NITROGEN. THE ATMOSPHERE 297 



the latter contained a little of some heavier gas. In 1894 Ramsay, 

 in consultation with Rayleigh, succeeded in separating this gas by 

 passing the " nitrogen " repeatedly over heated magnesium, and 

 so removing the real nitrogen as solid magnesium nitride Mg 3 N 2 . 

 The remaining gas, about 1 per cent of the whole, was named 

 argon (Greek, lazy or inactive], because it would combine with no 

 other element. 



Argon has a molecular weight of 39.9 (nitrogen only 28), and 

 when liquefied boils at -186 and freezes at -189.5. It is used 

 in filling electric light bulbs. 



Helium. An indifferent gas, previously known to be given 

 off when uranium ores were heated in a vacuum, was found by 

 Ramsay (1895) to be neither nitrogen, nor yet argon. By its 

 spectrum it was recognized to be helium (Greek, the sun), a sub- 

 stance shown in 1868 to be present in the sun. Its molecular 

 weight is 4, so that it is only twice as dense as hydrogen. It was 

 the last gas to be liquefied (by Onnes), and the liquid boils at 

 268.7 (4.3 Abs.). Like argon, it does not enter into chemical 

 combination. Helium is now being used to fill balloons, because 

 it is not combustible. 



Other Inert Gases. When liquefied argon was allowed to 

 evaporate, the first vapor coming off was found to contain another 

 gas, neon (Greek, new; Mol. wt. 20), along with helium. Care- 

 ful distillation of the remaining liquid gave two other gases 

 krypton (Greek, hidden; Mol. wt. 83) and xenon (Greek, stranger; 

 Mol. wt. 130). The total amount of these four gases, however, 

 was only 1 part in 80, the remaining 79 parts being pure argon. 

 None of these gases form any compounds. They do not combine 

 with themselves even, as do the more common gases such as H2, 

 62, Cl2. The molecule in each case is monatomic, for example 

 He, A. The valence throughout the group is therefore zero. 



