many chemical compounds, from some of which it can readily be prepared. The identification of atmospheric nitrogen with that contained in niter and nitric acid is due to Henry Cavendish, whose exact and skillful work not only established this fact, but led to an observation of great interest in connection with the discovery of argon. In a paper which appeared in 1785 Cavendish says: "As far as experiments hitherto published extend, we scarcely know more of the phlogisticated part of our atmosphere [nitrogen] than that it is not diminished by lime water, caustic alkalies, or nitrous air; that it is unfit to support fire, or maintain life in animals; and that its specific gravity is not much less than that of common air"; and raises the question "whether there are not in reality many different substances compounded by us under the name of phlogisticated air." He then describes an experiment for the purpose of deciding this point. By passage of electric sparks through a mixture of air and oxygen, the nitrogen was converted into a compound absorbed by the dilute alkali over which the gases were confined. The sparking was continued until no further diminution of volume took place, when, on removing the excess of oxygen by absorption in "liver of sulphur" "only a small bubble of air remained unabsorbed." From this he concludes that "if there is any part of the phlogisticated air of our atmosphere which differs from the rest, and can not be reduced to nitrous acid, we may safely conclude that it is not more than a hundred and twentieth part of the whole." Cavendish was apparently satisfied with this as a proof of the simple character of atmospheric nitrogen, and his work has been accepted as conclusive for more than a century; but we now know that this "small bubble of air" which survived his experiment must have been argon.
It seems strange that a substance present in the air all about us, and whose actual quantity is enormous, should have defied detection through so many years of exact and searching chemical work; but the explanation lies largely in the fact that argon forms no compounds, so far as is known, and thus fails to assert itself in the presence of the almost equally indifferent nitrogen with which it is mixed.
Indeed, the hint which led to its discovery was obtained in the course of a purely physical investigation. For some years Lord Rayleigh has been engaged in the exact determination of the densities of some of the more permanent gases. In dealing with nitrogen, it was found that this gas, when prepared from chemical compounds, was about one half per cent lighter than the nitrogen obtained from air. This discrepancy at once suggested contamination with some known impurities. A careful search proved, however, that this was not the case. The possible ex-
