536 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
we know that they exist in all subterranean waters and gases.^ Fur- 
thermore, helium can be derived from a series of rare minerals ; this 
observation was of great interest in what followed, after it was dis- 
covered that the same gas was given off in the disintegration of 
radium, as we shall see later on. 
Their resistance to any combination assigned to argon and helium 
a place apart among the elements, and they did not fit in any of the 
groups of Mendeleeff's table. Ramsay boldly suggested that they con- 
stituted the first two known terms of a new group, characterized by a 
valence of zero. Secure in observed analogies in the other groups of 
the periodic system, Ramsay, in a communication to the meeting of the 
British Association in Toronto in 1897 with the suggestive title, 
"An Undiscovered Gas," predicted the existence of at least one other 
inert element, situated between helium and argon, near fluorine and 
having an atomic weight not far from 20. 
Before another year had passed, not only had Ramsay's prediction 
been realized, but more, in collaboration with Morris Travers, two 
other elementary inert gases had been discovered, whose places he 
also fixed in the periodic system, near bromine and iodine, with the 
neighboring atomic weights of 82 and 130. 
Ramsay submitted to a close examination different thermal waters, 
such as those of minerals and of meteorites, without being able to 
discover any of the gases which he sought. Their presence in all the 
subterranean gases was to be demonstrated later,^ thanks to the use 
of a method of fractionating by means of cooled charcoal inaugurated 
by Sir James Dewar.* 
But if the three gases to be discovered really existed, ought they 
not to be found in considerable proportion in the atmospheric nitro- 
gen along with argon ? 
One hundred cubic centimeters of liquid air having been reduced 
through spontaneous evaporation to several cubic centimeters, Ram- 
say vaporized them in a gasometer, then eliminated from it the 
oxygen and nitrogen by appropriate means. The gaseous residue 
thus prepared furnished the spectrum of argon with, in addition, a 
yellow line and a very brilliant green line. Besides, the density was 
a little greater than that of pure argon; the residue examined was 
then argon mixed with a certain proportion of a heavier gas. 
In order to isolate this gas, Ramsay aided by Travers, prepared 15 
liters of argon, a task requiring several months, and liquified it by 
2 Charles Moureu, " Recherehes sur les gaz rares des sources thermales ; leurs enseigne- 
ments concernant la radioactivity et la physique du globe," Journal do Chimie 
physique, t. 11, no. 1, p. 63-152, 1913. Charles Moureu and Adolph Lepape, " Les gaz 
rares des Grisous," Annales de Chimie, 9" s., t. 4 et 5, 1915-1916. 
' Charles Moureu and Adolphe Lepape, loc. oit. 
* Separation directe, sans liquefaction, des gaz les plus volatils de I'air (Ann. Chim. 
Phys. 8« serie, t. 3, p. 12 ; 1904. 
