The analogies existing between elements belonging to one “family,” such, for instance, as the nitrogen family or the sulphur family, have long been recognised, and are pointed out and insisted upon even in elementary textbooks; but the very important analogies existing between substances of different quantivalence are apt to be forgotten or overlooked. For illustrations of such analogies we may point to boron and silicon, elements closely resembling one another in themselves and also in their compounds,—differing, indeed, in little else but that the one is triad and the other tetrad. A similar relation exists between gold and platinum. The elementary substances, sulphur and phosphorus, have many points of similarity: both fuse at a comparatively low temperature, both are transformed by heat into amorphous insoluble modifications, and both have anomalous vapour densities.