OcTOBER 1, 1897.] 
small quantity of a heavy gas mixed with 
a large quantity of the light gas. Repeated 
re-diffusion convinced us that there was 
only a very small amount of the heavy gas 
present in the mixture. The portion which 
contained the largest amount of heavy gas 
was found to have the density 2.275, and 
its refractive index was found to be 0.1333. 
On re-diffusing this portion of gas until 
only a trace sufficient to fill a Plucker’s 
tube was left, and then examining the 
spectrum, no unknown lines could be de- 
tected, but, on interposing a jar and spark 
gap, the well-known blue lines of argon be- 
eame visible; and even without the jar the 
red lines of argon and the two green 
groups were distinctly visible. Theamount 
of argon present, calculated from the den- 
sity, was 1.64 per cent., and from the re- 
fractivity 1.14 per cent. The conclusion 
had therefore to be drawn that the heavy 
constituent of helium, as it comes off the 
minerals containing it, is nothing new, but, 
so far as can be made out, merely a small 
amount of argon. 
If, then, there is a new gas in what is 
generally termed helium, it is mixed with 
argon, and it must be present in extremely 
minute traces. As neither helium nor argon 
has been induced to form compounds, there 
does not appear to be any method, other 
than diffusion, for isolating such a gas, if it 
exists, and that method has failed in our 
hands to give any evidence of the existence 
of such a gas. It by no means follows that 
the gas does not exist; the only conclusion 
to be drawn is that we have not yet stum- 
bled on the material which contains it. In 
fact, the haystack is too large and the 
needle too inconspicuous. Reference to the 
periodic table will show that between the 
elements aluminium and indium there oc- 
curs gallium, a substance occurring only in 
the minutest amount on the earth’s surface; 
and following silicon, and preceding tin, 
appears the element germanium, a body 
SCIENCE. 
499 
which has as yet been recognized only in 
one of the rarest of minerals, argyrodite. 
Now, the amount of helium in fergusonite, 
one of the minerals which yields it in rea- 
sonable quantity, is only 33 parts by weight 
to 100,000 of the mineral; and it is not 
improbable that some other mineral may 
contain the new gas in even more minute 
proportion. If, however, it is accompanied 
in its still undiscovered source by argon 
and helium, it will be a work of extreme 
difficulty to effect a separation from these 
gases. 
In these remarks it has been assumed 
that the new gas will resemble argon and 
helium in being indifferent to the action of 
reagents, and in not forming compounds. 
This supposition is worth examining. In 
considering it the analogy with other ele- 
ments is all that we have to guide us. 
We have already paid some attention to 
several triads of elements. We have seen 
that the differences in atomic weights be- 
tween the elements fluorine and manga- 
nese, oxygen and chromium, nitrogen and 
vanadium, carbon and titanium, is in each 
case approximately the same as that be- 
tween helium and argon, viz., 36. If ele- 
ments further back in the periodic table be 
examined, it is to be noticed that the differ- 
ences grow less, the smaller the atomic 
weights. Thus, between boron and scan- 
dium, the difference is 33; between bery]l- 
lium (glucinum) and calcium, 31; and be- 
tween lithium and potassium, 32. At the 
same time, we may remark that the elements 
grow liker each other the lower the atom- 
ic weights. Now helium and argon are 
very like each other in physical proper- 
ties. It may be fairly concluded, I think, 
that in so far they justify their position. 
Moreover, the pair of elements which show 
the smallest difference between their atomic 
weights is beryllium and calcium; there is 
somewhat greater difference between lithium 
and potassium, And, it is in accordance: 
