Upvote:1
Summa Theologica suppl. q. 91 a. 5 = Super Sent. lib. 4 d. 48 q. 2 a. 5.
By "they" at the end of the corpus of the article, St. Thomas means any mixed/corruptible (i.e., composite) body, such as plants, animals, or minerals, all of which are comprised of elements.
An element (στοιχεῖον) is "The inherent principle of which a thing is first composed and which is not divisible into another species is called an element." (Metaphysics ch. 3 1014a25; cf. St. Thomas's Metaphysica bk. 5 l. 4).
cf. St. Thomas's short work On the mixture of the elements (reprinted in the book Aquinas on Matter and Form and the Elements)
"New Earth" means—according to the best Catholic commentary on St. John's Apocalypse, The Book of Destiny by Fr. Herman Kramer—"the changed state of society, which before and during the days of Antichrist was antichristian and anti-God."
There will be work, according to Fr. Kramer: "The true and just relation of Labor and Capital and the rights of each [will] have been proclaimed."