THE PEEIODIC LAW. 167 



the spark was passed through the air, which consists chiefly of a mix- 

 ture of nitrogen and oxygen, and the resultant oxid of nitrogen was 

 absorbed in caustic potash. More oxygen was added from time to time 

 until the last of the nitrogen was used up. Now Cavendish noticed, 

 as have many chemists since his day, that it was always impossible to re- 

 move all the nitrogen; in every case about one per cent, of gas remained. 

 There is no record that any one ever suspected that this residue was 

 not nitrogen; such is, however, the fact, and the gases, argon, helium 

 and, perhaps, others are present. These can be best recognized by 

 passing an electric spark through the rarefied gas and examining the 

 spectrum. It seems now very strange to us that an element so abundant 

 that an ordinary sized room contains no less than a thousand liters 

 should have so long escaped discovery. The reason is not far to seek. 

 Argon and its congeners are distinguished by a most remarkable ex- 

 hibition of properties, in that they have apparently no chemical affinity, 

 and no compounds of them are known. From this fact it has been 

 argued by some that these gases cannot be considered chemical ele- 

 ments, for all elements hitherto known do form compounds with other 

 elements. It is, however, a curious fact that in the periodic table we 

 find, in the eighth group, place for several just such elements, as we 

 have seen, without affinity, and neither positive nor negative in electro- 

 chemical character. It may well be that helium, neon, argon and 

 xenon belong in these vacant spaces. 



If this be the case, there is still a difficulty which confronts us, and 

 this is that argon possesses an atomic weight slightly higher than the 

 next element in order, potassium, instead of lower. This would not, 

 however, be a unique instance of such a difficulty in the table. It was 

 formerly thought that the two metals, nickel and cobalt, had identical 

 atomic weights, and though the salts of nickel are generally green, and 

 those of cobalt red, in other respects these metals and their com- 

 pounds are very much alike. After the discovery of the Periodic Law, 

 when it was seen that cobalt belonged in the second series of the 

 eighth group and nickel in the third, it was supposed that further study 

 would necessarily show nickel to have an appreciably higher atomic 

 weight than cobalt. We have already seen that in this same group, 

 before the appearance of the periodic table, the accepted atomic weights 

 of osmium, iridium and platinum were incorrect, and it was the 

 fact of their mis-arrangement in the table which caused Seubert to 

 revise their weights. Very much labor has been spent upon the re- 

 vision of the atomic Aveights of nickel and cobalt. Gerhardt Kriiss 

 supposed he had found a new and heavier metal, hitherto unknown, 

 in ordinary cobalt, and that this caused the atomic weight to be esti- 

 mated too high. He called the new metal gnomium, but it was soon 

 shown that gnomium has no real existence. The more accurate the 



