of the Elements. 489 



of the oxygen to be 32. From the isomorphism of the sesqui- 

 oxide of chrome with the above sesquioxides, we should expect 

 to find that the atomic volume of its combined oxygen would be 

 also 32. Thus with sesquioxide of iron we have 



1000 ^ ..^ 



32x3 + 44x2 "^ ' 

 and consequently with sesquioxide of chromium should have 

 956-5 ^ g.„ 



32x3 + 44x2 ^ ' 

 Kopp, however, adopting the number 69 for chromium, is forced 

 to accord an atomic volume to the oxygen of chromic oxide, 

 different from that of the oxygen of the analogous ferric and 

 cobaltic oxides. Thi;s — 



— 956-5 ^ 5.^4 



16x3 + 69x2 

 The experimental specific gravity of chromic oxide is 5-21 

 (Wohler). Kopp appears to have assigned tlie number 69 to 

 chromium on account of its analogies with molybdenum and 

 tungsten, elements which likewise have the number 69 (or 

 66, Miller) as the expression of their atomic volumes. But in 

 the most similar of all the compounds, namely in the tungstates 

 and chromates, we certainly do not find the same atomic volume. 

 For instance, Kopp is obliged to assign the number 340 to the 

 chromatc of lead, and 358 to the tungstate of that metal. 



Chromous salts correspond generally to ferrous salts, parti- 

 cularly in their absorption of nitric oxide, with development of 

 a dark brown colour. They are of a red or bluish colour like 

 cobaltous salts. Chromous hydrate, like ferrous hydrate, is of 

 a pale colour when pure, and soluble in ammonia, but it rapidly 

 peroxidizes. The double sulphate of chromium with potassium is 

 isomorphous with the corresponding ferrous compound, CrKSO'*, 

 6H^0 with FeKSO"*, 6H-0. Chrome iron is isomorphous with 

 magnetic oxide of iron, FeCr^O'- with Fe'^0^. The sesquioxide 

 of chromium is isomorphous with sesquio'xide of iron, and chrome 

 alum with iron alum, &c. Chromic acid corresponds to man- 

 ganic and ferric acids, than which it is much more permanent. 

 Perchromic acid corresponds to permanganic acid, than which it 

 is much less permanent. Chromium, chromic oxide, and chromic 

 salts occur in allotropic modifications. One form of clu'omium 

 resists oxidation and solution in acids, but altogether chromium 

 has a greater tendency to the formation of highly oxidized com- 

 pounds than has either manganese or iron. Metallic chromium 

 is not magnetic. Iron, manganese, and chrome compounds 

 present no marked discrepancies. 



Phil. May. S. 4. No. 89. Bupjd. Vol. 13. 2 L 



