496 
In the nitrogen group— 
INTO UgNononpOODD DOO OooR>oou 14 47 
Phosphorus ...-.+--s2essecceeeeee 31 20.4 
MeEyEXsbwbNs Ge ogg BoooodKbDb0bGKK 51.4 
And in the carbon group— 
Carbon... 2... e eee weenie e ens 12 463 
Siliconeere enteritis r tt 28.3 19.8 
TE ANUMN 6osonomoobocacKddoDCNE 48.1 
These instances suffice to show that ap- 
proximately the differences are 16 and 20 
between. consecutive members of the corre- 
sponding groups of elements. The total 
differences between the extreme members 
of the short series mentioned are— 
Manganese-—Fluorine.................. 36 
Chromiura—Oxygen................. 36.3 
Vanadium—Nitrogen .... .-........ 37.4 
Titanium—Carbon .................. 36.1 
This is approximately the difference be- 
tween the atomic weights of helium and 
argon, 36. 
There should, therefore, be an undis- 
covered element between helium and argon, 
with an atomic weight 16 units higher than 
that of helium, and 20 units lower than 
that of argon, namely 20. And if this un- 
known element, like helium and argon, 
should prove to consist of monatomic mole- 
cules, then its density should be half its 
atomic weight, 10. And pushing the 
analogy still farther, it is to be expected 
that this element should be as indifferent to 
union with other elements as the two allied 
elements. 
My assistant, Mr. Morris Travers, has 
indefatigably aided me in a search for this 
unknown gas. There is a proverb about 
looking for a needle in a haystack ; modern 
science, with the aid of suitable magnetic 
appliances, would, if the reward were sufii- 
cient, make short work of that proverbial 
needle. But here isa supposed unknown 
gas, endowed, no doubt, with negative prop- 
erties and the whole world to find it in. 
Still, the attempt had to be made. 
We first directed our attention to the 
SCIENCE. 
[N.S. Vou. VI. No. 144. 
sources of helium—minerals. Almost every 
mineral which we could obtain was heated 
in @ vacuum, and the gas which was 
evolved examined. The results are inter- 
esting. Most minerals give off gas when 
heated, and the gas contains, as a rule, a 
considerable amount of hydrogen, mixed 
with carbonic acid, questionable traces of 
nitrogen, and carbonic oxide. Many of the 
minerals, in addition, gave helium, which 
proved to be widely distributed, though 
only in minute proportion. One mineral— 
malacone—gave appreciable quantities of 
argon ; and it is noteworthy that argon was 
not found except in it (and, curiously, in 
much larger amount than helium), and in 
a specimen of meteoric iron. Other speci- 
mens of meteoric iron were examined, but 
were found to contain mainly hydrogen, 
with no trace of either argon or helium. 
It is probable that the sources of meteorites 
might be traced in this manner, and that 
each could be relegated to its particular 
swarm. 
Among the minerals examined was one 
to which our attention had been directed 
by Professor Lockyer, named eliasite, from 
which he said that he had extracted a gas 
in which he had observed spectrum lines 
foreign to helium. He was kind enough to 
furnish us with a specimen of this mineral, 
which is exceedingly rare, but the sample 
which we tested contained nothing but un- 
doubted helium. 
During a trip to Iceland, in 1895, I eol- 
lected some gas from the boiling springs 
there ; it consisted, for the most part, of 
air, but contained somewhat more argon 
than is usually dissolved when air is shaken 
with water. In the spring of 1896 Mr. 
Travers and I made a trip to the Pyrenees 
to collect gas from the mineral springs of 
Cauterets, to which our attention had been 
directed by Dr. Bouchard, who pointed out 
that these gases are rich in helium. We 
examined a number of samples from the 
