CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, ETC. 303 



As the salts of uranic oxide are reduced in the absence of organic 

 matter by the action of light, and as they impart a characteristic 

 coloration to glass, 15 they find a certain application in photography and 

 glass work. 



If we compare together the highly acid elements, sulphur, selenium, 

 and tellurium, of the uneven series, with chromium, molybdenum, 

 tungsten, and uranium of the even series, we find that the resemblance 

 of the properties of the higher form RO 3 does not extend to the lower 

 forms, and even entirely disappears in the elements, for there is 

 not the smallest resemblance between sulphur and chromium and their 

 analogues in a free state. In other words, this means that the small 

 periods, like Na, Mg, Al, Si, P, S, Cl, containing seven elements, do 

 not contain any near analogues of chromium, molybdenum, &c., and 

 therefore their true position among the other elements must be looked 

 for only in those large periods which contain two small periods, and 

 whose type is seen in the period containing : K, Ca, Sc, Ti, V, Cr, Mn, 

 Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br. These large periods contain 

 Ca and Zn, giving RO, Sc, and Ga of the third group, Ti and Ge 

 giving R0 2 , V and As forming R 2 O 5 , Cr and Se of the sixth group, 

 Mn and Br of the seventh group, and the remaining elements, Fe, 

 fclo, Ni^ form connective members of the intermediate eighth group, to 

 the description of the representatives of which we shall turn in the 

 following chapters. We will now proceed to describe manganese, 

 Mn = 55, as an element of the seventh group of the even series, directly 

 following after Cr 52, which corresponds with Br=80 to the same 

 degree that Cr does with Se = 79. For chromium, selenium, and 

 bromine very close analogues are known, but for manganese as yet 

 none have been obtained that is, it is the only representative of the 

 even series in the seventh group. In placing manganese with the 



dominate in the form R0 5 , whilst the lower forms are green and blue. (7) Zimmermann 

 (1881) determined the vapour densities of uranous bromide, UBr 4 , and chloride, UC1 4 

 (19'4 and 13'2), and' they were found to correspond to the formulae giver* above that is, 

 they confirmed the higher atomic weight U = 240. Roscoe, a great authority on the 

 metals of this group, was the first to accept the proposed atomic weight of uranium, 

 U = 240, which since Zimmerraann's work has been generally recognised. 



15 Uranium glass, obtained by the addition of the yellow salt K 2 U 2 7 to glass, has a 

 green yellow fluorescence, and is sometimes employed for ornaments ; it absorbs the 

 violet rays, like the other salts of uranic oxide that is, it possesses an absorption spec- 

 trum in which the violet rays are absent. The index of refraction of the absorbed rays 

 is altered, and they are given out again as greenish-yellow rays ; hence, compounds of 

 uranic acid, when placed in the violet portion of the spectrum, emit a greenish-yellow 

 light, and this forms one of the best examples (another is found in a solution of quinine 

 sulphate) of the phenomenon of fluorescence. The rays of light which pass through 

 urauic compounds do not contain the rays which excite the phenomena of fluorescence 

 and of chemical transformation, as the researches of Stokes prove. 



