OF THE MINERAL WORK.

metals, especially iron of great weight, which refers to the form of many conglomerated drops, are condensed in that tract of air from dry exhalations and are sent down concreted. This matter is discussed at greater length elsewhere. Whence it is sufficiently clear that solar rays do not produce any heat from themselves in those places that they permeate freely without obstacle, but where they are detained and fixed having found solid matter, the harder the material, the greater the heat they cause. You will also see that wood or another porous body never contracts as much heat from the sun as stone, nor stone as much as metal, even though they are exposed to the sun side by side. The reason for this diversity is found only in the pores, of which some bodies have more, some less, granting a faster passage to the heat. For it is the property of heat, as has often been said, to hurry forward as long as it is not hindered, but very laboriously to carry its foot backward. Culinary as well as solar or lightning fire provides us with an example: for if anyone sitting by a fire happens to have some metal, whether it be a nail, a knife, or a coin, in a pouch, the heat easily permeates the rare clothing and falls upon the metal, to which it adheres as it increases, and brings about such heat that it sometimes cannot be held by the hand, yet it barely affects the clothing, even though it is closer to the fire. The same happens with lightning, whose fire, because it flies rapidly—not having space to explore the pores of obstructing solid bodies—momentarily dissipates and dissolves them, while it leaves porous things intact.