ything that filled my imagination with so many scientific visions as
these little harmless test-tubes at the back of Professor Ramsay's desk.
Perhaps I shall attempt to visualize some of these imaginings before
finishing this paper, but for the moment I wish to speak of the _modus
operandi_ of the discovery of these additions to the list of elements.

The discovery of argon came about in a rather singular way. Lord
Rayleigh, of the Royal Institution, had noticed in experiments with
nitrogen that when samples of this element were obtained from chemicals,
such samples were uniformly about one per cent, lighter in weight
than similar quantities of nitrogen obtained from the atmosphere.
This discrepancy led him to believe that the atmospheric nitrogen must
contain some impurity.

Curiously enough, the experiments of Cavendish, the discoverer of
nitrogen--experiments made more than a century ago--had seemed to show
quite conclusively that some gaseous substance different from nitrogen
was to be found mixed with the samples of this gas as he obtained it
from the atmosphere. This conclusion of Cavendish, put forward indeed
but tentatively, had been quite ignored by his successors. Now,
however, it transpired, by experiments made jointly by Lord Rayleigh
and Professor Ramsay, that the conclusion was quite justified, it being
shown presently that there actually exists in every portion of nitrogen,
as extracted from the atmosphere, a certain quantity of another gas,
hitherto unknown, and which now received the name of argon. It will
be recalled with what astonishment the scientific and the unscientific
world alike received the announcement made to the Royal Society in 1895
of the discovery of argon, and the proof that this hitherto unsuspected
constituent of the atmosphere really constitutes about one per cent, of
the bulk of atmospheric nitrogen, as previously estimated.

The discovery here on the earth of a substance which Professor Lockyer
had detected as early as 1868 in the sun, and which he had provisionally
named helium, excited almost equal interest; but this element was found
in certain minerals, and not as a constituent of the atmosphere.

Having discovered so interesting a substance as argon, Professor
Ramsay and his assistants naturally devoted much time and attention to
elucidating the peculiarities of the new substance. In the course of
these studies it became evident to them that the presence of argon alone
did