The force of the blast, as the compressed oxygen and acetylene were expelled, carried a fine spray of the disintegrated metal visibly before it.  And yet it was not a big hole that it made—­ scarcely an eighth of an inch wide, but clean and sharp as if a buzz-saw were eating its way through a plank of white-pine.

With tense muscles Kennedy held this terrific engine of destruction and moved it as easily as if it had been a mere pencil of light.  He was the calmest of all of us as we crowded about him, but at a respectful distance.

“I suppose you know,” he remarked hastily, never pausing for a moment in his work, “that acetylene is composed of carbon and hydrogen.  As it burns at the end of the nozzle it is broken into carbon and hydrogen—­the carbon gives the high temperature and the hydrogen forms a cone that protects the end of the blow-pipe from being itself burnt up.”

“But isn’t it dangerous?” I asked, amazed at the skill with which he handled the blow-pipe.

“Not particularly—­when you know how to do it.  In that tank is a porous asbestos packing saturated with acetone, under pressure.  Thus they carry acetylene safely, for it is dissolved and the possibility of explosion is minimized.

“This mixing chamber, by which I am holding the torch, where the oxygen and acetylene mix, is also designed in such a way as to prevent a flash-back.  The best thing about this style of blow-pipe is the ease with which it can be transported and the curious purposes—­like this—­to which it can be put.”

He paused a moment to test what had been burnt.  The rest of the safe seemed as firm as ever.

“Humph!” I heard one of them, I think it was Alfonso, mutter.  I resented it, but Kennedy affected not to hear.

“When I shut off the oxygen in this second jet,” he resumed, “you see the torch merely heats the steel.  I can get a heat of approximately sixty-three hundred degrees Fahrenheit, and the flame will exert a pressure of fifty pounds to the square inch.”

“Wonderful!” exclaimed Lockwood, who had not heard the suppressed disapproval of Alfonso, and was watching, in undisguised admiration at the thing itself, regardless of consequences.  “Kennedy, how did you ever think of such a thing?”

“Why, it’s used for welding, you know,” answered Craig, as he continued to work calmly in the growing excitement.  “I first saw it in actual use in mending a cracked cylinder in an automobile.  The cylinder was repaired without being taken out at all.  I’ve seen it weld new teeth and build up worn teeth on gearing, as good as new.”

He paused to let us see the terrifically heated metal under the flame.

“You remember when we were talking to the watchman down there at the station, Walter?” he asked.  “I saw this thing in that complete little shop of theirs.  It interested me.  See.  I turn on the oxygen now in the second nozzle.  The blow-pipe is no longer an instrument for joining metals together, but for cutting them asunder.