Complete Works, Vol. V

stone, as was made clear above. For it is either from this, that the hot has drunk up the moisture from earthy matter, and therefore the earthy part has remained hardened. Or it is from this, that the most cold dryness has vehemently seized the transparent moisture, and in converting it to its own property, it has squeezed out the moisture, compressing the matter vehemently, and has hardened it and excellently compacted it, as is the case in transparent stones. For this reason, they are very hard; and when struck, they even emit fire, and they do not sustain a file, but must be polished as if by rubbing and scouring. In other stones made from earthy matter, however, the cause of greater hardness is nothing other than greater dryness, which is caused by a stronger or less strong heat from the side of the agent, and from moisture that is more easily or less easily separable from the matter, as far as the matter is concerned. For oily, very moist material is easily continuous, and entirely watery material easily evaporates. For this reason, stones that are like chalk, or softer than chalk, are very white and leave a white stain on things they touch; they are mixed with a certain very evaporable moisture, and they have been burned by an excellent heat beyond the measure of coagulation, and have already begun to be calcined reduced to a powdery state by heat. For this reason, they do not remain in walls, because being dry and calcined on the outside, they are always rough, and this part, receding into the tenacity of the mortar, leaves another part of the stone not glued to the mortar. And therefore those stones fall from walls, and the wall made of them, after a short time, becomes like a dry-stone wall. Flints, however, are very hard, because their moisture, not being separable from the matter, has been much consumed and has been hardened by a vehement earthy dryness. And for that reason, they also do not take mortar well, because it is not imbibed by their constricted pores; for which reason, in architectural works, stonemasons rarely use these, and they say that these stones split walls. Other genera of marbles, however, are perfectly mixed and vehemently cooked, and therefore they are hard and suitable for walls. But squa-

-re stones are more suitable than others for buildings, and when they are vehemently hard, then their dryness is great, and there is little moisture connecting the parts. When this is frozen by cold, it leaves the exterior parts and flees to the interior, and such moisture is not well incorporated into the parts, and therefore it is easily moved to the interior and exterior. For this reason, after the cold has made it mobile by compressing it, it is consumed by the nearby heat of the sun, and then the stone is divided in different ways. Conversely, however, stones that are somewhat moist, with moisture well glued to the members of the stone, are vehemently dried in the air, and therefore they become harder and better in buildings over the length of time. In tufa, however, the cause of softness is moisture not vehemently extracted, nor excellently mixed with the earthy part; and therefore tufa is soft, and when placed near fire, it is not cooked like a brick, but is formed into earthy ash. These, therefore, are the things said about the hardness of stones; for from these, other diversities can also easily be recognized.

CHAPTER V.

From these, the cause of hewability and unhewability can now be clear. For stones that are excellently hard are not hewable, but are comminutable able to be broken into small pieces into small parts, and when they do not have orderly pores, they are not split to a rule. Just as in wood there happens to be knottiness from the diverse flow of the dry material from which the body of the wood is generated, so it happens from diverse mixing in stones and the confusion of matter: and that knottiness causes the stone to be broken inordinately and not to a rule. Nevertheless, the very hard and very dry stones, whether they are knotty or