For convenience we will call the mixture above mentioned, at which the water had become thoroughly saturated with the salt, “cementite”, because this is the name which our friends, the metallurgists, have given to a similar mixture of iron and carbon. They call the water, “ferrite”; the salt, “carbide” and the resulting mixture of brine, “cementite”. This mixture of iron and carbon always exists in exactly the same ratio, namely, 93.4% iron and 6.6% carbon, and is expressed chemically by the symbol Fe3C, which means, in other words, that three “atoms” of iron have united with one “_atom_” of carbon to form the “chemical compound”, “iron carbide”, which the metallurgists, as above mentioned, desire to term “Cementite”. Now let us go back to the brine solution with which we are already familiar, and suppose that we added a little more salt than the water could absorb, and which therefore would exist in a “solid solution”, and then bring this “mechanical mixture” to such a low temperature that it would actually “freeze”. For convenience, and in order to agree with the metallurgists again, let us call the resulting structure “pearlite”. That is the name which they have given to a corresponding “mechanical mixture” of cementite and ferrite. This new constituent “pearlite” contains approximately O.9% carbon and consists of inter-stratified layers or bands of ferrite and cementite. It is regarded as a separate and distinct constituent of steel, and takes its name from the fact that it has a mother of pearl-like appearance under the microscope. It always occurs at a definite range of temperature and always contains the above mentioned definite percentage of carbon. From the above it may be suspected that a steel containing O.9% carbon, consisting entirely of pearlite, forms rather a special and particular class of steels, which the metallurgists have decided to dignify with the title “Eutectoid Steels”. Having done this much to properly impress the unsuspecting probers of their secrets, they decided to call steels containing less than this Eutectoid ratio of carbon (0.9% C) “Hypo-eutectoid Steels”. These steels, of course, contain certain definite amounts of pearlite with other amounts of free or excess ferrite. Likewise, if the carbon content is greater than O.9% there will be an excess of cementite over the ferrite and we will then have a structure of pearlite plus free cementite. And these steels are spoken of as “hyper-eutectoid” steels. [Illustration: Hypo-eutectoid Steel. Carbon .11%. Structure: Light—Ferrite; Dark—Pearlite. Mag. 500x] [Illustration: Hypo-eutectoid Steel. Carbon .37%. Structure: Light—Ferrite; Dark—Pearlite. Mag. 500x] [Illustration: Eutectoid Steel. Carbon .90%. Structure: Fine uniform Pearlitic condition. Mag. 500x] [Illustration: Hyper-eutectoid Steel. Carbon 1.20%. Structure: Dark—Pearlitic; White boundaries—Cementite. Mag. 500x] CHAPTER III. AN EXPERIMENT PERFORMED WITH A PIECE OF PEARLITIC STEEL.