7. No one knows just when, with increase of carbon, steel ceases to be steel and becomes white cast iron. There is no definite dividing line either in chemical or physical properties. The changes are extremely gradual throughout the scale. Aided by the microscope, modern physical chemistry has disclosed the fact that alloys of iron with carbon “freeze” from molten to solid condition according to two different laws. The change from one to the other occurs somewhere between 1.7 per cent and 2.2 per cent of carbon as is described in Chapter XXII. This is our only basis for calling alloys with less than 2 per cent of carbon, steels, and those with greater amounts, cast irons. 8. For our immediate purposes the other “metalloids” or constituents are of secondary importance and will not be taken up now. From this it must not be understood that they can be slighted by the metallurgist and furnace man in his work. They cannot. Every one of them is of importance and must be accounted for in the final product or trouble results. Volumetric Analysis of the Iron Alloys There is a way in which we may visually get a very intimate idea of the relative composition of these alloys. The cabinet of which a photograph is given is partitioned into four sections. Each one of these contains a bar and six specimen jars. As you may or may not be able to read from the labels, the bars, all of exactly the same size, are soft cast iron, semi-steel (a stronger cast iron), annealed malleable iron and cast steel. The six jars above each bar contain the exact amounts of the various constituents other than iron which are in the bar beneath. As none of the constituents except manganese are as heavy as iron, their volumes per unit of weight are correspondingly greater. Putting it into approximate figures we have the percentages by weight and by volume shown in Table C. This means, of course, that _of the cast iron plates of your cook stove or steam or water radiators fully one-quarter (26 per cent by volume) is not iron at all_ but brittle substances of little or no strength. These elements, silicon, sulphur, phosphorus, and carbon, are commonly called “metalloids.” While the first three named are not in “free” form in the alloy and therefore allow of some doubt as to just the space they require, we have good reason to suppose that the figures given are not far from correct. With such a volume of weakening constituents and particularly with the graphite flakes cutting through and separating the iron grains as the photomicrographs show, can one wonder that cast iron is fragile—more so than steel or wrought iron? To sum up, naming only the most familiar alloys and the two or three qualifying features of each which stand forth with particular boldness, we have: _Pig Iron_—Very High Carbon. Brittle. _Gray Cast Iron_—High Carbon. Brittle. _Malleable Cast Iron_—High Carbon. Made Malleable by Annealing.