Glass! He had cut his hand.

The beggars had covered the trap with cement and broken glass, sealing it.

It would take time to cut round the trap; and even then he wouldn't be sure; they might have nailed it down from the inside.

The worst of it was he would have to do the work himself; and in the meantime Karlov would have a fair wind for his propaganda gas, and perhaps the disposal of the drums to some collector who wasn't above bargaining for smuggled emeralds.
Odd, though, that Karlov should have made a prisoner of Coles.

What lay behind that manoeuvre?
Well, this trap must be liberated; no getting round that.
Hang it, he wasn't going to be dishonest exactly; it would be simply a double play, half for Uncle Sam and half for himself.