702 



SCIENCE. 



[N. S. Vol. I. No. 26. 



red-hot copper ; and I thought that I ought 

 at least to give that method a trial, fully 

 expecting to obtain forthwith a value in 

 harmony with that already afforded bj^ the 

 ammonia method. The result, however, 

 proved otherwise. The gas obtained by 

 the copper method, as I may call it, proved 

 to be one-thousandth part heavier than that 

 obtained by the ammonia method ; and, on 

 repetition, that difference was only brought 

 out more clearly. This was about three 

 years ago. Then, in order, if possible, to 

 get further light upon a discrepancy which 

 puzzled me very much, and which, at that 

 time, I regarded only with disgust and im- 

 patience, I published a letter in Nature in- 

 viting criticisms from chemists who might 

 be interested in such questions. I obtained 

 various useful suggestions, but none going 

 to the root of the matter. Several persons 

 who wrote to me privately were inclined to 

 tliink that the explanation was to be sought 

 in a partial dissociation of the nitrogen 

 derived from ammonia. For, before going 

 further, I ought to explain that, in the nit- 

 rogen obtained by the ammonia method, 

 some— about a seventh part — is derived 

 from the ammonia, the larger part, however, 

 being derived as usual from the atmos- 

 phere. If the chemically derived niti'ogen 

 were partly dissociated into its component 

 atoms, then the lightness of the gas so pre- 

 pared would be explained. 



The next step in the enquiry was, if pos- 

 sible, to exaggerate the discrepancy. One's 

 instinct at first is to try to get rid of a dis- 

 crepancy, but I believe that experience 

 shows such an endeavor to be a mistake. 

 What one ought to do is to magnify a small 

 discrepancy with a view to finding out the 

 explanation ; and, as it appeared in the 

 present case that the root of the discrepancy 

 lay in the fact that part of the nitrogen pre- 

 pared by the ammonia method was nitrogen 

 out of ammonia, although the greater part 

 remained of common origin in both cases. 



the application of the principal suggested a 

 trial of the weight of nitrogen obtained 

 wholly from ammonia. This could easily . 

 be done by substituting pure oxygen for at- 

 mospheric air in the ammonia method, so 

 that the whole, instead of only a part, of 

 the nitrogen collected should be derived 

 from the ammonia itself The discrepancy 

 was at once magnified some five times. The 

 nitrogen so obtained from ammonia proved 

 to be about one-half per cent, lighter than 

 nitrogen obtained in the ordinary waj^ from 

 the atmosphere, and which I may call for 

 brevity ' atmospheric ' nitrogen. 



That result stood out prettj' sharply from 

 the first ; but it was necessarj' to confirm 

 it by comparison with nitrogen chemically 

 derived in other ways. The table before 

 you gives a summary of such results, the 

 numbers being the weights in grams actually 

 contained under standard conditions in the 

 globe employed. 



ATMOSPHEEIC NITROGEN. 



By hot copper (1892) 2.3103 



By hot iron (1893) 2.3100 



By ferrous hydrate (1894) 2.3102 



Mean 2.3102 



CHEMICAL NITROGEN. 



From nitric oxide 2.3001 



From nitrous oxide 2.2990 



From ammonium nitrite purified at a red heat . 2.2987 



From urea 2.2985 



From ammonium nitrite purified in the cold . 2.2987 



Mean 2.2990 



The difference is about 11 milligrams, or 

 about one-half per cent.; and it was suffi- 

 cient to prove conclusive^ that the two 

 kinds of nitrogen — the chemically derived 

 nitrogen and the atmospheric nitrogen — 

 differed in weight, and therefore, of course, 

 in quality, for some reason hitherto un- 

 known. 



I need not spend time in explaining the 

 various precautions that were necessaiy iu 

 order to establish surely that conclusion. 

 One had to be on one's guard against im- 



