Objection 1. It would seem that the fire in question is not of the same species as elemental fire. For nothing consumes itself. But that fire will consume the four elements according to a gloss on 2 Peter 3:12. Therefore that fire will not be of the same species as elemental fire.
Objection 2. Further, as power is made known by operation, so is nature made known by power. Now that fire will have a different power from the fire which is an element: because it will cleanse the universe, whereas this fire cannot do that. Therefore it will not be of the same species as this.
Objection 3. Further, in natural bodies those that are of the same species have the same movement. But that fire will have a different movement from the fire that is an element, because it will move in all directions so as to cleanse the whole. Therefore it is not of the same species.
On the contrary, Augustine says (De Civ. Dei xx, 16), and his words are contained in a gloss on 1 Corinthians 7:31, that "the fashion of this world will perish in the burning of worldly flames." Therefore that fire will be of the same nature as the fire which is now in the world.
Further, just as the future cleansing is to be by fire, so was the past cleansing by water: and they are both compared to one another, 2 Peter 3:5. Now in the first cleansing the water was of the same species with elemental water. Therefore in like manner the fire of the second cleansing will be of the same species with elemental fire.
I answer that, We meet with three opinions on this question. For some say that the element of fire which is in its own sphere will come down to cleanse the world: and they explain this descent by way of multiplication, because the fire will spread through finding combustible matter on all sides. And this will result all the more then since the virtue of the fire will be raised over all the elements. Against this, however, would seem to be not only the fact that this fire will come down, but also the statement of the saints that it will rise up; thus (2 Peter 3:10) it is declared that the fire of the judgment will rise as high as the waters of the deluge; whence it would seem to follow that this fire is situated towards the middle of the place of generation. Hence others say that this fire will be generated towards the intervening space through the focusing together of the rays of the heavenly bodies, just as we see them focused together in a burning-glass; for at that time in lieu of glasses there will be concave clouds, on which the rays will strike But this again does not seem probable: for since the effects of heavenly bodies depend on certain fixed positions and aspects, if this fire resulted from the virtue of the heavenly bodies, the time of this cleansing would be known to those who observe the movements of the stars and this is contrary to the authority of Scripture. Consequently others, following Augustine, say that "just as the deluge resulted from an outpouring of the waters of the world, so the fashion of this world will perish by a burning of worldly flames" (De Civ. Dei. xx, 16). This burning is nothing else but the assembly of all those lower and higher causes that by their nature have a kindling virtue: and this assembly will take place not in the ordinary course of things, but by the Divine power: and from all these causes thus assembled the fire that will burn the surface of this world will result. If we consider aright these opinions, we shall find that they differ as to the cause producing this fire and not as to its species. For fire, whether produced by the sun or by some lower heating cause, is of the same species as fire in its own sphere, except in so far as the former has some admixture of foreign matter. And this will of necessity be the case then, since fire cannot cleanse a thing, unless this become its matter in some way. Hence we must grant that the fire in question is simply of the same species as ours.
Reply to Objection 1. The fire in question, although of the same species as ours, is not identically the same. Now we see that of two fires of the same species one destroys the other, namely the greater destroys the lesser, by consuming its matter. In like manner that fire will be able to destroy our fire.
Reply to Objection 2. Just as an operation that proceeds from the virtue of a thing is an indication of that virtue, so is its virtue an indication of its essence or nature, if it proceed from the essential principles of the thing. But an operation that does not proceed from the virtue of the operator does not indicate its virtue. This appears in instruments: for the action of an instrument shows forth the virtue of the mover rather than that of the instrument, since it shows forth the virtue of the agent in so far as the latter is the first principle of the action, whereas it does not show forth the virtue of the instrument, except in so far as it is susceptive of the influence of the principal agent as moving that instrument. In like manner a virtue that does not proceed from the essential principles of a thing does not indicate the nature of that thing except in the point of susceptibility. Thus the virtue whereby hot water can heat is no indication of the nature of water except in the point of its being receptive of heat. Consequently nothing prevents water that has this virtue from being of the same species as water that has it not. In like manner it is not unreasonable that this fire, which will have the power to cleanse the surface of the world, will be of the same species as the fire to which we are used, since the heating power therein arises, not from its essential principles but from the divine power or operation: whether we say that this power is an absolute quality, such as heat in hot water, or a kind of intention as we have ascribed to instrumental virtue (Sent. iv, D, 1, qu. 1, 4) [Cf. III, 62, 4, ad 1]. The latter is more probable since that fire will not act save as the instrument of the Divine power.
Reply to Objection 3. Of its own nature fire tends only upwards; but in so far as it pursues its matter, which it requires when it is outside its own sphere, it follows the site of combustible matter. Accordingly it is not unreasonable for it to take a circular or a downward course, especially in so far as it acts as the instrument of the Divine power.
Source: Summa Theologica (New Advent)