By heating molybdenite in a combustion tube in a current of air, we obtain the trioxide MoO 3 (molybdic acid) as a white crystalline sublimate. This substance, when heated to redness in close vessels, fuses without much volatilization into a yellow liquid, which, on cooling, freezes into a crystalline radiated mass of 4 ’ 39 specific gravity. It dissolves in 500 parts of cold, and in 960 of hot water. It dissolves readily in aqueous ammonia or alkalies, forming molybdates. Like silica, it combines with bases in a great variety of proportions. Of these many salts, an ammonia salt of the composition 3(NH 4 ) 2 O. 7Mo0 3 + 4H 2 0 (known in laboratory parlance simply as molybdate of ammonia) is the most important, affording, as it does, the most delicate, characteristic, and widely applicable precipitant for ortho-phosphoric acid. To detect phosphoric acid in any substance soluble in water or nitric acid, add first to a solution of molybdate of ammonia an excess of nitric acid, and then (not too much) of the nitric solution of the phosphate, and keep the mixture at 40° C.; the whole of the phosphoric acid gradually separates out in the shape of a canary-yellow crystalline precipitate of “phospho-molybdate of ammonia," of the composition 24MoO 3 . P 2 O 5 . 3(NH 4 ) 2 O Ij-IRHO + 24MoO 3 . P 2 O 8 . 2(NH 4 ) 2 O. H 2 O J^ + l6li 2 υ

(according to Gibbs), which is insoluble in the reagent, even in the presence of dilute nitric acid, but soluble in excess of phosphoric acid. By treatment of this complex ammonia salt with aqua regia we can eliminate its acid 24MoO 3 . P 2 O 5 .3H,0 as a substance soluble in water and crystallizing from this solution with 59 molecules of water.

This phospho-molybdic acid plays a great part in chemical toxicology, being a generically characteristic precipitant for all (organic) alkaloids, which combine with it, pretty much as ammonia does, into precipitates insoluble in dilute mineral acids. A solution of the acid sufficient for this purpose may be obtained by saturating carbonate of soda solution with molybdic acid, adding phosphate of soda, one part for every five of MoO 3 , evaporating to dryness, fusing, dissolving in water, filtering, and adding nitric acid until the liquid becomes yellow.

Metallic molybdenum is obtained by reduction of the trioxide in hydrogen gas at very high temperatures. It is thus obtained in small crystalline granules which are infusible even in the oxyhydrogen flame. An alloy of the metal with four or five per cent. of carbon (formerly accepted as molybdenum) fuses in the oxyhydrogen flame into a silver-white metal, of 8 ’ 6 specific gravity, which is harder than topaz (Debray).

Analysis.— Molybdenum in all its forms is readily converted'into molybdic acid by oxidizing agents, such as nitric acid; or if in nonvolatile forms into alkaline molybdate by fusion with carbonate of alkali and nitre. Alkaline molybdate is soluble in water; the solution, on a gradual addition of hydrochloric acid, gives first a white precipitate, which then dissolves in the excess of acid. When a piece of zinc is added to such a solution, the latter, through gradual reduction of its MoO 3 to lower oxides, assumes first a blue, then a green, and lastly a deep blackish-brown colour. Molybdic acid colours the blowpipe flame yellowish green. It dissolves in fused borax, forming a head which in the oxidizing flame becomes yellow in the heat, but almost colourless on cooling; the reducing flame colours it dark brown, and may cause the separation of brown flakes of MoO 2 ∙ Compare Chemistry, vol. v. pp. 541, 542.