270 Lord Ray lei gh and Prof. Ramsay. [Jan. 31, 



accepting this explanation, even provisionally, we had to face the 

 improbability that a gas surrounding ns on all sides, and present in 

 enormous quantities, could have remained so long unsuspected. 



The method of most universal application by which to test whether 

 a gas is pure or a mixture of components of different densities is that 

 of diffusion. By this means Graham succeeded in effecting a partial 

 separation of the nitrogen and oxygen of the air, in spite of the com- 

 paratively small difference of densities. If the atmosphere contain 

 an unknown gas of anything like the density supposed, it should be 

 possible to prove the fact by operations conducted upon air which had 

 undergone atmolysis. This experiment, although in view from the 

 first, was not executed until a later stage of the inquiry ( VI), when 

 results were obtained sufficient of themselves to prove that the atmos- 

 phere contains a previously unknown gas. 



But although the method of diffusion was capable of deciding the 

 main, or at any rate the first question, it held out no prospect of 

 isolating the new constituent of the atmosphere, and we, therefore, 

 turned our. attention in the first instance to the consideration of 

 methods more strictly chemical. And here the question forced itself 

 upon us as to what really was the evidence in favour of the prevalent 

 doctrine that the inert residue from air after withdrawal of oxygen, 

 water, and carbonic anhydride, is all of one kind. 



The identification of " phlogisticated air " with the constituent of 

 nitric acid is due to Cavendish, whose method consisted in operating 

 with electric sparks upon a short column of gas confined with potash 

 over mercury at the upper end of an inverted (J tube.* 



Attempts to repeat Cavendish's experiment in Cavendish's manner 

 have only increased the admiration with which we regard this won- 

 derful investigation. Working on almost microscopical quantities of 

 material, and by operations extending over days and weeks, he thus 

 established one of the most important facts in chemistry. And what 

 is still more to the purpose, he raises as distinctly as we could do, ancl 

 to a certain extent resolves, the question above suggested. The 

 passage is so important that it will be desirable to quote it at full 

 length. 



" As far as the experiments hitherto published extend, we scarcely 

 know more of the phlogisticated part of our atmosphere, 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 : so that 

 though the nitrous acid, by being united to phlogiston,f is converted 

 into air possessed of these properties, and consequently, though it 



* " Experiments on Air," ' Phil. Trans.,' vol. 75, p. 372, 1785. 

 t I.e., deprived of oxygen. " Pblogisticaied air " = nitrogen - " dephlogisti- 

 gated air " = oxygen. 



