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Rh consists of this acid united to phlogiston, yet it might fairly be doubted whether the whole is of this kind, or whether there are not in reality many different substances compounded together by us under the name of phlogisticated air. I therefore made an experiment to determine whether the whole of a given portion of the phlogisticated air of the atmosphere could be reduced to nitrous acid, or whether there was not a part of a different nature from the rest which would refuse to undergo that change. . . . If there is any part of the phlogisticated air of our atmosphere which differs from the rest, and cannot be reduced to nitrous acid, we may safely conclude that it is not more than $1⁄120$ part of the whole." This is of vast importance, if we bear in mind the discovery of argon as a constituent of the atmosphere by Rayleigh and Ramsay in 1894, and the later work of Ramsay and Travers on krypton, neon, and metargon.

Although Cavendish's work on atmospheric gases was of great importance, it led him to a still more famous discovery, namely, the composition of water in 1781; and in his paper of 1784 he says: "By the experiments with the globe it appeared that when inflammable air (hydrogen) and common air are exploded in a proper proportion, almost all the inflammable air, and nearly one-fifth of the common air, lose their elasticity and are condensed into dew. And by this experiment it appears that this dew is plain water, and, consequently, that almost all the inflammable air, and