xviii THE CLASSIFICATION OF THE ELEMENTS 471 



evidence to support this view ; on the contrary, the few 

 measurements that have been made of the atomic volumes 

 of rare-earth elements suggest that these would fall on a 

 smooth curve running down gently from barium to tantalum 

 and tungsten. It is remarkable that Lothar Meyer, with 

 the help of his atomic volume curve should have antici- 

 pated at so early a date the modern classification of the 

 elements into five complete periods and two fragments. 



Later measurements have added a number of additional 

 points to the curve. The caesium peak has been realised 

 experimentally, and the discovery of niton and radium 

 has rendered probable the existence of yet another peak 

 beyond the region covered by Lothar Meyer's curve. The 

 most striking additions to the curve are points corresponding 

 with 10 elements which are gaseous at ordinary tempera- 

 tures. 1 Some risk attaches to the inclusion of liquefied gases 

 in a comparison which is primarily concerned with solid 

 elements, but the additional points are by no means devoid of 

 interest. Thus the inert gases of Group o are found on the 

 ascending portions of the curves, just below the alkali metals, 

 with the solitary exception of helium, which rises above and 

 dominates the point representing lithium. If the atomic 

 volume were the only periodic property, it would be con- 

 venient to place the inert gases at the ends of Periods I to 

 VI, instead of at the beginning of Periods II to VII, but 

 the point is not important and there are definite advantages 

 in placing the gases of zero valency at the beginning of the 

 table. Liquid oxygen and fluorine fall into position on 

 the second catenary, but liquid nitrogen produces a small 

 secondary peak on the curve. 



Periodicity of atomic heat (Dewar, 1913). According 

 to the Law of Dulong and Petit, the ATOMIC HEAT is a 

 constant quantity which does not vary with the atomic 

 weight of the element. No better illustration could be 



1 Chlorine was the only substance studied by Meyer in the liquid 

 state. 



