Thus, according to the theory, there is no essential difference in the proportion of carbon between grey and white cast iron, but the former is a mechanical mixture of crystals of carbon, nearly pure, with iron containing a less proportion of carbon than the white, while the white iron is a homogeneous alloy of carbon and iron. Upon this theory may be explained all the facts which have been found wholly irreconcilable with the other. 1. The more intense the heat of the furnace, the deeper the colour, and consequently the higher quality of the cast iron. 2. The changes that take place from grey to white cast iron, merely by difference in the rate of cooling. 3. The reconversion of the white variety into grey, by simply heating it above its melting temperature, and allowing it to cool gradually. 4. The formation of imperfect crystals of plumbago (_kish_) on the surface of grey iron. 5. The approach to malleability of the grey iron, which is utterly irreconcilable with its being a homogeneous compound, more charged with carbon than the white. The basis of white cast iron, appears to be a definite chemical compound, of two atoms of iron to one of carbon, and is therefore analogous in its chemical constitution to carburet of hydrogen and carburet of sulphur, but like all metallic alloys it is capable of containing an excess of one of the substances in a state of mixture during fusion, and which does not separate on rapid cooling. The iron alone is found in excess in this substance. Steel appears to contain but half the quantity of carbon in its chemical proportions that white cast iron does, but, like it, is susceptible of a variety of mixtures; if the proportion of carbon amount to three per cent., it loses the property of malleability, if the proportion fall as low as one per cent. it can no longer be tempered, and is identical with the harder varieties of bar-iron. As the carburets of iron, whether in the form of pig or of steel, may be considered as alloys, if they be presented to other metals, the results must necessarily be different from what occurs when pure iron is exposed to the same substance. The union that may take place in the one instance may not occur in the other. It may often happen, that when the iron is pure, a true chemical combination will occur, while in the other case, no more than a mechanical mixture can be effected. For the same reason, the consequence may be totally different when the third substance is presented to the iron when first deoxidated, in the presence merely of an excess of carbon, and when the combination with that substance has actually occurred. If reduced at the same time with the iron, the other metals will unite with it more readily than with the carburet, and they may afterwards prevent its union with carbon, for there are few, if any metals, besides iron, which have any affinity for carbon.