1nCnVqgFI

ELEMENTS AND NON-ELEMENTS. 597
opinion among chemists had the respective solutions of these bodies and their
compounds presented identical colours, instead of colours which, approximatelyspeaking,
are mutually complementary. Would their distinct nature have even
now been recognized When we pass further and come to the so-called rare earths
the ground is less secure under our feet. Perhaps we may admit scandium, ytterbium,
and others of the Uke sort to elemental rank; but what are we to say in the
case of praseo- and neo-dymium, between which there may be said to exist no wellmarked
chemical diflference, their chief claim to separate individuality being slight
diflFerences in basicity and crystallizing powers, though their physical distinctions,
as shown by spectrum obsen^ations, are very strongly marked
Even here we mayimagine
the disposition of the majority of chemists would incline toward the side
of leniency, so that they would admit these two bodies within the charmed circle.
Whether in so doing they woiild be able to appeal to any broad principle is an open
question. If we admit these candidates how in justice are we to exclude the series
of elemental bodies or meta-elements made known to us by Kriiss and Nilson
Here the spectral differences are well marked, while my own researches on didymium
show also a slight difference in basicity between some at least of these
doubtful bodies.
In the same category must be included the numerous separate
bodies into which it is probable that yttrium, erbium, samarium, and other
"elements"—commonly so-called—have been and are being split up. Where then
are we to draw the line The different groupings shade off so imperceptibly the
.
one into the other that it is impossible to erect a definite boundary between any
two adjacent bodies and to say that the body on this side of the line is an element,
while the one on the other side is non-elementary, or merely something which
simulates or approximates to an element. Wherever an apparently reasonable line
might be drawn it would no doubt be easy at once to assign most bodies to their
proper side, as in all cases of classification the real difiiculty comes in when the
border-line is approached. Slight chemical differences, of course, are admitted,
and, up to a certain point, so are well-marked physical differences. What are we
to say, however, when the only chemical difference is an almost imperceptible
tendency for the one body—of a couple or of a group—to precipitate before the
other
Again, there are cases where the chemical differences reach the vanishing
point, although well-marked physical differences still remain. Here we stumble
on a new difiiculty: in such obscurities what is chemical and what is physical
Are we not entitled to call a slight tendency of a nascent amorphous precipitate
to fall down in advance of another a "physical difference" And may we not call
coloured reactions depending on the amount of some particular acid present and
varying, according to the concentration of the solution and to the solvent employed,
"chemical differences" I do not see how we can deny elementary character to a
body which differs from another by well-marked colour, or spectrum-reactions,
while we accord it to another body whose only claim is a very minute difference
in basic powers. Having once opened the door wide enough to admit some spectrum
differences, we have to inquire how minute a difference qualifies the candidate
to pass I will give instances from my own experience of some of these doubtful
candidates.