CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 297 



with it hydrogen is given off, which shows, as does its whole character, 

 the acid properties of the metal. The properties of tungsten are almost 

 identical ; it is infusible, has an iron-grey colour, is exceedingly hard, so 

 that it even scratches glass. Its specific gravity is 19'1 (according to 

 Roscoe), so that, like uranium, platinum, &c., it is one of the heaviest 

 metals. 9 bis Just as sulphur and chromium have their corresponding 

 persulphuric and perchromic acids, H 2 S 2 O 8 and H 2 CrO 8 , having the 

 properties of peroxides, and corresponding to peroxide of hydrogen, so 

 also molybdenum and tungsten are known to give permolybdic and per- 

 tungstic acids, H 2 Md 2 O 8 and H 2 W,>0 8 , which have the properties of true 

 peroxides, i.e. easily disengage iodine from KI and chlorine from HC1, 

 easily part with their oxygen, and are formed by the action of peroxide 

 of hydrogen, into which they are readily reconverted (hence they may 

 (be regarded as compounds of H 2 2 with 2Mo0 5 and 2WO 3 ), <kc. Their 

 formation (Boerwald 1884, Kemmerer 1891) is at once seen in the 

 coloration (not destroyed by boiling), which is obtained on mixing a 

 solution of the salts with peroxide of hydrogen, and on treating, for in- 

 stance, molybdic acid with a solution of peroxide of hydrogen (Pochard 

 1892). The acid then forms an orange-coloured solution, which after 

 evaporation in vacuo leaves Mo 2 H 2 8 4H 2 O as a crystalline powder, 

 and loses 4H 2 O at 100, beyond which it decomposes with the evolu- 

 tion of oxygen. 9trl 



Uranium, U = 240, has the highest atomic weight of all the 

 analogues of chromium, and indeed of all the elements yet known. Its 



9 bis We ma y conclude our description of tungsten and molybdenum by stating that 

 their sulphur compounds have an acid character, like carbon bisulphide or stannic sul- 

 phide. If sulphuretted hydrogen be passed through, a solution of a molybdate it doea 

 not give a precipitate unless sulphuric acid be present, when a dark brown precipitate of 

 molybdenum trisulphide, MoS 3 , is formed. When this sulphide is ignited without access 

 of air it gives the bisulphide MoS.^ ; the latter is not able to combine with potassium sulphide 

 like the trisulphide MoS 3 , which forms a salt, K^MoS^ corresponding with K 2 Mo0 4 . 

 This is soluble in water, and separates out from its solution in red crystals, which have a 

 metallic lustre and reflect a green light. It is easily obtained by heating the native 

 bisulphide, MoS 2 , with potash,,sulphur, and a small amount of charcoal, which serves for 

 deoxidising the oxygen compounds. Tungsten gives similar compounds, R^WS^ where 

 E = NH 4 , K, Na. They are decomposed by acids, with the separation of tungsten trisul- 

 phide, WS-5, and molybdenum trisulphide, MoS 3 . Eideal (1892) obtained W S N 5 by heating 

 WO 3 in NH 3 . This compound exhibited the general properties of metallic nitrides. 



9n-j When peroxide of hydrogen acts upon a solution of potassium molybdate well- 

 formed yellow crystals belonging to the triclinic system separate out in the cold. When 

 these crystals are heated in vacuo they first lose water and then decompose, leaving a 

 residue composed of the salt originally taken. They are soluble in water but insoluble 

 in alcohol. Their composition Is reprasented by the formula K 4 Mo 2 O s 2H 2 O. An am- 

 monium salt is obtained by evaporating peroxide of hydrogen with ammonium molybdate. 

 The following salts haice also been obtained by the action of peroxide of hydrogen upon the 

 corresponding molybdates : Na^MojOeeHjO in yellow prismatic crystals ; MgMo 2 O 8 10H 2 O 

 stellar needles; BaMo..O 8 2H 2 O in microscopic yellow octahedra. A corresponding 

 sodium pertungstate has been obtained by Pe'chard by boiling sodium tungstate with a 



