x NITRE, NITRIC ACID, AND NITROGEN 191 



(A.C.R. III. 45). It was free from vitriolic and muriatic 

 acids, but gave a precipitate with a solution of silver owing 

 to the fact that a part of the nitre was in the " phlogisti- 

 cated" (i.e. reduced or deoxidised) condition, to which it 

 is brought when heated so as to drive off a part of its oxygen 

 (see below, p. 199). 



The composition of air. It has been seen that air contains 

 about one-fifth of oxygen and a trace (about 1/2500) of fixed 

 air or carbonic anhydride, as well as a variable quantity of 

 moisture. The inert residue, of " azote " is much less easy 

 to analyse. Cavendish writes : 



" As far as the experiments hitherto published extend, we 

 scarcely know more of the nature of the phlogisticated part 

 of our atmosphere, than that it is not diminished by lime- 

 water, caustic alkalis, 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" (A.C.R. 

 III. 49). 



His experiments had shown that it consisted largely of 

 NITROGEN, 1 i.e. of a gas which can be converted into nitre 

 by sparking with oxygen in presence of potash. "Yet it 

 might fairly be doubted whether the whole is of this kind, 

 or whether there are not in reality many different substances 

 confounded together by us under the name of phlogisticated 

 air. 2 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 [nitric] acid, or whether 

 there was not a part of a different nature from the rest, 

 which would refuse to undergo that change" (A.C.R. III. 



49)- 



For this purpose Cavendish took the usual mixture of air 

 and oxygen and diminished it as much as possible by sparking. 



1 The name was invented by the French chemist and calico- 

 printer Chaptal about 1790 (see the Preliminary Discourse of his 

 Elements of Chemistry, tr. 1791, pp. xxxiv. to xxxvi.). 



2 i.e. Azote. 



