The Groundwork of the Universe, Sec. 45 



Hydrogen has been omitted fi'om the table on account of the 

 difficulty in placing it properly. From its chemical ])roperties it 

 should be situated above lithium, but for physical reasons it 

 seems -allied to fluorine at the other end of the line. 



The table is so arranged that the vertical columns contain 

 those elements which show a family resemblance to each other in 

 their properties. Thus the inert gases are ranged together, and 

 in the next column we find lithium, sodium, potassium, rubidium 

 and caesium showing close family likenesses, while with them are 

 copper, silver and gold which have many chemical characteristics 

 in common with the other five. 



This periodicity is also found in the volumes of the atoms. 

 The curve at the end of this section shows a regular periodicity 

 in volume and in other physical properties, Avhen arranged in the 

 same order as in the Table. 



From this arrangement we get the conception of an "atomic 

 order" in which hydrogen is the first element, helium the second, 

 lithium the third, and so on to uranium, which is the ninety- 

 second. The place which the element occupies is called its 

 atomic number. 



When the rays from the cathode of a Crookes' vacuum tube 

 are allowed to fall upon a specimen of an element, an X-ray is 

 produced, and the wave-length of this ray is characteristic of the 

 element. Moseley has shown that the wave-length is reciprocally 

 proportional to A{N - if, in which .4 is a constant and N the 

 atomic number of the element used. We can thus calculate the 

 atomic number of the element and establish its place in the 

 periodic system by this measurement alone. If from a com])lete 

 series of such measurements we find any blanks in our table, we 

 are able to say where the corresponding unknown elements will 

 take their places when actually discovered. Moseley found there 

 were five blanks upon the roll: (l) between molybdenum and 

 Ruthenium ; (2) between tungsten and osmium ; (3) between 

 polonium and niton ; (4) between niton and radium ; (5) one 

 in the rare earth group, which is not shoAvn in Table II. When 



