De primo frigido

Then again, congelation, which is the most forcible and violent of all things that befall our bodies by reason of cold, is the affection of water, but the action of air. For water of itself is easily diffused, loose in its parts, and not readily congealed together; but it is thickened and compressed by the air, by reason of the coldness of it. Which is the reason of the proverb:

But if the southern wind provoke the north,

Snow straight will cover all the earth.

For the southern wind preparing the moisture as matter, presently the north wind receives and congeals it. And this is manifest from the consideration of snow; for ere it falls, you shall observe a thin and sharp cold air breathing before it. Aristotle also tells us, that whetstones of lead [?] will melt and run in the winter through excess of freezing cold, merely upon the setting of the water near them. For it is probable that the air compresses and gripes the bodies so close together, that at length it breaks and crumbles them in pieces.

And therefore water drawn from a fountain soonest congeals; for the more of cold in the air overcomes the less of cold in the water. Thus if a man takes cold water out of a well and puts it into a vessel, and then lets the vessel down again into the well, so that it may not touch the water but hang for some time in the air, the water will be much colder. Whence it is apparent, that the coldness of the water is not the first cause of coldness, but the coldness of the air. For you do not find that any of your great rivers are ever thoroughly frozen, by reason of their depth. For the air doth not pierce through the whole; only so much as it can seize and embrace with its cold quality generally freezes, and no more. Therefore the barbarians never cross over frozen rivers till they have sent a fox before to try the depth of the ice. For if the ice be not very thick, but only superficial, the fox, perceiving it by the noise of the water floating underneath, returns. And some there are that melt the ice with hot water to make way for their lines, when they go to catch fish in winter. So that nothing suffers from cold in the depth of the water. Nevertheless, so great has been the alteration of the upper parts of the water by congelation, that several vessels riding in the stream have been bruised and broken by the forcible compressure and griping of the congelation; as we have heard from them who lately had their winter quarters with Caesar upon the Danube. And indeed, what happens to ourselves is sufficient to demonstrate the truth of this. For after hot bathings and sweatings, we are most sensible of cold, at what time, our bodies being open and the skin relaxed, we give a freer entrance to the cold together with the ambient air. And after the very same manner the water itself suffers. For

it sooner freezes if it be first heated, as being thereby rendered more easy for the air to work upon. And therefore they who lade out scalding water, and let it fall again from a good height in the air, do it to no other purpose than to mix it with a great deal of air. And therefore, Favorinus, the arguments that attribute the first power of cold to the air are grounded upon these probabilities.