The object in Nick's mouth was a "choke pear!"

This vicious instrument of torture dates back to the time of Palioly,
the notorious French robber and renegade, when it was very worthily
called "the pear of anguish."

It consists of a solid gag, so to speak, yet it is so constructed, with
interior springs, that, once thrust into a person's mouth, it expands as
fast as the mouth is opened, and rigidly distends the victim's jaws.

The more widely the victim gapes to eject the "choke pear," or to cry
out for aid, the larger the hideous object becomes, until torture,
suffocation and death speedily ensue.

Had this infernal device been generally available to modern criminals,
Nick would have been warned by the significant words he had heard, and
would have guarded himself against it.

As it was, however, he had been caught; and in the mouth of any ordinary
man the "choke pear" would have been irresistible.

But the muscles of Nick Carter's jaws were like fibers of steel, and the
instant he realized his situation he opened his mouth no wider. Instead,
while hands and arms were still engaged in the furious conflict with his
assailants, he brought his jaws together as if with superhuman power,
and with a force that crushed the infernal device between them, much as
if it had been little more than an eggshell.

One of the ruffians heard the snapping crunch, and uttered a cry of
amazement.