GENERAL REMARKS REGARDING ELEMENTS. 129 



their properties. Fortunately his predictions have, in several cases, 

 been verified, a number of the missing elements having since been 

 discovered. Among them may be mentioned scandium, gallium, and 

 ycntianium. These elements not only fitted in the previously blank 

 spaces by virtue of their atomic weights, but their general properties 

 also assigned to them the places which they now occupy. 



When the table was first arranged it did not show group 0. The 

 elements forming this group have all been discovered since the year 

 1894, and their discovery necessitated the addition of an extra group. 

 In order to avoid renumbering the previously known groups the new 

 group was designated by zero. 



Another graphic representation of the periodic law is obtained by 

 arranging the elements according to the increase in their atomic 

 weights on a spiral line, as shown on the diagram (Fig. 36). 

 From the centre of the spiral extend 20 radii, and in placing the 

 elements on the intersections of the spiral and a radius three im- 

 portant facts are noticed, viz. : 1. The distances of the elements 

 from the centre of the spiral are proportionate (or nearly so) to 

 their atomic weights. 2. From left to right the elements, arranged 

 on the diametric lines, follow one another according to the periodic 

 grouping. 3. The elements belonging to the even series of one of 

 the groups are on one radius, while the elements belonging to the 

 uneven series of the same group find their position on the radius 

 opposite. For example, calcium, strontium, and barium, of the 

 even series 4, 6, and 8 of group II., are on a radius opposite the 

 one on which we find magnesium, zinc, and cadmium, of the uneven 

 series 3, 5, and 7 of group II. 



Physical properties of elements. Most elements are, at the 

 ordinary temperature, solid substances, two are liquids (bromine and 

 mercury), and of the more important elements five are gases (oxy- 

 gen, hydrogen, nitrogen, chlorine, and fluorine). 



Most, if not all, of the solid elements may be obtained in the crys- 

 tallized state; a few are amorphous and crystallized, or polymorphous. 

 The physical properties of many elements in these different states 

 differ widely. For instance : Carbon is known crystallized as diamond 

 and graphite, or amorphous as charcoal. The property of elements to 

 assume such different conditions is called allotropy, and the different 

 forms of an element are termed allotropic modifications. 



Some of the gaseous elements are also capable of existing in allo- 

 tropic modifications For instance: Oxygen is known as such and as 

 ozone, the latter differing from the common oxygen both in its physi- 

 cal and chemical properties. The explanation given for this surprising 



