Such curves have been constructed for a great many alloys since the discovery of the analogy between their behavior and that of aqueous solutions led us to study alloys after the manner which physical chemists found so satisfactory for the study of ordinary solutions. Since the study of binary or two metal alloys is often very difficult, it can be readily understood why the determination and interpretation of the curves of alloys which contain three, four or more metals is a very much more serious matter. Much of it has to be done by methods which are long and tedious, such as quenching and microscopic study of innumerable specimens taken during the freezing and subsequent cooling of various alloys of each series. The value of the results depends upon the skill, devotion and clear sightedness of those who carry out the work. [Illustration: THE FREEZING-POINT CURVES OF THE IRON-CARBON ALLOYS ] The “freezing-point” and “decomposition” curves of the iron-carbon series of alloys have been brought to their present stage of development after something like twenty years of labor by investigators in many lands. If we look at the diagram on page 345, we note at once that the curves are quite complicated. Even yet they are not complete for all percentage combinations of iron and carbon, and those who have given the most time and study to the subject have not yet been able to interpret with entire satisfaction to all concerned all of the discoveries so far made. Without endeavoring to take up in detail the technique of the manner of their production, which would be unprofitable for us without a great deal more of preliminary study than we have time and space to give, we will at once examine the freezing curves of the iron-carbon alloys as now developed. The works named on page 354 as references for Chapters XXII and XXIII may be consulted for the various types and methods of construction and for explanation of freezing curves by those who desire to study them. Referring to the freezing-point diagram on page 336, the upper or broad V-shaped line, ABC, indicates the temperatures at which the alloys of various percentages of iron and carbon begin to freeze, and the lower one, AED, the temperatures at which the freezing of these alloys ends. From the diagram it is readily seen that pure iron (100%), has a very high freezing-point and solidifies at once. Iron which contains about 2% of carbon begins to freeze at a much lower temperature and has a long period of solidification, while iron with 4.3% of carbon has the lowest freezing-point of the series with an extremely short solidification period or range.