Since we have been unable to go sufficiently into the methods and technique of freezing-curve construction to be able to understand their general classification, we must accept the statement that the curve of the iron-carbon series is really a double one. The part of it that lies to the left of the dividing line UV of the diagram on page 336, is of the type exhibited by liquids which freeze from “_liquid solutions_” into what are known as “_solid solutions_,” which by aid of the microscope are found to be homogeneous mixtures of crystals. On the other hand, alloys which lie to the right of UV, are of the type which form “_eutectics_.” This will be described later. This dividing line UV, which occurs at about 1.7% of carbon, divides the iron-carbon alloys into these two natural divisions. It was the basis for calling those having 2% of carbon or less, “steels,” and those with over this amount, “cast irons.” Molten iron is so greedy for carbon, that, when it can get it, it readily holds in solution from 7% to 10% of this element. But solid (frozen) iron cannot retain anything like this amount. As we learned in the last chapter, gamma iron is the only variety which can exist above our lines of loss of conductivity, magnetism and recalescence, i. e., Ar_{3}, Ar_{3·2}, etc. It is, too, the only variety of solid iron which is able to retain carbon in solution, and it can retain only about 1.7% of it. So when molten steel containing 1.5% of carbon, say, cools until it reaches the temperature represented by the line, AB, which, at its intersection with the 1.5% carbon line would be at about 2582° F., particles or crystals begin to freeze out and float in the molten alloy. As the temperature falls, more crystals separate until, when the temperature determined by intersection of the 1.5% carbon line with the lower freezing curve, AE, is reached, the last of the now mushy alloy solidifies. Alloys of all other compositions below 1.7% of carbon do just this way except that the temperatures at which freezing begins and ends are different and distinctive for each composition.[10] Upon freezing, every one of them retains in “solid solution” in the “gamma” iron whatever carbon it had in the liquid or molten solution. But, as stated above, it can not be over the 1.7% limit. Footnote 10: Temperatures of beginning and end of freezing may always be ascertained by locating on the freezing-point diagram the points at which the vertical line representing the desired composition intersects and crosses the lines of the freezing-point curves—in these cases, AB and AE.