The more advanced nature of the views of Theophrastus is evident when his statements are compared with Plato's theories about the formation of stones and the reason for the differences in their physical properties. This is what Plato says: "But when earth is compressed by air into a mass that will not dissolve in water, it forms stone, of which the transparent sort made of uniform particles is fairer, whereas the opposite kind is coarser."10
3. Some things are solidified through heat, others through cold.
Theophrastus seems to follow the theories of Aristotle about the solidification of mineral substances. According to Aristotle,11 anything capable of being solidified was either water or a mixture of water and earth, and the agent that brought this about was either heat or cold. Ordinary water and analogous substances like molten metals were solidified by cold. On the other hand, a salt solution, since it left a solid residue after being treated at an elevated temperature, was considered to be solidified by heat. Aristotle did not recognize, as we do, any radical difference in the two phenomena. Cold was supposed to act by driving off the heat, and the moisture of the liquid was believed to accompany the heat in the form of vapor. Heat was supposed to act directly in driving off the moisture and leaving the earthy or solid matter behind. In other words, solidification of substances by either agent was considered to be a drying process. Aristotle also showed that the solidification of some substances could occur in both ways. Certain mixtures of earth and water, such as common mud, were in this class, for either heat or cold could readily bring them to a solid condition.
3. it would seem that all the types of earth are produced by fire, since things become solid or melt as a result of opposite forces.
The formation of ashes when materials are set on fire and burnt, and the resemblance of these to natural earthy substances, may well have been the basis of the idea that all earthy substances owed their origin to fire. In subsequent passages (sees. 50, 54, 69) Theophrastus refers again to the part that fire plays in the forma-
10 Timaeut, 6oC.
J1 Meteorologica, IV, 6-12.
