134 THE PATH OF SCIENCE 



considerations, D. I. Mendeleev, professor of chemistry at 

 St. Petersburg, was led to classify the elements by plotting 

 properties which could be measured quantitatively, such as 

 the atomic volumes, against the atomic weights. The curves 

 showed that the same properties repeated periodically, and 

 Mendeleev classified the elements in what is known as the 

 periodic table. By extrapolating this table, he was able to 

 prophesy the existence of elements that had not yet been dis- 

 covered and to state their approximate properties. Several 

 of these prophesies were justified by the discovery of the ele- 

 ments that he had foreseen. 



In the last years of the nineteenth century, two discoveries 

 were made that disclosed the existence of elements for which 

 there seemed to be no room in the periodic table. The first 

 was the discovery by Sir William Ramsay of the rare gases 

 of the atmosphere. In 1882 Lord Rayleigh started to re- 

 determine the density of oxygen and hydrogen and later ex- 

 tended the work to nitrogen, whose atomic weight is of 

 fundamental importance in connection with the determina- 

 tion of the atomic weights of many elements. He used ni- 

 trogen prepared from the atmosphere by the elimination of 

 the oxygen and of all other reactive gases, such as carbon 

 dioxide and water vapor, and also nitrogen prepared by the 

 decomposition of ammonia. To his astonishment, the at- 

 mospheric nitrogen was appreciably heavier than that pre- 

 pared chemically. After many checks, he discussed the matter 

 in 1894 with Ramsay, who investigated the nature of the 

 atmospheric nitrogen by causing it to react with metals, such 

 as magnesium, which combine with nitrogen. About one per 

 cent of the gas would not react, and this proved to be a new 

 gas having a higher density than nitrogen and a different spec- 

 trum. Moreover, this new gas ^vould not react with anything 

 at all, for which reason it was named argon, the "lazy" gas. 

 Following this discovery, Ramsay succeeded in isolating four 

 other gases having properties similar to argon— helium,* 



* Chapter V, p. 116. 



