40 
ABBE. 
The next step in the separation of the gases in our atmos- 
pheric mixture was due to Rutherford, a pupil of Black, who 
in 1772 announced the discovery of nitrogen as the residual 
gas after the combustion of carbon and the absorption of the 
resulting fixed air. The discovery of oxygen was made inde- 
pendently and nearly simultaneously by Priestley and 
Scheele; but Priestley published his results in 1775, a year 
before Scheele. The recognition of oxygen as an inde- 
pendent gaseous element and the establishment of our mod- 
ern view of the air as a simple mixture is due to Lavoisier, 
who published several memoirs on the subject in 1777 and 
1778, while Cavendish was carrying on a parallel system of 
experiments in England, experiments that he began in 1777, 
but published only some years later. 
With the discovery of nitrogen and oxygen in the atmos- 
phere and the measurement of the exact ratios of these and 
various so-called impurities, the chemistry of the atmosphere 
halted until, in 1882, Lord Rayleigh began a research on the 
relative densities of hydrogen, oxygen, and other gases, con- 
cluding in 1893 by the statement that nitrogen obtained 
from the atmosphere was somewhat denser than nitrogen 
prepared from ammonia, and that the difference, though 
slight, was so far beyond all question that it demanded an 
explanation. This explanation was announced in a prelim- 
inary way in August, 1894, but was received with such in- 
credulity that one chemist sarcastically inquired “whether 
the name of the new gas had also been discovered.” But the 
matter was brought to a clear demonstration by diffusing the 
mixed gases slowly through a long train of tobacco pipes of 
the variety known as the “church-warden pipe,” which is 
made of a very fine clay through which diffusion proceeds 
very slowly. Thus argon was discovered. Of course you will 
recognize the fact that this last step in the analysis of atmos- 
pheric air is not a chemical, but a physical process, illus- 
trating the general statement that no one branch of science 
can endure or progress without the assistance of correlated 
friendly branches. 
The discovery of argon paved the way to new ideas in re- 
