“..for charity is a spiritual fire which, when inflamed, is called devotion. “

(St. Francis de Sales, An Introduction to the Devout Life, Chapter I: Description of True Devotion)

Charity as used above by St. Francis could also be thought of as love.  So another way of understanding the analogy is to say that our love for the Lord is a fire, and when that fire is burning hot, white hot, that love can be considered a devotion.