The Black Gang by H.C. McNeile

Some ten minutes later he emerged from the bathroom carefully carrying a saucer in his hand. The girl's announcement that Number 13 had started at once had been received with a satisfied grunt, but he had spoken no word. And the girl, glancing through the door, saw him with his shirt sleeves rolled up above his elbows, carefully mixing two liquids together and stirring the result gently with a glass rod. He was completely absorbed in his task, and with a faint smile on her face she went back to the sofa and waited. She knew too well the futility of speaking to him on such occasions. Even when he came in, carrying the result of his labours with a pair of india-rubber gloves on his hands, she made no remark, but waited for him to relieve her curiosity.

He placed the mixture on the table and glanced round the room. Then he pulled up one of the ordinary stuff armchairs to the table and removed the linen headrest, which he carefully soaked with the contents of the saucer, dabbing the liquid on with a sponge, so as not to crumple the linen in any way. He used up all the liquid, and then, still with the same meticulous care, he replaced the headrest on the chair, and stood back and surveyed his handiwork.