e tested
sample as was used to make the well-known sample. The color of the
unknown sample is then compared with the known sample, and the quantity
determined by the difference of color in the two."

"What do you mean by reagent?"

"A substance used to effect a chemical change in another substance. For
instance, what is called Nessler's Reagent is a substance which, if put
into water, will detect one part of ammonia in twenty million parts of
water, and give a perceptible reddish-yellow tinge."

"Well, Red Angel has certainly made a good test sample for us; isn't
that a good color?"

The blue color was still wanting when the other parts were ready, and
the Professor came to the laboratory with a sample of bitter-sweet, the
common hedge plant of North America. The boys both recognized the plant
and were surprised to learn that it contained a dye suitable for their
purpose.

"We can use this or take some of the copper ore, that is, the blue
vitriol part of the ore, and by putting it in a lime-water solution a
beautiful blue color can be obtained."

The flag, when completed, was really a work of art. They gave many days
to the task, and were proud of it. The question of a suitable pole or
flagstaff was one which now absorbed their energies. As nothing of the
kind was found in the immediate neighborhood, it was suggested that on
their regular hunting day it should be made a part of their duty to find
a staff worthy to be installed. The trips to the cave, and the absorbing
work of preparing the flag, had so taken up the time, that they had
entirely forgotten the regular outings.

On this occasion they insisted that the Professor should accompany them,
and George declared that the only place available for a good pole would
be in the forest below the South River, where they had shot the ocelots.

The yaks were prepared, as usual, and it was a merry party which started
off on that bright morning for the forest. They did not, you may be
sure, forget the spears and the guns, and before leaving home Harry
thought it would be a good idea to provide a small two-wheeled truck,
which could be used as a trailer, for the pole.

On the way down, Harry said: "How large a pole shall we get?"

George had his opinions, as usual: "Let us get a pole at least 50 feet
long. We must have something which will match the flag."

"A pole that length will be a difficult matter to raise; have you
thought of that?" was the Professor's obser