, the composition of the chemical
compound. He had made it as strong as possible, and he had spared no
pains to insure a hot fire, so that the test would not be too simple.

"What's the matter, Tom?" asked Ned, as his chum appeared to hesitate
about giving the word that would send the chemicals hurtling down into
the fire.

"Nothing. I was just making sure I hadn't forgotten anything," Tom
answered. "I guess I haven't."

He paused a moment, looked up at his assistant on the overhanging arm
of the tower, glanced down at the flames, now at their height, and then
suddenly cried:

"Let her go!"

"Right!" came back the man's voice, and then a dark object, like a
bomb, was seen descending from the skeleton framework above the flames.

There was a scattering of the fire in the pit as the extinguisher bomb
fell among the blazing embers. Then followed a slight explosion when
the bomb broke, as it was intended it should.

Tom and Ned leaned forward to peer through the pall of smoke which
swirled this way and that. Here was to come the real test of the
device. Would the fumes of the liberated chemicals choke the fire, or
would it burn on in spite of them? That was the question to be settled
for Tom Swift.

Almost immediately he had his answer. For after a fierce burst of the
tongues of fire following the fall of the bomb, there was a distinct
dying down of the conflagration in the pit. Great clouds of smoke
arose, but the fire was quenched in a great measure, and as the
fire-blanketing gas continued to be generated from the chemicals
liberated from the bomb, there was a further dying down of the
crackling fire.

"Tom, you've struck it!" yelled Ned in delight. "You have the right
combination this time!"

Tom did not answer. He leaned forward and looked eagerly down into the
pit. He was about to join with Ned in agreeing that he had, indeed,
solved the problem, when, to his surprise, the flames started up again.

"What's this?" asked the young financial manager. "Are you going to
have a second test, Tom?"

"Not that I know of," was the puzzled answer. "I don't exactly
understand this myself, Ned. By all calculations this fire ought to
have died a natural death, but now it is breaking out again. I think
what must have happened is that a quantity of the oil they poured on
collected in a pool and didn't get all the effects of the chemicals
from the bomb. Then the oil started to blaze."

"What can you do about it?" Ned