Therefore as the becoming of a thing cannot continue when that action of the agent ceases which causes the "becoming" of the effect: so neither can the "being" of a thing continue after that action of the agent has ceased, which is the cause of the effect not only in "becoming" but also in "being." This is why hot water retains heat after the cessation of the fire's action; while, on the contrary, the air does not continue to be lit up, even for a moment, when the sun ceases to act upon it, because water is a matter susceptive of the fire's heat in the same way as it exists in the fire. Wherefore if it were to be reduced to the perfect form of fire, it would retain that form always; whereas if it has the form of fire imperfectly and inchoately, the heat will remain for a time only, by reason of the imperfect participation of the principle of heat. On the other hand, air is not of such a nature as to receive light in the same way as it exists in the sun, which is the principle of light. Therefore, since it has not root in the air, the light ceases with the action of the sun.
Now every creature may be compared to God, as the air is to the sun which enlightens it. For as the sun possesses light by its nature, and as the air is enlightened by sharing the sun's nature; so God alone is Being in virtue of His own Essence, since His Essence is His existence; whereas every creature has being by participation, so that its essence is not its existence. Therefore, as Augustine says (Gen. ad lit. iv, 12): "If the ruling power of God were withdrawn from His creatures, their nature would at once cease, and all nature would collapse." In the same work (Gen. ad lit. viii, 12) he says: "As the air becomes light by the presence of the sun, so is man enlightened by the presence of God, and in His absence returns at once to darkness."
Henry of Ghent paraphrases this passage in his Quodlibet I q.9, on whether a creature's essence is its being (my translation):
Those who say that in creatures the essence of a creature is one thing and its being another thing think that a creature participates in being. Whence they say that creatures are related to God as air to the sun illuminating it, for as the sun which shines by its nature, so that it is nothing other than light itself, so God has being through his nature and essence, for he is nothing other than being. And as air is of itself obscure, and of its nature is not altogether a participant in light unless it be illumined by the sun, participating through this light from the sun, so a creature of itself and of its essence does not have the character of being, but is in the darkness of nonentity, unless it be lightened by God and the being in which it participates be given to it.
After noting a different sense in which we might understand "participation", Henry goes on:
The first way of understanding the participation of a creature in being is mistaken; it is not an understanding but a phantastical imagination. For the essence of a creature should not be imagined like the air indifferent to obscurity and luminosity, but like a certain ray in itself apt to subsist, produced by the sun, not by the necessity of nature but by free will. Whence, if the sun by free will could produce a subsistent ray, that ray, inasmuch as its own nature is concerned, would be indifferent to being and non-being, and of itself would be a certain kind of non-being.
Henry goes on to explain the reason for the correction of St Thomas' image. In the image of the air being illumined by the sun the nature of the air is something different from the nature of the light or its illumination, whereas in a luminous body and the ray of light the nature of light is the same, though one light is dependent on and participates in the other. The ray which reaches our eye is not the same as the sun but is its similitude, as the creature is the similitude of God - but the air is not the similitude of the sun at all. (This seems to me to agree with the way Thomas elsewhere characterizes the essence of creatures as modes of imitability of the divine essence.) Thomas' image of the air's illumination is an image of one sort of thing being poured into another sort of thing to make it actual in a certain way, but for Henry (and, I might add, the Franciscan tradition in general along with him) existence can't be understood as a different sort of thing than the existing nature and added to it in order that it can be.
Of course, whether Henry's own account of the relation of essence and existence in terms of his intentional distinction is ultimately successful is another matter.