| Scandium, | Sc | = | 44. | Germanium, | Ge | = | 72. | |
|---|---|---|---|---|---|---|---|---|
| Gallium, | Ga | = | 70. | Rubidium, | Rb | = | 86. | |
| Yttrium, | Y | = | 89. | Cæsium, | Cs | = | 133. | |
| Niobium, | Nb | = | 94. | Lanthanum, | La | = | 138. | |
| Ruthenium, | Ru | = | 102. | Didymium, | Di | = | 142. | |
| Rhodium, | Rh | = | 103. | Ytterbium, | Yb | = | 173. | |
| Indium, | In | = | 114. | Tantalum, | Ta | = | 183. | |
| Tellurium, | Te | = | 125. | Thorium, | Th | = | 232. |
Besides these 66 elements there have been discovered:—Erbium, Terbium, Samarium, Thullium, Holmium, Mosandrium, Phillipium, and several others. But their properties and combinations, owing to their extreme rarity, are very little known, and even their existence as independent substances29 is doubtful.
It has been incontestably proved from observations on the spectra of the heavenly bodies that many of the commoner elements (such as H, Na, Mg, Fe) occur on the far distant stars. This fact confirms the belief that those forms of matter which appear on the earth as elements are widely distributed over the entire universe. But we do not yet know why, in nature, the mass of some elements should be greater than that of others.30
The capacity of each element to combine with one or another element, and to form compounds with them which are in a greater or less degree prone to give new and yet more complex substances, forms the fundamental character of each element.
Thus sulphur easily combines with the metals, oxygen, chlorine, or carbon, whilst gold and silver enter into combinations with difficulty, and form unstable compounds, which are easily decomposed by heat.
The cause or force which induces the elements to enter into chemical change must be considered, as also the cause which holds different substances in combination—that is, which endues the substances formed with their particular degree of stability. This cause or force is called affinity (affinitas, affinité, Verwandtschaft), or chemical affinity.31
Since this force must be regarded as exclusively an attractive force, like gravity, many writers (for instance, Bergmann at the end of the last, and Berthollet at the beginning of this, century) supposed affinity to be essentially similar to the universal force of gravity, from which it only differs in that the latter acts at observable distances whilst affinity only evinces itself at the smallest possible distances.
But chemical affinity cannot be entirely identified with the universal attraction of gravity, which acts at appreciable distances and is dependent only on mass and distance, and not on the quality of the material on which it acts, whilst it is by the quality of matter that affinity is most forcibly influenced.
Neither can it be entirely identified with cohesion, which gives to homogeneous solid substances their crystalline form, elasticity, hardness, ductility, and other properties, and to liquids their surface tension, drop formation, capillarity, and other properties, because affinity acts between the component parts of a substance and cohesion on a substance in its homogeneity, although both act at imperceptible distances (by contact) and have much in common. Chemical force, which makes one substance penetrate into another, cannot be entirely identified with even those attracting forces which make different substances adhere to each other, or hold together (as when two