks next in number of advocates is the “carbon theory.” Its supporters contend that by quenching, alpha iron is made to hold carbon or iron carbide in solid solution and that it is this solution of carbon or carbide which gives to the steel hardness in proportion to its carbon content. Others hold that the great density or strain under which quenched steel exists accounts for the hardness. Like many other great problems of the universe this one is not yet conclusively or satisfactorily solved, so reluctantly does Nature yield her secrets. But while it may not have explained all of the “whys” and “wherefores,” the work which investigators have done has brought about great improvement in methods of manufacture and quality of the alloys which are making our civilization greater and this Age more wonderful. CHAPTER XXIII THE EQUILIBRIUM DIAGRAM OF THE IRON-CARBON ALLOYS It was a great day for metallurgy when it was discovered that molten mixtures were governed by the same natural laws which govern our ordinary liquid solutions. Probably it was because most metallic alloys are solid at ordinary temperatures and liquid only at very high ones that for so long a time we failed to suspect their similarity. If a sensitive pyrometer is inserted into an ordinary solution or a molten alloy which is being gradually cooled, it indicates the first instant that solidification (freezing) begins as well as the termination of the freezing period. Unlike the freezing of water or a pure metal, complete solidification ordinarily does not take place at a definite single temperature but over a greater or lesser range of temperature. By taking such upper and lower freezing-point measurements for many different percentage compositions of a binary (two metal) alloy, for instance, curves can be plotted which show accurately the habits of any and all of the possible combinations (i.e., alloys) of those two metals. Such are called “freezing-point” curves, and, as we shall see later on, a study of them will give us much valuable and interesting information.