It was but twenty years ago that the first outline was drawn and the whole “fusibility” or “equilibrium diagram” of the iron-carbon alloys given in the next chapter has practically been developed within this time. But over this period of twenty or so years the points upon which these lines Ar_{1}, Ar_{2}, Ar_{3}, Ar_{3·2} and Ar_{3·2·1} are based have been checked many times and they are now well substantiated. These lines form but a small part of the complete “iron-carbon diagram.” The Meaning of the Points You remember that wrought iron and steels having less than .10% carbon showed no point Ar_{1}, and that in all other steels this point becomes stronger as they contain higher and higher carbon. There is little doubt that the point Ar_{1} exists or results from and because of the carbon of the alloy. In wrought iron there is no carbon, hence there is no point Ar_{1}. If the extremely low carbon steels have an Ar_{1} it is so weak that it cannot be detected. Had we tested the .45% carbon steel for magnetic properties we would have found that it lost magnetism at about 1395° F., instead of at 1290° F., at which temperature the .90% steel became non-magnetic. The point Ar_{2}, then, shows the temperature at which loss or gain of magnetism occurs. The electrical conductivity change comes at neither of these points, Ar_{1}, nor Ar_{2}, but at Ar_{3}. However, with increase of carbon the line Ar_{3}, which was drawn through the points, Ar_{3}, rapidly descends. At about .45% or .50% carbon content, this line Ar_{3}, representing the changes in conductivity, joins line Ar_{2}. Hence in steels having .45% carbon or more, there is a common point, or one which in reality is made up of both points. At this common point the phenomena peculiar to each of the points occur. This common line, now called Ar_{3·2}, itself lowers with further increase of carbon until, in steels of around .90% carbon, there is but the single point Ar_{3·2·1}, and the phenomena corresponding to all three of the points occur at this one point at 1290° F., as we found in our experiments. As points Ar_{2} and Ar_{3} occur in carbonless iron, they cannot result in any way from carbon but must have to do with the iron itself. From their experiences with other materials, chemists and physicists are well acquainted with such evolutions of heat as occur at Ar_{2} and at Ar_{3}. These heat absorptions and evolutions, with the sudden dilatation, gain in conductivity, etc., indicate that some internal change or reorganization takes place in the iron itself.