82 AN INTRODUCTION TO SCIENCE 



atoms ; with the exception of the gas, argon, which Lord 

 Raleigh and Professor Ramsay have recently discovered in 

 the atmosphere, which is monatomic, and sodium and potas- 

 sium (and probably other elementary gases), which become 

 monatomic at high temperature. In the cases of elements 

 which cannot be converted into gas, the molecular weight 

 must of course be determined by indirect methods. 



A study of the numerical relations between the atomic 

 weights of the elements led Mendele"ef to the greatest 

 generalisation of Modern Chemistry the formulation of 

 the "Periodic Law." This generalisation more than any 

 other has given rise to speculation as to the ultimate con- 

 stitution of matter. It seems to be a logical inference from 

 the periodic relations between them, that the atoms of all 

 chemical elements are really clusters of atoms of the funda- 

 mental substance, protyle. If this be true, the indestructi- 

 bility and immutability of an element means the indivisibility 

 by chemical means of the protyle cluster. Neglecting all 

 qualifications, the Periodic Law may be explained as 

 follows : 



Mendeleef s Law (1869). The atomic weights of the 

 elements (on the hydrogen scale) range from hydrogen, i, to 

 uranium, 240. They might therefore be arranged in a linear 

 series. But a consideration of their properties shows that 

 the elements fall into groups. If any property common to 

 all elements be considered, and a band varying in width 

 according to the degree in which they severally exhibit this 

 property be drawn down the full length of the list, it will be 

 found that prominences and subsidences occur at intervals 

 on the band. No matter whether we are comparing the 

 elements with regard to the melting points and boiling 

 points of certain of their compounds, the heat evolved dur- 

 ing their union with chlorine, their spectra, the colours of 

 certain of their salts, their magnetic properties, or their oc- 

 currence in Nature, we find that the line representing the 

 quality of the character or the amount of its development 



