Bern, Burgerbibliothek, Cod. 402

...does not make a flame, except for a little, because it does not have much moisture being removed, which the flame itself consumes when it is being ignited. It is reasonable to see a sign of the aforementioned irregularity in the fact that it heats [better] where it is more [intense] and faster, being hot, just as the flame of reeds [heats] water, and for men [it is better] than that from woods. And yet, all fire from solids is generally very hot, but coal, being most solid, heats least. And the sun's heat darkens bodies, but the fire's does not. The cause of all these and such things is the thinness and the density; the flame of reeds heats flesh and water most quickly because of its thinness and density. It is thin because it is light, and continuous because it is dense. That which is thin is penetrative, and heating [occurs] through connection and mixture. The flame of all coals [heats] more for this reason, and at the same time, the efflux is greater and more concentrated. In sum, it is not equally heating through the same [means] for each, just as it is not [acting] on the thing itself, nor is it the same for each, but coal is the sharpest, and the flame is the thinnest and densest. For the flame heats as much as these [things] are there, but again it does not burn in the same way because it is thinner and weaker. For fire in a bare and solid [substance] is hottest, because it is straight in the same way, but it is accepted by the heat, having a combustible body. And the solid [substance], because of the density and hardness, is what is ignited, such as iron, copper, stone. Shells are naturally almost the coldest, for heat is held within them...
Why coal does not make a flame.

Why the sun's heat darkens bodies, but that of fire does not.

it heats

likewise

solid

that solids are most caustic.