ACETATE OF SODA. The crystals are Na 0, Ac 03, 6 H 0 = 137. This is the most convenient acetate for the photographer, and he may make it by saturating aommon washing soda (carbonate of ' soda) with acetic acid, or vinegar, and afterwards crystallizing and recrystallizing. It is important also as the sourc,e of pure acetic acid.
Acme Acrn. Ac 03 H 0 ; or C, H3 03, H 0 = 60. This acid is used in the developer in the calotype, wax-paper, albumen and collodion processes, to check the spontaneous decomposition and too rapid action of the developing compound, to keep the lights clean, and also to make the developer flow evenly and bite uni formly into the film ; it is used in most solutions of N. S. which are intended to excite iodised surfaces on paper, albumen, collodion, and other vehicles, and wherever N. S. is employed it is more or less useful : it is added to gallic acid to lessen its tendency to become mouldy : it has been used as a solvent of xyloidine, to form a fibn, and is altogether of very extensive and important ser vice in Photography. It is therefore of moment to get it pure and understand its action ; and it is of the more importance to know what ends it answers, because it iS so generally useful, that it has been recommended in cases where it would be injurious, as in the N. S. bath for collodion positives, where all organic matter, whe ther acid or otherwise, should be avoided, and where nitric is the proper acid ; and also in collodion, which should be preserved in a strictly neutral condition, except in special cases. In order to see the effects it produces, let a person try a collodion negative from a bath made of commercial nitrate of silver, smelling strongly of nitric acid, yet otherwise pure : the resulting imago is slowly impressed, feeble in character, and blue in tone, but clear and distinct. The addition of carbonate of soda, until slight alkalinity is produced, makes the picture much redder, much quicker in its formation, with much fog and but little increase of strength. Acetic acid in slight excess of the alkali now shows itself, by restoring clearness entirely, by reproducing slovrness and blue tone in a very slight degree, and at the same time by a marked improvement,in vigour and beauty : and up to a certain point every drop increases these effects ; is remarkable how little the sensitiveness is impaired. Its effect in development is best studied by comparing two solutions of N. S. mixed with pyrogallic acid, one containing acetic acid, the other containing none, and observing how much the acid retards the blackening which such solutions undergo.
Other acids exercise an influence similar to, but not identical with, that manifested by acetic.
Acetic acid is formed, under the name of vinegar, by the ferment ation of malt liquors, and of wines and spirits : it is also pro duced in the distillation of wood, when it is called pyroligneous acid.
In both cases it is contaminated with organic matters, which colour it, and, in wood vinegar, make it empyreumatic and tarry. If added, in these states, to N. S., the nitrate is discoloured by light, and a black precipitate gradually formed. The pure acid is obtained from these vinegars by saturating them with oxide of lead, carbonate of soda, or similar substances, crystallizing out the pure acetates, and submitting the pure salts to distillation with sulphuric or hydrochloric acid. The acetic is driven off by the stronger acid, a portion of which is apt to go over with it : and when this is the case, the acetic acid, when added to nitrate of silver, will throw down chloride or other salts of silver, and weaken the solu tion ; in development the same cause will weaken or even arrest the appearance of the image. It should therefore be tried with N. S., and with chloride of barium, before use.
The strength of pure acetic acid is very variable, and cannot be determined by its specific gravity. The most constant is the " glacial," which below 40° of temperature becomes solid. When it does not solidify, the only means of knowing its strength is by observing the quantity of crystallized carbonate of soda which is necessary to neutralize it. The dilute acetic acid of the London Pharmacopoeia, sold by the druggists, should be such that one fluid ounce saturates 57 grains : the acetic acid fortior, of the Phar macopoeia, or the acid called Beaufoy's, should take 390 grains to the fluid ounce ; and the glacial requires 1036 to the ounce, or 129 to the fluid drachm. When pure, any of these may be used by calculating from these numbers the equivalent measures. The num bers 1, 7, and 18, are near enough ; but it would be better if a standard acid were used by photographers, easily verified by a standard solution of carbonate of soda.
The disadvantages attending the employment of this acid are with attention. As used, it is 50 times as dear as citric; it is liquid and bulky, and inconvenient in travelling; its fumes are inju rious to the brass work of the apparatus, and unpleasantly strong,.
and it is of very uncertain strength. It is apt, by solvent action on the size of paper, to render it absorbent, and to cause the picture formed on it to " sink" and lose brilliancy. And it is apt, when present with alcohol, as in the N. S. bath and collodion, to form acetic ether, which injures the transparency and strength of the film, mars the sensitiveness of the plate, and impairs the detail of the image. On these accounts citric acid may possibly supersede it.