C.-FOSITIVE PHOTOGRAPHS BY MEANS OF IIYDRIODIC SALTS.
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Muriate of Baryta,....Sol. Chloride of Lime,Sol. Chloride of Soda,Iodide of Potassium, ...
Chlorate of Potash,...
Phosphate of Soda,...
Tartrate of Soda,.
Urate of Soda.
Bromide of Sodium.
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Colour of Picture.
A rich brown, inclining to purple.
Very red.
A brick red.
Yelloivish brown.
Variable, sometimes yellowish, often of asteel blue.
Mouse colour.
Dark brown.
Yellowish brown.
Deep brown , which blackens.
Red brown, of a peculiarly rich tint.
The change mentioned in the colour of the finished picture is thatwhich arises from a fresh exposure to the solar rays; where no changeis mentioned, it is too slight to be worth notice. This phenomenon willpresently occupy our attention.
When papers prepared with any of the above, except the phosphates,are soaked for a little time in water, and dried in the sunshine, thepicture produced, it matters not what hydriodate is used, is renderedpeculiarly red, and does not change by re-exposure. By washing someof the papers with weak solution of ammonia, this peculiarity is pro-duced in a very striking manner.
The Solution of Silver. Take of crystallized nitrate of silver 120grains, distilled water 12 fluid drachms, when the salt is dissolved, addof alcohol 4 fluid drachms, which renders the solution opaque; after afew hours, a minute quantity of a dark powder, which appears to be anoxide of silver, is deposited, and must be separated by the filter. Theaddition of the alcohol to the solution was adopted from an observationI made of its influence in retarding the chemical action of the hydrio-dates on the salt of silver, which goes on in the shade. Its use istherefore to make the action depend more on luminous influence thanwould be the case without it.
Nitric Ether. The swoet spirits of nitre not only checks the bleachingprocess in the shade, but acts with the hydriodic salts to exalt the oxid-ation of the silver, or increase the blackness of it In copying laco orany fine linear object, it is a very valuable agent but it is useless forany other purposes, as all the faintly lighted parts are of the same tint.
Hydrochloric Ether, used as tlio solvent of the silver, and appliedwithout any saline wash, has a similar property to the nitric ether, butas it is readily acted on by faint light, it is of greater value. However,papers prepared with it must be used within twenty-four hours, as afterthat they quickly lose their sensitiveness, and soon become nearlyuseless.
To fix with any degree of certainty the strength of the solution of thehydriodic salts, which will in all cases produce the best effects, appearsto me impossible; every variety of light to which it has been exposedto darken, requiring a solution of different specific gravity.