Obj. 2 - Something that works with a certain power indicates that its nature has this power earthly fire by nature can not do much.
Resp. 2 - This is only true when a nature operates by its own power, not when it is being used as an instrument and receives a power higher than its own. This purifying action comes from the divine power
Obj. 3 - The the various earthly fire work the same but this fire will move everywhere unlike it does now, so must be not earthly.
Resp. 3 - The fire as instrument of divine power it will have the power to move everywhere
The generation of this fire will be different but variety of fires will be the same insofar as it affects all matter as an instrument of and by the special intervention of God.
This purging by fire will probably be a grouping together by divine power of all causes capable of causing fire (fire from heaven and below)