586 



POPULAR SCIENCE MONTHLY. 



Fig. 1. 



as nitrogen. Two methods were adopted for isolating this gas. One 

 was a repetition of a process which had been employed by Cavendish in 



1785; it consisted in passing 

 electric sparks for a long time 

 through air, confined over mer- 

 cury, in presence of caustic 

 alkali. The accompanjdng 

 woodcut gives an idea of the 

 apparatus he employed. This 

 experiment had been made by 

 Priestley ten years previously, 

 but not with quantitative ac- 

 curacy. It was Cavendish's ob- 

 ject to inquire whether the 

 nitrogen of the atmosphere had 

 any claim to be regarded as a 

 homogeneous substance, but he left the question undecided. Hav- 

 ing continued to pass sparks through a measured volume of air, with 

 the occasional addition of oxygen, imtil no 

 further diminution in volume occurred, he 

 found that after the excess of oxygen had 

 been removed, the residue amounted to not 

 more than 1/1 20th part of the whole of 

 the nitrogen. The actual volume of the 

 inactive gases in the nitrogen of the atmos- 

 phere is one eighty-fourth. Cavendish did 

 not pursue the investigation further, and 

 the discovery of argon was postponed for 

 a century. 



A convenient form of apparatus for re- 

 peating Cavendish's experiment is shown 

 in the accompanying figure. The gas, 

 air mixed with oxygen, is confined over 

 mercury in an inverted test-tube, in contact 

 with a few drops of a solution of caustic 

 potash; and by connecting the rings with 

 wires from the secondary coil of an induc- 

 tion apparatus, sparks pass between the 

 platinum terminals in the interior of the 

 test-tube. The volimie of the gas rapidly 

 diminishes; and in a few hours, the gas is 

 removed to a clean tube, and the excess of 



oxygen absorbed by burning phosphorus ; the inert gases remain behind. 

 On a larger scale, the apparatus used by Lord Eayleigh, consisting 



