We shall first consider numbers (5) and (10). In phosphorous trichloride, PC1₃, phosphorus is undoubtedly a triad. On heating this compound to 6o°, and passing over it a current of dry air, and subsequently leading the air through ice-cold water, crystals separate, which are washed with ice-water and dried in a vacuum. They have the formula H₃PO₃. The acid, however, is di-basic ; the formula of its sodium salt, for example, is Na₂HPO₃. Again, phosphorous anhydride P₄O₆, produced by the combustion of phosphorus in a limited supply of air, constitutes a crystalline substance, melting at 22.5⁰ ; it is acted on only very slowly by cold water, and then yields phosphorous acid. These facts point to a molecular change from P(OH)₃ to 0=PH(OH)₂. This view is rendered certain by consideration of the ethyl salts of the acids.