re wonderful than the fire
fly," said George.

"But what do you think makes it so wonderful?" asked John.

"Well, I suppose the wonderful part is that it has strength enough to
make a light," answered Harry.

"No, the remarkable thing is that the light which it emits is absolutely
cool. Experiments which have been made go to show that there is no heat.
In every form of light which man has been able to produce thus far
artificially, a great heat is evolved, and it would be a most valuable
discovery to find out why these insects are able to do it without
raising the temperature."

[Illustration: _Fig. 3. The Fire-fly._]

"But what difference does it make if heat is produced?" asked Harry.

"The production of heat means the loss of power. The heat generated
takes up more of the power than the light which is produced, so that it
would be a great economy if the heat could be dispensed with."

"But if there was no heat in the light produced would it make any
difference in the lamps themselves?"

"Unquestionably. The lamps would last much longer."

"What are the things which must be learned in order to get the secret of
cold light?"

"Well, there are number of questions which must be determined. While it
is known that the fire fly and the glow worm emit what is called a
phosphorescent light, this fact is a mere prelude to the knowledge of
what is the exact color of daylight."

"Color of Daylight? Why, I supposed it was white."

"But the light of the glow worm and fire fly are not white."

They watched them, and soon appreciated that John's statement was true.

"You asked what were some of the things to be solved? Well, to find out
the secret of the phosphorescent glow. That is one thing. What is the
best artificial light, is the next. Then, what substance will have the
most intense glow when a current passes through it, and give out the
least heat."

"Well, has no one attempted to explain any of these things?"

"Yes; many explanations have been offered, but all of them leave the
subject dark somewhere." And John laughed as he saw that the boys
appreciated his little attempt at witticism.

"But the time will come when man will find out this, as everything in
his way. When you think of it, that electricians, chemists,
metallurgists, physiologists, engineers, physicists and microscopists,
are all working on the problem, we should be able to extract the secret
sooner or later."

"I am going to have some of t