OcToBER 1, 1897. ] 
iridium and osmium. Thereis apparently 
room for a fourth group of three elements 
in this column, and it may be a fifth. And 
the discovery of such a group is not un- 
likely, for when this table was first drawn 
up Professor Mendeléeff drew attention to 
certain gaps, which have since been filled 
up by the discovery of gallium, germanium 
and others. 
The discovery of argon at once raised the 
curiosity of Lord Rayleigh and myself as to 
its position in this table. With a density 
of nearly 20, if a diatomic gas, like oxygen 
and nitrogen, it would follow fluorine in 
the periodic table; and our first idea was 
that argon was probably a mixture of three 
gases, all of which possessed nearly the 
same atomic weights, like iron, cobalt and 
nickel. Indeed, their names were sug- 
gested, on this supposition, with patriotic 
bias, as Anglium, Scotium and Hibernium ! 
But when the ratio of its specific heats had, 
at least in our opinion, unmistakably shown 
that it was molecularly monatomic, and 
not diatomic, as at first conjectured, it was 
necessary to believe that its atomic weight 
was 40, and not twenty, and that it followed 
chlorine in the atomic table and not fluo- 
rine. But here arises a difficulty. The 
atomic weight of chlorine is 35.5, and that — 
of potassium, the next element in order in 
the table, is 39.1; and that of argon, 40, 
follows, and does not precede, that of potas- 
sium, as it might be expected to do. It 
still remains possible that argon, instead of 
consisting wholly of monatomic molecules, 
may contain a small percentage of diatomic 
molecules; but the evidence in favor of 
this supposition is, in my opinion, far from 
strong. Another possibility is that argon, 
as at first conjectured, may consist of a 
mixture of more than one element; but, un- 
less the atomic weight of one of the ele- 
ments in the supposed mixture is very high, 
say 82, the case is not bettered, for one of 
the elements in the supposed trio would 
SCIENCE. 
495, 
still have a higher atomic weight than po- 
tassium. And very careful experiments, 
carried out by Dr. Norman Collie and my- 
self, on the fractional diffusion of argon, 
have disproved the existence of any such 
element with high atomic weight in argon, 
and, indeed, have practically demonstrated 
that argon is a simple substance and not a 
mixture. 
The discovery of helium has thrown a 
new light on this subject. Helium, it will 
be remembered, is evolved on heating cer- 
tain minerals, notably those containing 
uranium; although it appears to be con- 
tained in others in which uranium is not 
present, except in traces. Among these 
minerals are cléveite, monazite, fergusonite, 
and a host of similar complex mixtures, 
all containing rare elements, such as 
niobium, tantalum, yttrium, cerium, ete. 
The spectrum of helium is characterized by 
a remarkably brilliant yellow line, which 
had been observed as long ago as 1868 by 
Professors Frankland and Lockyer in the 
spectrum of the sun’s chromosphere, and 
named ‘ helium’ at that early date. 
The density of helium proved to be very 
close to 2.0, and, like argon, the ratio of its 
specific heat showed that it, too, was a 
monatomic gas. Its atomic weight, there- 
fore, is identical with its molecular weight, 
viz., 4.0, and its place in the periodic table 
is between hydrogen and lithium, the atomic 
weight of which is 7.0. 
The difference between the atomic weights 
of helium and argon is thus 36, or 40 — 4. 
Now there are several cases of such a dif- 
ference. For instance, in the group the 
first member of which is fluorine we have : 
Wltorine... 2.5... -ccsesese seen 19 
Chorin esazpyercjsrmserepelefereieia sieteiciele tale 35.5 ee 
Mamganese....-......sccccesseos 55 p 
In the oxygen group— 
Ob /FBNs so o5cgc0np000bG 50500000 16 16 
Swileeheit cs GonsogasaoggaDn005050 32 20.3 
Chrominmseperrrypeereiecessie noe om 
