Complete Works, Vol. V

is either a species of certain earth, or a certain species of water. For one of these elements prevails in stones, and in those in which certain species of water seem to dominate, there is also some earth dominating at the same time. A sign of this is that the genera of almost all stones sink under water, which we said in the science of On the Heavens and the World original: "Cœli et Mundi" to abound in the matter of earth. If the superior elements were dominating in them, without a doubt they would float upon the water: but now no genus of stone floats, unless it is spongy, or burned, and made spongy and porous by burning, such as pumice pumex volcanic rock, and A stone of Vulcan, although he himself is not beneath the earth. the stone which hot springs or the fire of Vulcan the god of fire, referring here to volcanic activity vomits forth, which even if it is reduced to powder, the powder itself sinks under water. Furthermore, if in transparent stones there were not an earthy element limiting the moisture mixed with water, they would not sink under water, such as crystal crystallus quartz and beryl beryllus a silicate mineral, just as ice and other things in which water is pure or superabundant do not sink. Furthermore, all stones that are generated in the kidneys and bladders of animals are generated from viscous, thick, and earthy moisture. It is necessary, therefore, that such be the matter of stones.

Speaking specifically about those stones in which the matter is earth, it can easily be seen that not only is earth the matter in them: for that would not be continued into the solidity of a stone. For we say that the cause of continuation and mixture is moisture: for this, by its subtlety, makes one part of the earth flow to another, and this is the cause of the good mixture of the parts of the matter. If, however, this moisture were not well infused everywhere into the earthy parts, and were not retentive, and evaporated during the coagulation of the stone, then a powder of earth would remain, not continued: and for this reason it is necessary that it be viscous and glutinous, so that its parts lie together with the earthy parts like the hooks of chains. Then the dry earthy matter retains the moisture, and the aqueous moisture existing within the dry

causes the continuation. And Avicenna testifies to this, when he says that pure earth does not become a stone: because earth does not make continuation by its dryness, but rather fragmentation. For the dryness prevailing in it does not permit conglutination to occur. The same philosopher gives the reason, that sometimes clay is dried and becomes a medium between stone and clay, and then in the passage of time it becomes a stone. He says again, that That clay is more apt to be converted into stone. clay is more apt to be transmuted into stone, which is unctuous: for what is not of such a kind is fragmentable or reducible into powder, because of the easy separability of moisture from it. A sign of this is that in the stones themselves, veins of earth frequently remain, and it is hard dry earth, which, when it is compressed or crushed, becomes powder. The cause of which is none other than that its moisture, not being sufficiently unctuous and viscous, evaporated from it in the coagulation of the stone: and therefore hard, fragmentable earth remained because of the virtue of the coagulation of the surrounding stone. There is also another sign of this: because when stones are not generated continuously, but by way of layers, so that one is generated upon another, the interposed such earth, suffering fragmentation in compression or concussion, is not continuously united, and is hard: the cause of which is what we said above. That it is, however, viscous and unctuous moisture which is the continuative of the matter of the stone is signified by this, that animals which are called tortoises testudines or turtles are very frequently generated with their shells in stones: and this is very frequent in stones that are found to be small, which have many holes having the shapes of the shells of tortoises, which some call lunar. The cause of this is the moisture which Why there are many holes in stones in the likeness of shells. evaporated to that place, and, retained by the surrounding matter, was rolled upon itself, and first being consistent externally, it received a vital spirit circled within itself, as we said in the fourth book of Meteorology. This, therefore, in general, is the matter of the stones that are not transparent nor simi-