The percentages of carbon carried by the particles of iron freezing at any particular temperature of the solidification range may be determined from the diagram but the works named in the reference list should be consulted for method and explanation. This we must now consider another alloy with a lower freezing-point, the reason being, of course, its higher carbon content. At the next lower temperature, more iron containing carbon is frozen out and the remaining liquid is again left a little higher in carbon than before. In this way the continually diminishing amount of remaining liquid keeps concentrating, forming thereby a continuous succession of alloys of higher and higher carbon content as the temperature continuously drops. Eventually, of course, the concentration of this remaining liquor becomes 4.3% of carbon just before completion of the freezing at 2066° F. [Illustration: THE “EUTECTIC,” THE PART OF THE ALLOY WHICH SOLIDIFIES LAST (_Magnification 700 Diameters_) ] Now with alloys containing more than 4.3% of carbon, almost the opposite occurs. Let us choose the one having 5% carbon and 95% of iron. This molten alloy cools until at 2215° F., small crystals begin to freeze and form in the molten mass. But, as the liquid already has more than the favored 4.3% of carbon, it is not free iron which freezes out, but instead, the chemical compound, Fe_{3}C, which contains 6.6% of carbon. This, of course, takes out carbon proportionally faster than iron, hence, at each very slightly lower temperature, the liquid which remains unfrozen contains just a little less of carbon than did its predecessor. So the constantly decreasing amount of remaining liquid progresses through a succession of compositions each containing just a little less of carbon than the previous one, and eventually, just before freezing we get back to the mixture which contains 4.3% of carbon. Of course there is left unfrozen by this time only a very small amount of the alloy and it is this which has the composition stated. The “Eutectic” Now, having just the composition which she wants, whether arrived at from alloys lower or higher than 4.3% in carbon, Nature lets this composition freeze at once in thin alternating plates which lie side by side about and among the earlier frozen crystals of the alloy. The appearance of this typical eutectic formation under the microscope is shown on page 341. Had we chosen the 4.3% alloy itself, neither any of the solid solution of carbon in iron nor the chemical compound, Fe_{3}C, would have frozen out, but the whole mass would have remained liquid down to 2066° F., where the whole would have solidified at once in the plate-like eutectic formation just described.