Theophrastus states here that they were fusible when placed in contact with certain other material. Since the two kinds of stones are mentioned together, it is likely that they were either similar in nature or had some common property or use. Though there is no direct evidence that would enable us to identify the first of these two classes of stones, several references in ancient literature make it possible to identify at least some of the class of millstones. Strabo,47 for example, observes that the lava of Etna becomes millstone on cooling, and in another place48 he mentions tliat an abundance of millstone was found on the volcanic island of Nisyros. Pliny,49 on the other hand, states that a superior kind of quicklime was prepared from the stone commonly used for millstones (molares). Although it is very likely that a variety of stones were used for millstones in ancient times, the remarks of these authors indicate that they were usually made either from highly siliceous volcanic rocks or from common hard compact limestone. From the name it may be inferred that fire-resisting stones were varieties of limestone, and since Aristotle50 mentions the fusibility of such stones immediately after referring to the manufacture of iron, this identification is even more likely; for limestone is generally added as an essential fluxing agent in the smelting of iron ore. Although there is no necessary connection between the sentence in Theophrastus dealing with these two stones and the two preceding sentences, it seems quite likely that such a connection was intended. If the passage is understood in this manner, then it also seems justifiable to assume that, because of the chemical nature of the two kinds of stones, the material that was burnt with them was the mixture of ore and fuel used in making up the charge for the smelting furnaces. For these two kinds of stones supply the two different types of fluxes needed for the reduction of ores, the acid type in the form of highly siliceous rocks and the usual basic type in the form of limestone. Though they lack precision, the brief allusions to ore-smelting in this section of Theophrastus are of considerable interest, since they are the earliest historical reference to the process.
" VT, 2, 3. *8X, 5, 16. «XXXVI, 174.
80 Mctcorologica, IV, 6, 383B.
