THE ATOMS OF THE ELEMENTS. 23 



That if you know the weight of the atom of the element 

 you may know, if you like, its properties, for they are 

 fixed. 



Just as the pendulum returns again in its swing, just as 

 the moon returns in its orbit, just as the advancing year ever 

 brings the rose of spring, so do the properties of the ele- 

 ments periodically recur as the weights of the atoms rise. 

 To demonstrate this fact, take some one specific property, 

 for example, the atomic volume, which is the atomic weight 

 divided by the specific gravity of the solid element, and 

 arrange a table on a piece of engineering paper in which 

 the atomic weights read from left to right (the abscissas), 

 while the atomic volumes read from bottom to top (the 

 ordinates) . Now construct a curve by pricking out the posi- 

 tion of the different elements in accordance with both their 

 atomic volumes and atomic weights, and you will find your- 

 self in possession of a table such as Fig. 2. We see at once 

 from this curve that the atomic volume is a periodic func- 

 tion of the atomic weight. As the atomic weight increases, 

 the atomic volume alternately increases and decreases. The 

 periodicity proclaims itself in the regularly recurring hills 

 and valleys which constitute the curve. Elements which 

 occupy similar positions on the five hills and valleys have 

 markedly similar properties. Thus, you will notice at the 

 summit of each of the five hills, the symbols of the ele- 

 ments lithium, sodium, potassium, rubidium and caesium, 

 all of these elements possessing amazingly similar properties. 

 Or, again, find the little dot marked S (signifying sulphur) 

 on the slope of the third hill, and you will then notice a little 

 dot marked Se (selenium) and another Te (tellurium) in a 

 correspondingly similar position on the other two hills re- 

 spectively. These elements have strikingly similar proper- 

 ties. Take now another property altogether, let us say the 



