OcTOBER 1, 1897. ] 
various springs, and confirmed Dr. Bou- 
chard’s results, but there was no sign of 
any unknown lines in the spectrum of these 
gases. Our quest was in vain. 
We must now turn to another aspect of 
the subject. Shortly after the discovery of 
helium its spectrum was very carefully ex- 
amined by Professors Runge and Paschen, 
the renowned spectroscopists. The spec- 
trum was photographed, special attention 
being paid to the invisible portions, termed 
the ‘ultra-violet’ and ‘infra-red.’ The 
lines thus registered were found to have a 
harmonic relation to each other. They ad- 
mitted of division into two sets, each com- 
plete in itself. Now, a similar process had 
been applied to the spectrum of lithium and 
to that of sodium, and the spectra of these 
elements gave only one series each. Hence, 
Professors Runge and Paschen concluded 
that the gas, to which the provisional name 
of helium had been given, was, in reality, 
a mixture of two gases, closely resembling 
each other in properties. As we know no 
other elements with atomic weights between 
those of hydrogen and lithium, there is no 
chemical evidence either for or against this 
supposition. Professor Runge supposed 
that he had obtained evidence of the sepa- 
ration of these imagined elements from each 
other by means of diffusion; but Mr. 
Travers and I pointed out that the same 
alteration of spectrum, which was appar- 
ently produced by diffusion, could also be 
caused by altering the pressure of the gas 
in the vacuum tube; and shortly after Pro- 
fessor Runge acknowledged his mistake. 
These considerations, however, made it 
desirable to subject helium to systematic 
diffusion, in the same way as argon had 
been tried. The experiments were carried 
out in the summer of 1896, by Dr. Collie 
and myself. The result was encouraging. 
It was found possible to separate helium 
into two’ portions of different rates of dif- 
fusion, and consequently of different 
SCIENCE. 
497 
density by this means. The limits of sep- 
aration, however, were not very great. 
On the one hand, we obtained gas of a 
density close on 2.0; and on the other, a 
sample of density 2.4 or thereabouts. The 
difficulty was increased by the curious be- 
havior, which we have often had occasion 
to confirm, that helium possesses a rate of 
diffusion too rapid for its density. Thus, 
the density of the lightest portion of the 
diffused gas, calculated from its rate of dif- 
fusion, was 1.874 ; but this corresponds to 
a real density of about 2.0. After our 
paper, giving an account of these experi- 
ments, had been published, a German in- 
vestigator, Herr A. Hagenbach, repeated 
our work and confirmed our results. 
The two samples of gas of different den- 
sity differ also in other properties. Differ- 
ent transparent substances differ in the rate 
at which they allow light to pass through 
them. Thus, light travels through water 
at a much slower rate than through air, 
and at a slower rate through air than 
through hydrogen. Now Lord Rayleigh 
found that helium offers less opposition to 
the passage of light than any other sub- 
stance does, and the heavier of the two por- 
tions into which helium had been split 
offered more opposition than the lighter 
portion. And the retardation of the light, 
unlike what has usually been observed, was 
nearly proportional to the densities of the 
samples. The spectrum of these two sam- 
ples did not differ in the minutest particu- 
lar ; therefore it did not appear quite out of 
the question to hazard the speculation that 
the process of diffusion was instrumental, 
not necessarily in separating two kinds of 
gas from each other, but actually in remov- 
ing light molecules of the same kind from 
heavy molecules. This idea is not new. 
It had been advanced by Prof. Schttzen- 
berger (whose recent death all chemists 
have to deplore), and later, by Mr. Crookes, 
that what we term the atomic weight of an 
