86 OUTLINES OF PHYSICAL CHEMISTRY
is, however, no less true that some of the groups in this table have a certain artificiality about them. The strange positions of copper and of gold in the first vertical column and of thallium in the third are self-evident. And the positions of chromium in the sulphur group and man- ganese in the chlorine group can hardly be justified by the analogous composition and isomorphism of certain of their salts (chromates and sulphates, permanganates and perchlorates).
Tellurium falls into its proper place if we assume that its atomic weight is 125 ; but the latest determination of this points to the value 127*6, which places tellurium after iodine (126*5) in the table — a position extremely difficult to explain. Again, the atomic weights of some other elements — particularly beryllium, lanthanum, and cerium — are more or less doubtful, and their position in Mendelteff's table is a much-discussed question. Lastly, it is to be noted that there are no missing members of the first three periods, and the positions of argon l (atomic weight 40 ?) and of helium (atomic weight 4 ?) are very problematic.
��1 As well as metargon, krypton, and neon.
Note by Translator. — In a paper read before the Royal Society (June 1898) Crookes proposes an arrangement of the elements on a descending figure-of-eight spiral. In this arrangement the elements helium, neon, argon, and krypton are placed at the points where ' the lemniscate track crosses the neutral line.'
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