INERT CONSTITUENTS OF THE ATMOSPHERE. 583 



fluorine column; and the long and short periods are kept separate by 

 a fold in the diagram. A diagonal line, too, divides the 'metallic' 

 from the 'non-metallic' elements. 



Other devices have been suggested in order to represent diagram- 

 matically the relations between the atomic weights ; but it must be borne 

 in mind that whatever system is employed, such plans are merely aids 

 to thought, and have no real significance. They are on a par with the 

 representation of numerical relations as curves, and can convey nothing 

 which is not already contained in the actual numbers. 



It was Mendeleef who first drew attention to the progressive alter- 

 ation of the valency of the elements in passing from left to right along 

 the table. While the metals of the alkalies, lithium, sodium, potassium, 

 rubidium and caesium, are all monads, in as much as one atom of any 

 one of these elements is able to replace one atom of hydrogen, the 

 typical monad, elements of the beryllium group are dyads ; hence while 

 the formula of sodium chloride is NaCl, that of calcium chloride is 

 CaClg, that of boron chloride, BCI3, for boron is a triad; the chloride 

 of the tetrad, carbon, CCI4, and so on. And considering the com- 

 pounds with hydrogen, where these exist, we have BH3, corresponding 

 to the chloride; CH^, NH3, OH2, and finally CIH and FH. As the 

 valency alters by unity in each ease, it appeared reasonable to place 

 the elements on the table in equidistant columns; or, as in Dr. 

 Stone/s diagram, on equidistant lines, dividing the spiral curve into 

 eight equal segments. 



Meyer, however, showed that if the elements be mapped on square 



M to iO 40. so 60 70 00 30 100 110 IZO _ liO- . 1*9 



Atomic Weights. 



paper, so that the vertical divisions correspond to the volumes occupied 

 by unit weight of the solid or liquid elements, while the horizontal 

 divisions correspond with the atomic weights, a certain amount of 



