50 



KNOWLEDGE. 



[Makoh 1, 1895. 



^J-gth part of the nitrogen employed) remained unabsorbed, 

 and this led him to surmise the possible existence of still 

 another constituent in the atmosphere. The passage in 

 Cavendish's paper which relates to this is so important 

 that it was quoted by Lord Eayleigh and Prof. Eamsay at 

 full length, and it is reproduced here for the benefit of 

 those who are curious with regard to the history of the 

 subject. It runs as follows (for phlogisticated air read 

 nitrogen, and for dephlogisticated air read oxygen) : — 



"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 alkalis, or nitrous air ; that it is unfit to support 

 fire, or maintain life in animals ; and that its specific 

 gra^'ity is not much less than that of common air : so 

 that, though the nitrous acid, by being united to 

 phlogiston, is converted into air possessed of these 

 properties, and consequently, though it was reasonable to 

 suppose, that part at least of the phlogisticated air of the 

 atmosphere 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 difl^erent 

 substances confounded 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. The foregoing experiments indeed in some measure 

 decided this point, as much the greatest part of the air let 

 up into the tube lost its elasticity ; yet, as some remained 

 unabsorbed, it did not appear for certain whether that was 

 of the same nature as the rest or not. For this purpose I 

 diminished a similar mixture of dephlogisticated and 

 common air, in the same manner as before, till it was 

 reduced to a small part of its original bulk. I then, in 

 order to decompound as much as I could of the 

 phlogisticated air which remained in the tube, added some 

 dephlogisticated air to it, and continued the spark till no 

 further diminution took place. Having by these means 

 condensed as much as I could of the phlogisticated air, I 

 let up some solution of liver of sulphur to absorb the 

 dephlogisticated air ; after which only a small bubble of 

 air remained unabsorbed, which certainly was not more 

 than yl-j of the bulk of the phlogisticated air let up into 

 the tube ; so that 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 J^ part of the whole." . 



The above passage serves to throw in clear relief the 

 marvellous gift of observation and experiment by which 

 Cavendish was distinguished. There can thus be no doubt 

 that he actually separated from the nitrogen of the air, 

 and had in his hands a minute quantity of the new gas, but 

 more than a century was to come and go before anything 

 further was to be heard about it. The experimental 

 facilities which Cavendish could command were so primitive 

 that we can only marvel at his having made this observa- 

 tion, and not at his carrying the matter no further. But 

 why, it may be asked, was a century and more allowed to 

 pass before the thread was again taken up, and argon 

 discovered '? This no doubt arose in part from the fact that 

 comparatively few chemists were aware of Cavendish's 

 surmise, while those who were aware of it neglected to 

 give it its proper weight, on the assumption that Cavendish 

 was mistaken in his experiment and deductions ; and 

 further, as we shall see presently, the experimental 

 diiSculties to be overcome were of necessity very great. 



It is not my purpose to enter into minute detail with 



regard to the discovery and the investigation of argon. 

 I shall merely touch here on some of the more salient 

 points. The observation which ultimately led to the 

 discovery of the new gas was one made by Lord Eayleigh, 

 who had been engaged for a long time in re-determina- 

 tions of the densities of various gases, including nitrogen, 

 those determinations being made with all possible 

 precautions, so as to reduce experimental errors to the 

 lowest practicable limit. He observed that the nitrogen 

 prepared by various methods from different chemical 

 compounds (nitric oxide, nitrous oxide, ammonium nitrite 

 and urea) was lighter than atmospheric nitrogen by about 

 half a per cent., under equal conditions of temperature 

 and pressure ; or, to put the matter in other words, he 

 found that two hundred and thirty volumes of atmospheric 

 nitrogen weighed as much as two hundred and thirty-one 

 volumes of "chemical" nitrogen.'' This discrepancy in 

 weight was so unlooked for that he at first attempted to 

 account for it on the supposition that there had been 

 contamination of the nitrogen in question with some other 

 gas, or that there had been a partial dissociation of the 

 nitrogen molecules into atoms. But these hypotheses 

 having been put to the test of actual exjieriment without 

 result, it was felt that the only remaining explanation must 

 lie in one of the two nitrogens being a mixture of gases, 

 and that in all probability it was the nitrogen of the air. 

 Prof. Eamsay now joining forces with Lord Eayleigh, they 

 set to work upon a rigorous search for the suspected gas, 

 and after much arduous and patient labour on many 

 different lines, their efforts were crowned with success. 



A variety of methods were tried in order to isolate the 

 gas, but of these we need only mention three here, the first 

 two being found to answer well : (1) Cavendish's old 

 method on an immensely larger scale, or, to speak more 

 correctly, at a rate of combination of the oxygen and 

 nitrogen far beyond what was possible with Cavendish's 

 appliances, the highest rate yet attained being about three 

 thousand times that of Cavendish. This method, it will 

 be remembered, was to go on passing electric sparks through 

 a mixture of atmospheric nitrogen and excess of oxygen 

 contained in a vessel inverted over a solution of caustic 

 alkali, the gases being fed in alternately as required. 

 After no further diminution in volume took place on 

 continued sparking, the excess of oxygen was absorbed by 

 alkaline pyrogallate, and there remained a residue of a gas 

 whose spectrum showed it to be neither nitrogen nor 

 oxygen. (2) Atmospheric nitrogen was passed backwards 

 and forwards from one gas-holder to another over red-hot 

 magnesium turnings contained in combustion tubes, 

 whereby the nitrogen was somewhat slowly taken up as 

 magnesium nitride, while there remained a residue of 

 unabsorbed gas. This, like the sparking, was a very 

 tedious operation, and necessitated the use of a some- 

 what compUcated series of tubes containing soda-lime, 

 phosphoric anhydride, red-hot copper, and red-hot copper 

 oxide, in order to get rid of every trace of other possible 

 gases, the whole arrangement being worked by a Sprengel 

 pump. During the first stage of the absorption large 

 gas-holders containing water were used, and finally, 

 after most of the nitrogen had been absorbed, smaller gas- 

 holders containing mercury. Both of these methods (1) 

 and (2) were found suitable for the preparation of argon 

 on a large scale, i.e., in quantities of a litre or more at 

 a time. In their paper the authors enter minutely into 

 the various steps of the operations, and detail all the 

 precautions which they employed so as to insure getting 

 the argon pure. (3) The third method to which we have 



* Eoi/al Society. 1 Proceedings, LIU., 134 (1&93); LIV., 340 (1894). 



