Amongst the metals accompanying zinc which have been named, but not authentically separated, must be included the _actinium_ of Phipson (1881). He remarked that certain sorts of zinc give a white precipitate of zinc sulphide which blackens on exposure to light and then becomes white in the dark again. Its oxide, closely resembling in many ways cadmium oxide, is insoluble in alkalis, and it forms a white metallic sulphide, blackening on exposure to light. As no further mention has been made of it since 1882, its existence must be regarded as doubtful. CHAPTER XVII BORON, ALUMINIUM, AND THE ANALOGOUS METALS OF THE THIRD GROUP If the elements of small atomic weight which we have hitherto discussed be placed in order, it will be clearly seen that, judging by the formulæ of their higher compounds, one element is wanting between beryllium and carbon. For lithium gives LiX, beryllium forms BeX_{2}, and then comes carbon giving CX_{4}. Evidently to complete the series we must look for an element forming RX_{3}, and having an atomic weight greater than 9 and less than 12. And _boron_ is such a one; its atomic weight is 11, and its compounds are expressed by BX_{3}. Lithium and beryllium are metals; carbon has no metallic properties; boron appears in a free state in several forms which are intermediate between the metals and non-metals. Lithium gives an energetic caustic oxide, beryllium forms a very feeble base; hence one would expect to find that the oxide of boron, B_{2}O_{3}, has still more feeble basic properties and some acid properties, all the more as CO_{2} and N_{2}O_{5}, which follow after B_{2}O_{3} in their composition and in the periodic system, are acid oxides. And, indeed, the only known _oxide of boron_ exhibits a feeble basic character, together with the properties of a feeble acid oxide. This is even seen from the fact that a solution of boron oxide reddens blue litmus and acts on turmeric paper as an alkali, and these reactions may be used for determining the presence of B_{2}O_{3} in solutions. By themselves the alkali borates have an alkaline reaction, which clearly indicates the feeble acid character of boric acid. If they are mixed in solution with hydrochloric acid, boric acid is liberated, and if a piece of turmeric paper be immersed in this solution and then dried, the excess of hydrochloric acid volatilises, while the boric acid remains on the paper and communicates a _brown coloration_ to it, just like alkalis.