Bern, Burgerbibliothek, Cod. 402

that motion hinders combustion.

...and the heat, so it is reasonably most caustic to the flesh. At the same time, the motion of the flame makes a certain difference. For this reason, those who move their hands quickly in the flame are not burned. For the motion of the hand being faster, and at the same time changing the parts, hinders the burning. For it does not burn if it does not remain for the same time. For this reason, those walking in the sun are burned less than those sitting. And those running [are burned] less than those walking. At the same time, perhaps there is also a certain cooling resulting from the motion of the air. And that [concerning] congealings, or boilings, and all other such things, all have great differences according to the underlying [materials]: for instance, whether it is soft or very hot, or hard, or again sharp, and more violent and more concentrated. From soft and porous [things], the heat is softer. From hard and dense [things], it is more violent. For this reason, physicians use the suns original: "ἡλίοις" — likely referring to sunlight or solar-heated applications more than the [artificial] fires. For the heat from these is softest. Similarly, this is [observed] in fumigations and ointments. For they seek these heats almost exclusively from such things. Similarly, the other arts [follow] accordingly. Some use it as if it were most violent, such as smithing, using the most violent [heat]. For this reason, they take the earthiest and densest coals. And they make [them] for the earth-workers original: "γεηπόνοις" — perhaps workers of earth/smelters for the sake of the strength. And they use bellows. For in this way, the heat is more violent and more concentrated as the pneuma burns along with it. In this category, then, the puzzle is solved: that the sun darkens the flesh...
difference.

that the sun darkens the flesh, being soft.