I believe no happiness can be found worthy to be compared with that of a soul in Purgatory except that of the saints in Paradise; and day by day this happiness grows as God flows into these souls, more and more as the hindrance to His entrance is consumed. Sin's rust is the hindrance, and the fire burns the rust away so that more and more the soul opens itself up to the divine inflowing. A thing which is covered cannot respond to the sun's rays, not because of any defect in the sun, which is shining all the time, but because the cover is an obstacle; if the cover be burnt away, this thing is open to the sun; more and more as the cover is consumed does it respond to the rays of the sun
It is in this way that rust, which is sin, covers souls, and in Purgatory is burnt away by fire; the more it is consumed, the more do the souls respond to God, the true sun. As the rust lessens and the soul is opened up to the divine ray, happiness grows; until the time be accomplished the one wanes and the other waxes. Pain however does not lessen but only the time for which pain is endured. As for will: never can the souls say these pains are pains, so contented are they with God's ordaining with which, in pure charity, their will is united.
But, on the other hand, they endure a pain so extreme that no tongue can be found to tell it, nor could the mind understand its least pang if God by special grace did not shew so much. Which least pang God of His grace shewed to this Soul, but with her tongue she cannot say what it is. This sight which the Lord revealed to me has never since left my mind and I will tell what I can of it. They will understand whose mind God deigns to open.
Bl. John Henry Newman, Dream of Gerontius, which, of course, is a poem about Purgatory:
Take me away, and in the lowest deep
There let me be,
And there in hope the lone night-watches keep,
Told out for me.
There, motionless and happy in my pain,
Lone, not forlorn,—
There will I sing my sad perpetual strain,
Until the morn.
There will I sing, and soothe my stricken breast,
Which ne'er can cease
To throb, and pine, and languish, till possest
Of its Sole Peace.
There will I sing my absent Lord and Love:—
Take me away,
That sooner I may rise, and go above,
And see Him in the truth of everlasting day.
I did some brief and inadequate comparison of Catherine and Newman on this subject last year.