S.L. chp. 14

#48

It seemed to be his wish and purpose to mask this expression with a smile; but the latter played him false, and flickered over his visage so derisively, that the spectator could see his blackness all the better for it. Ever and anon, too, there came a glare of red light out of his eyes; as if the old man's soul were on fire, and kept on smouldering duskily within his breast, until, by some casual puff of passion, it was blown into a momentary flame.

Chilingsworth is on his way out. He can hardly hide his satanic influences and this goes to show that too much of a "good" thing can be bad. He forces himself to be fake to hide from society since he'd pretty much be burned at the stake if they knew what he was really experiencing.

The use of two colors, black and red, give I guess an opening view of the spectrum of this society. White is pure, red is considered touched by Satan, and black means you have no hope of escaping his wrath. The glare of red light shines through Roger's eyes because the eyes are the window to the soul. Since he the man's soul is compared to a fire, his blackness could be similar to that of ashes.

A casual puff of passion is referring to his new found passion of destroying his enemies with small discrete strokes.