ON ARC40N AND ITS COMPANIONS. 
83 
krypton, and xenon, yet many determinations by the Kimdt method were carried out 
with the approximately pure gases. It is hardly necessary to point out that if the 
theoretical ratio of 1'66 is attained with a mixture of monatomic gases, it applies also 
to the constituents of that mixture. This reasoning supports the above assertion. 
(2.) Supposing that the atomic weights, as in the case of oxygen, nitrogen, &c., be 
taken as identical with the numbers expressing density, there is no place for these 
elements in the periodic table. This argument, of course, does not necessarily 
apply to helium, for it occupies the gap between hydrogen (I) and lithium (7). 
But neon would occupy a place between beryllium (9) and boron (11) ; argon 
between fluorine (19) and sodium (23), taking the place we have assigned to 
neon ; krypton between calcium (40) and scandium (44) ; and xenon, between 
copper (63‘6) and zinc (65’4). But if it be granted that they are monatomic, they 
form a group by themselves. Their atomic weights would then be :— 
Helium. Neon. Argon. Krypton. Xenon. 
4 20 40 82 128 
The group of elements, the atomic weights of which come next in order, is :— 
Hydrogen. 
Helium. 
Lithium. 
Berj'llium. 
1 
4 
7 
9 
Fluorine. 
Neon. 
Sodium. Magnesium. 
19 
20 
23 
24 
Chlorine. 
Argon. 
Potassium. Calcium. 
35-5 
40 
39 
40 
Bromine. 
Krypton. 
Rubidium. Strontium. 
80 
82 
85 
87 
Iodine. 
Xenon. 
Cffisium. 
Barium. 
127 
128 
133 
137 
It may appear remarkable that hydrogen is placed as the first member of tbe 
chlorine-group ; but arguments may be adduced in favour of this position, which 
need not be here recapitulated, and which have been well summed-up by Professor 
Orme Masson (‘ Chem. News,’ vol. 73, 1896, p. 283). It is also to be noticed that 
the group ends here; for no elements of the fluorine, sodium, and magnesium series 
with higher atomic weights than iodine, caesium and barium have been discovered. 
We have no absolute proof that “xenon” does not contain some element of still 
higher weight ; the same could have been said of argon at the time of publication of 
the ‘ Phil. Trans.’ memoir ; but it may be confidently stated that if xenon contains 
elements of higher atomic weight, they must be present in an extremely minute 
amount; inasmuch a§ the pressure does not rise during the process of its condensation 
M 2 
