has a peculiarity shared
only by kalon, that _x_ and _y_ are asymmetrical, the centre of one having
three atoms and the centre of the other two. Is this done in order to
preserve the difference of seven from its comrade?

XENON: Six arms of 363 atoms 2178
Central tetrahedra 120
-----
Total 2298
-----
Atomic weight 127.10
Number weight 2298/18 127.66
META-XENON differs from xenon only by the substitution of two _z_'s for _x_
and _y_.

META-XENON: Six arms of 370 atoms 2220
Central tetrahedra 120
-----
Total 2340
-----
Atomic weight -----
Number weight 2340/18 130
KALON (Plate XXI, 3 and 4, and Plate XX, 6 and 7) has a curious cone,
possessing a kind of tail which we have not observed elsewhere; _x_ and _y_
show the same asymmetry as in xenon.

KALON: Six arms of 489 atoms 2934
Central tetrahedra 120
----
Total 3054
----
Atomic weight ----
Number weight 3054/18 169.66
META-KALON again substitutes two _z_'s for _x_ and _y_.

META-KALON: Six arms of 496 atoms 2976
Central tetrahedra 120
----
Total 3096
----
Atomic weight ----
Number weight 3096/18 172
Only a few atoms of kalon and meta-kalon have been found in the air of a
fair-sized room.

It does not seem worth while to break up these elements, for their
component parts are so familiar. The complicated groups--_a_ 110, _b_ 63
and _c_ 120--have all been fully dealt with in preceding pages.

* * * * *

There remains now only radium, of the elements which we have, so far,
examined, and that will be now described and will bring to an end this
series of observations. A piece of close and detailed work of this kind,
although neces