CHROMIC ACID AND CHROMATES. Chromic acid is a teroxide of chromium Cr. : and resembles ferric acid Fe. The com pounds of iron and chromium are, indeed, generally so similar in their reactions, that we might almost expect them to act, photo graphically, in the same manner. Chromic acid is more stable than ferric, and peroxide of chromium than peroxide of iron : chromium, therefore, appears to have the stronger affinity for oxygen. Ferric acid is much more easily decomposed than chromic, though it is very difficult to keep either in the free state : the former, indeed, can not be isolated, for it splits up into and 0. It is only in combination with very strong bases, such as potassa, that it can exist. Chromic acid, on the other hand, will form salts with a great number of bases, and, even, with such a weak base as : but these salts are more easily decomposed in proportion to the weakness of the base : the acid then, like the ferric, splits up into sesquioxide and oxygen. The strong bases generally will combine with either one or two equivalents of chromic acid, but the second equivalent is extremely liable to undergo the same change as free acid. Chromic s acid is decomposed instantly by the contact of organic matter, which takes the oxygen, and it may be, therefore, employed as a bleaching agent, and, as this change is favoured by light, it may be used in a great variety of ways photographically : the principle of the opera tions being the exaltation by light of the affinity of oxygen for car bon. Oxygen and carbon are thus affected in a degree only second t,o chlorine and hydrogen. Alcohol is sufficiently active to decom pose chromic acid : it is itself oxydized into aldhehyde and forthic acid, which forms formic ether. This unstable body will not even allow itself to be filtered through paper without reduction.
There is a strong analogy between the photographic processes by means of the salts of chromium, iron, uranium, and aluminium, and the operations of dyeing. These salts are in both cases decomposed by the organic matter to be printed on, and the reduced oxide, which combines with the oxidized organic fabric, becomes a basis on which other oxides will subsequently adhere. The dyer, even, becomes at times altogether a photographer, when he makes use of the sun to impart greater brilliancy to his colours than their unaided affinities would produce. And the photographer becomes a dyer,
when, having reduced his oxides on Ms paper by the light, he pro ceeds to deposit on them, as a mordant, other oxides by means of nitrate of silver, chknide of gold, ferrocyanide of potassium, and other substances used as developers. The colour of the dye is also often developed in a way precisely analogous. The mordant oxide appears to be such a one as can play the part of both base and acid, and hold out a hand both to the organic matter on the one side and the metallic oxide on the other.
It is often difficult to point out the exact formula which takes place when the salts of chromium and iron are reduced photo graphically and afterwards developed. When bichromate of copper, for instance, is decomposed, the reaction would be easy to under stand if the sesquioxide formed were immediately precipitated in the solid state. But this oxide appears to form with the elements of the undecomposed compound complex double salts. The sesqui oxides of iron, chromium, aluminum, manganese, are isomorphic, and form aluminoid salts with sulphuric acid and potassa of a com plex kind : and such aluminoid double salts, with some slight modifications, appear to be formed in the decomposition of the bichromates. Chromic acid salts are isomorphous with the sul phates, and double chromates and chromosulphates are easily formed. When a neutral chromate is deoxidized, the decomposition is, pro bably, first into subchromate, sesquioxide of chromiunt, and oxygen, and, perhaps, afterwards, entirely into sesquioxide and protoxide ; but, when a bichromatic solution is used, these double aluminoid compounds are probably first formed. The first effect of light upon them is to deepen the colour, because of the formation of sesqui oxide; but further action often bleaches this colour, and this, most likely, indicates the formation of a double salt, or of a salt composed of sesquioxide as the acid in the place of chromic acid. The action upon a developer is in both cases the same in kind, and depends upon the developer used : with nitrate of silver, chromate of silver appears to be formed in the parts NOT changed by light, and contain ing the original chromate, and with ferrocyanide of potassium com pounds resembling Prussian blue, but containing chromium, in place of iron, as the base. Chromic acid will destroy a photograph more quickly than nitric.