166 ESSAYS BIOGRAPHICAL AND CHEMICAL 



irregular, and vary between fairly wide limits ; and it is 

 quite probable that these differences may occasionally be 

 negative. 



In 1894 a new constituent of the atmosphere, which 

 was named ' argon/ was discovered by Lord Rayleigh and 

 Ramsay; this was followed in 1895 by the discovery by 

 Ramsay of helium in certain minerals. This gas gives a 

 spectrum in which a brilliant yellow line is conspicuous. 

 So long ago as 1868 this line had been observed in the 

 solar spectrum by Jansen; it was attributed by Frank- 

 land and Lockyer to the presence of a new element in 

 the sun, and they named the then unknown element 

 ' helium.' These discoveries were followed by that of three 

 other gaseous elements in atmospheric air, by Ramsay and 

 Travers in 1898; thus five elements were added to the 

 list. All these elements are distinguished by their inert- 

 ness, for none of them forms compounds with other 

 elements. 



The Roman figures at the head of the columns of the 

 periodic table have a certain significance. They show the 

 maximum number of atoms of hydrogen which the ele- 

 ments in each column can combine with or replace, or, as 

 it is termed, their 'valency.' Thus an atom of lithium 

 combines with one atom of hydrogen ; it can also replace 

 one atom, as when it forms lithium hydroxide, LiOH, in 

 which it has replaced one atom of hydrogen in water, 

 H 2 0. So also magnesium can replace two atoms of 

 hydrogen, for it forms the hydroxide Mg(OH) 2 . Boron 

 combines with three atoms of hydrogen; carbon with 

 four; phosphorus, although it can combine with only 

 three atoms of hydrogen, can replace five ; for it forms a 

 chloride PC1 5 , in which it has replaced the five atoms of 

 hydrogen in five molecules of hydrogen chloride, 5HC1. 

 Sulphur forms a hexafluoride, and iodine a pentafluoride, 

 in which they replace six and five atoms of hydrogen 



