The very rapid rubbing together of two dense bodies generates fire. It is proved that the sun in its nature is hot and not cold, as has already been said.

a - e c - b

c - d g - f

The concave mirror, being cold in receiving the rays of the fire, reflects them hotter than the fire itself. The glass ball full of cold water sends out from itself the rays taken from the fire hotter still than the fire itself. From these two said experiments it follows that such heat of the rays issuing from the mirror or from the ball of cold water are hot by virtue, and not because the mirror or ball is hot. The like happens in this case with the sun passed through these bodies, which heats by virtue; and for this reason they have concluded that the sun is not hot. But by these same alleged experiments it is proved that the sun is very hot, by the said experiment of the mirror and the ball, which, being cold, on taking the rays from the heat of the fire, render them hot rays, because the first cause is hot; and the like happens with the sun which, being itself hot, in passing through such cold mirrors reflects great heat.

Therefore these effects partake of the first cause and not of the intermediate one. If greater light with less heat makes concave mirrors reflect rays of more powerful heat than does a body of greater heat and lesser light: in such an experiment let a loaf of copper be made red-hot, and let it be seen through a round hole of size and distance from the mirror equal to the red-hot copper. Here you will have two bodies equal in distance, varying in heat and varying in splendour, and you will find that the greater heat here will make the mirror produce a reflection of greater heat than the flame already mentioned. Therefore we shall say: it is not the splendour of the sun that heats, but its natural heat.