THE ELEMENTS OF THE CHEMIST 139 



as belonging to a constituent of the sun's chromosphere and 

 designated D 3 . This line had been attributed by Professors 

 Janssen and Lockyer, thirty years previously, to a hypothetical 

 solar element which was named by them helium. 



Clevite, the mineral originally used, is a variety of uraninite, 

 and contains beside uranium and lead a considerable quantity 

 of the rare earths. The significance of the association of helium 

 with uranium and lead will be referred to on a later page. 



Helium has been found to be a constituent of a large number 

 of minerals, but it is especially found in connection with com- 

 pounds of uranium, thorium, and the rare earths. It is a colour- 

 less inert gas like argon, but very much lighter, being, in fact, 

 with the exception of hydrogen, the lightest gas known. Its 

 density is 2-0, and hence its molecular and atomic weight is 4-0. 

 It was found to be very sparingly soluble in water, one volume 

 of the gas being soluble in between 130 and 140 volumes of 

 water, at atmospheric temperatures. This low degree of solu- 

 bility indicated a probably low boiling-point. The subsequent 

 history of helium confirmed this conjecture, for it was not till 

 1908 that helium was reduced to the liquid state by Professor 

 Kamerlingh-Onnes of Leiden. The boiling-point was found to 

 be about 4-5 absolute or 268 to 269 C. below C. This result, 

 by which the last of the known gases was made to change its 

 state, was only accomplished by pressure with the aid of liquid 

 hydrogen as a cooling agent. By causing the liquid helium to 

 boil under reduced pressure the lowest temperature ever attained 

 was reached. This was estimated to be less than 2*5 on the 

 absolute scale. 



These two remarkable elements helium and argon presented 

 a difficult problem as to their position in relation to the rest of 

 the chemically active elements. 



On reviewing MendeleefFs periodic scheme of the elements it is 

 noticeable that, if we regard for the moment only the elements 

 known prior to the discovery of helium and argon, the valency 

 or combining capacity of the atoms in Group I is represented by 

 unity. These elements are univalent. The same is the character- 

 istic valency of the halogens, fluorine, chlorine, bromine, and 

 iodine, which stand, along with manganese, somewhat doubt- 

 fully in the Group VII. If then, following the order from left 

 to right, it is found that the valency steadily increases as shown 

 below, it appears that if a column is provided to the left of 



