_Chrome tannages_, from a practical point of view, stand on a very different footing to those which have just been mentioned; having established their position in the manufacture of almost all sorts of light leathers, in competition with all the older methods, and making a serious claim to a share in the production of belting and even of sole leathers. Chromium is a grey, and very infusible metal, which chemically much resembles iron in its compounds, and has an atomic weight of 52, or a little over. Like iron, it possesses a divalent and a trivalent form, but the divalent has so strong an affinity for oxygen, and passes so readily into the trivalent form, that until easier means are found for its preparation, it is of little practical interest. Its salts are blue. On the other hand, salts of the trivalent form, corresponding to the ferric salts of iron, are very stable, and powerful tanning agents. They are mostly green, but violet modifications are known, corresponding to the violet crystals of iron-alum, but of a much deeper tint. There is also a hexavalent form, probably corresponding to that of iron in the unstable ferrates, but in the case of chromium, of considerable stability. Its oxide is chromic anhydride, CrO₃, commonly called chromic acid, which combines with bases, and especially with the alkalies to form yellow or orange-red salts, and the anhydride itself is almost crimson in the solid form, though dissolving to orange or yellow solutions. Chromic acid though it hardens and preserves animal tissues, has no tanning properties till it becomes reduced to chromic oxide. There is also a higher, but very unstable oxide, perchromic acid, possibly corresponding to persulphuric acid, which is soluble in ether to an intensely blue solution. The name chromium is derived from the intense colour of many of its compounds. Our supplies of chromium are derived from chrome-iron-ore, a mineral which contains oxides both of chrome and iron. This is furnaced with a mixture of lime, and soda or potash, when it absorbs oxygen from the air, the chromium becoming converted into chromic acid which combines with the alkali present, while the iron remains undissolved as ferric oxide. Lixiviating the mass, and evaporating the solution, lime and potassium or sodium chromates are obtained, according to the alkali used, and on adding sufficient sulphuric acid to combine with half the base, potassium or sodium dichromate (or as it is commonly called “bichromate”) can be crystallised out. Potassium dichromate is most commonly made, because it crystallises well, and is not deliquescent, but sodium dichromate is somewhat cheaper, though less convenient. Dichromates, at least in the crystallised state, are not hydric salts like bisulphates, but anhydrochromates corresponding to the potassium anhydrosulphate obtained by fusing ordinary bisulphate, and to fuming sulphuric acid. Thus the formula of potassium dichromate is { CrO₂OK { O , { CrO₂OK