236 LORD RAYLEIGH AND PROFESSOR W. RAMSAY ON ARGON, 
cular weight is therefore 2. Argon is approximately 20 times as heavy as hydrogen, 
that is, its molecular weight is 20 times as great as that of hydrogen, or 40. But its 
molecule is monatomic, hence its atomic weight, or, if it be a mixture, the mean of 
the atomic weights of the elements in that mixture, taken for the proportion in which 
they are present, must be 40. 
This conclusion rests on the assumption that all the molecules of argon are mon- 
atomic. The result of the first experiment is, however, so nearly that required by 
theory, that there is room for only a small number of molecules of a different 
character. A study of the expansion of argon by heat is proposed, and would 
doubtless throw light upon this question. 
There is evidence both for and against the hypothesis that argon is a mixture: for, 
owing to Mr. Crookes’s observations of the dual character of its spectrum ; against, 
because of Professor OLszEwskI’s statement that it has a definite melting-point, a 
definite boiling-point, and a definite critical temperature and pressure ; and because 
on compressing the gas in presence of its liquid, pressure remains sensibly constant 
until all gas has condensed to liquid. The latter experiments are the well-known 
criteria of a pure substance ; the former is not known with certainty to be character- 
istic of a mixture. The conclusions which follow are, however, so startling, that in 
our future experimental work we shall endeavour to decide the question by other 
means. 
For the present, however, the balance of evidence seems to point to simplicity. 
We have, therefore, to discuss the relations to other elements of an element of atomic 
weight 40. We inclined for long to the view that argon was possibly one, or more 
than one, of the elements which might be expected to follow fluorine in the periodic 
classification of the elements—elements which should have an atomic weight between 
19, that of fluorine, and 28, that of sodium. But this view is apparently put out of 
court by the discovery of the monatomic nature of its molecules. 
The series of elements possessing atomic weights near 40 are :— 
C@hlovincee ieee ere oo 
IROLASSIUM sy ye eee Ol! 
Calcium: eee eee OO) 
eEniebibt, Gag Ey on la A) Zuo), 
There can be no doubt that potassium, calcium, and scandium follow legitimately 
their predecessors in the vertical columns, lithium, beryllium, and boron, and that 
they are in almost certain relation with rubidium, strontium, and (but not so 
certainly) yttrium. If argon be a single element, then there is reason to doubt 
whether the periodic classification of the elements is complete; whether, in fact, 
elements may not exist which cannot be fitted among those of which it is composed. 
On the other hand, if argon be a mixture of two elements, they might find place in 
the eighth group, one after chlorine and one after bromine. Assuming 37 (the 
