"My dear fellow! How on earth are we to understand what the Martians
say, and how on Mars are they to understand what we say? We have no
common code."

"True; but the chemical bodies have certain well-defined properties,
have they not?"

"Yes. Each has a peculiarity marking it from all the rest. For example,
two or more may resemble each other in colour or hardness, but not in
weight."

"Precisely. Now, by comparing their spectra can we not be led to
distinguish a particular quality, and grasp the idea of it? In short,
can the Martians not impress that idea on us by their
spectro-telegraph?"

"I see what you mean," said Professor Gazen; "and, now I think of it,
all the spectra we have seen belong to the group called 'metals of the
alkalies and alkaline earths,' which, of course, have distinctive
properties."

"At first, I should think the Martians would only try to attract our
notice by striking spectra."

"Lithium is the lightest metal known to us."

"Well, we might get the idea of 'lightness' from that."

"Sodium," continued the professor, "sodium is a very soft metal, with so