"It may be electrical--for instance, an aurora. It may be a volcanic
eruption, or a lake of fire such as the crater of Kilauea. Really, I
can't say. Let me see if I can identify the bright lines of the
spectrum."

I yielded the spectroscope to him, and scarcely had he looked into it
ere he cried out--

"By all that's wonderful, the spectrum has changed. Eureka! It's
thallium now. I should know that splendid green line amongst a
thousand."

"Thallium!" I exclaimed, astonished in my turn.

"Yes," responded Gazen, hurriedly. "Make a note of the observation, and
also of the time. You will find a book for the purpose lying on the
desk."

I did as directed, and awaited further orders. The silence was so great
that I could plainly hear the ticking of my watch laid on the desk
before me. At the end of several minutes the professor cried--

"It has changed again: make another note."

"What is it now?"

"Sodium. The yellow bands are unmistakable."

A deep stillness reigned as before.