In Revelation 5:1, this signifies the states of all people in heaven (within) and on earth (on the back). (Apocalypse Explained 299)

6314. As it is with light, so it is with the heat that is vital to a person's existence. In no way at all does that vital heat originate in the heat radiated by the sun in the world; rather, it originates in spiritual heat, which is love and radiates from the Lord. This heat is what angels have. Consequently the amount of vital heat a person has is determined by the amount of love he has. His body however is warmed by worldly heat, as is his sensory awareness within it; yet vital heat flows into that worldly heat and imparts life to it. Differences in the purity and intensity of this heat are akin to variations in the clarity of the light. This vital heat is what one should understand by the holy fires mentioned in the Word, and therefore heavenly loves are meant by those fires. But in a contrary sense that worldly heat is what one should understand by the fires of hell, and therefore hellish loves and the desires that go with them are meant when these fires are mentioned in the Word.