PERIODIC ARRANGEMENT OF ELEMENTS 171 



were entirely negative ; hence it may be concluded, con- 

 firmatory of the experiments of Landolt, that no change 

 in mass is produced by a chemical reaction. A variation 

 in weight or in inertia has not been observed. 



There is one curious discrepancy which still remains 

 unexplained. The density of nitrogen gas has been very 

 accurately determined by two very competent observers 

 Lord Rayleigh and Leduc. They both agree in their 

 results to one part in 10,000. Now it is known, for reasons 

 into which we cannot enter here, that the molecules of 

 both nitrogen and oxygen consist each of two atoms ; and 

 as it is also certain that equal volumes of gases contain 

 nearly equal numbers of molecules, when measured under 

 similar conditions of temperature and pressure, the rela- 

 tive weights of these gases correspond to the relative 

 weights of the atoms. The word ' nearly ' has been used ; 

 for a slight correction must be introduced in order to 

 secure exact correspondence. Hence the atomic weight 

 of nitrogen, referred to that of oxygen taken as 16, as is 

 now customary, must be 14r008, since that is the density of 

 nitrogen referred to oxygen as 16, after the necessary cor- 

 rection has been made. But this number does not corre- 

 spond with the atomic weight of nitrogen obtained by the 

 celebrated chemist Stas, as the result of the analysis of such 

 compounds as potassium nitrate, when he determined the 

 ratio between the quantities of nitrogen and oxygen in 

 the molecule KN0 3 . Both he and, quite recently, one 

 of the most skilful of analysts, to whom we owe in recent 

 years many exact determinations of atomic weights, 

 Theodore Richards, agreed in ascribing the number 14'04 

 to nitrogen as its atomic weight. The difference does not 

 appear very great ; but yet it amounts to one part in 370 : 

 and the error of experiment is not likely to be greater 

 than one part in 10,000. This discrepancy is one of. the 

 most curious of chemical facts, and it would well repay 



