a sort of
metallic combination that is a peculiar conductor of electricity.
The more light that shines on it the better conductor it is, and
the less light, the poorer."

"It must be queer stuff," said Ned.

"It is," declared Tom. "I think it is the only thing to use in
this photo telephone experiment, though I might try the metal
plate method, as they did between Monte Carlo and Paris. But I am
not trying to make newspaper pictures."

"What is selenium, anyhow?" asked Ned. "Remember, Tom, I'm not up
on this scientific stuff as you are."

"Selenium," went on Tom, "was discovered in 1817, by J. J.
Berzelius, and he gave it that name from the Greek word for moon,
on account of selenium being so similar, in some ways, to
tellurium. That last is named after the Latin word tellus, the
earth."

"Do they dig it?" Ned wanted to know.

"Well, sometimes selenium is found in combination with metals, in
the form of selenides, the more important minerals of that kind
being eucharite, crooksite, clausthalite, naumannite and zorgite--"

"Good night!" interrupted Ned, with a laugh, holding up his hands.
"Stop it, Tom!" he pleaded. "You'll give me a headache with all
those big words."

"Oh, they're easy, once you get used to them," said the young
inventor, with a smile. "Perhaps it will be easier if I say that
sometimes selenium is found in native sulphur. Selenium is usually
obtained from the flue-dust or chamber deposits of some factory
where sulphuric acid is made. They take this dust and treat it
with acids until they get the pure selenium. Sometimes selenium
comes in crystal forms, and again it is combined with various
metals for different uses."

"There's one good thing about it. There are several varieties, and
I'll try them all before I give up."

"That's the way to talk!" cried Ned. "Never say die! Don't give up
the ship, and all that. But, Tom, what you need now is a little
fun. You've been poking away at this too long. Come on out on the
lake, and have a ride in the motor boat. It will do you good. It
will do me good. I'm a bit rusty myself--been working hard lately.
Come on--let's go out on the lake."

"I believe I will!" exclaimed Tom, after thinking it over for a
moment. "I need a little fresh air. Sitting in that telephone
booth, trying to get an image on a plate, and not succeeding, has
gotten on my nerves. I want to write out an order for Koku to take
to town, though. I want to get some fresh selenium,