VANADIUM (Symbol V; atomic weight 51.2).—The sensitiveness to light of the vanadium compounds, which the Brothers Lumiere have so thoroughly studied, and which Gibbons records in the Chemical News, 1874, p. 267, has, I believe, not much practical importance in connection with positive printing. Those vanadates which I myself investigated (Photo Archiv, 3893, p. only show sensitiveness on unsized paper, none on sized paper.
An aqueous solution of these salts may, however, be employed in actinometry. If a con centrated aqueous solution of vanadate of ammonia is mixed with its own volume of a 25 per cent. aqueous solution of tartaric acid, we obtain a clear, reddish-yellow fluid which undergoes no change in the dark, but on exposure to sunlight becomes dark olive, this occurring in about twenty minutes. During the next ten minutes the fluid becomes green, and finally blue, these stages being quite definite, and requiring no analytical means for indicating them. Should, how ever, a chemical reaction be required for tracing the changes, the following may be taken advan age of. The unexposed mixture of vanadate and tartaric acid gives, with diamidophenol, a solu ble intensely brown-red coloring matter, and with hydrochlorate of paramidaphenol an intense violet color. When the vanadate mixture has been brought to the blue stage by exposure, these reactions do not take place, the mixture remaining almost colorless—or, at any rate, only slightly blue. Titration with permanganate of potassium may be adopted as a means of quantitatively determining the extent to which the light has acted.
A mixture of the vanadate of ammonium and citric acid is much less sensitive to light than the mixture containing tartaric acid, and the addition of alcohol still further reduces the sensitive ness. Mixtures prepared with sulphuric acid or with hydrochloric acid are insensitive.t VARNISHING.—Negatives from which a number of prints are required or which are in tended to be kept for probable future use should be varnished with a protecting coating of a lac solution. Suitable varnishes for all kinds of work will be found under Varnish.
The negative should be varnished as soon as it is quite dry, and should be handled as little as possible previous to the operation. Touching the surface of the film makes it repellent to the varnish, and consequently it is more difficult to get an even flow over the surface.
Negatives from which extra large quantities of prints are required should be given a pre liminary coating of plain collodion.
Although the operation of coating a negative with varnish may seem simple enough, yet few manage it properly without considerable practice. First of all the varnish should be quite clean, and kept free from impurities, and before putting it on the plate should be carefully wiped over with a broad camel-hair brush. Another important consideration is that the room where the coating takes place be quite free from dust, as it is astonishing how soon particles of all sorts of matter floating in the air settle on the sticky plate. The first operation is to gently warm the negative, but not by any means to make it too hot. A little of the varnish is then poured on about the right hand top corner, and with a little manceuvering this pool can be made to run over the whole of the plate, and the surplus run off into a separate bottle kept for the purpose. The plate is then rocked in front of a clear fire or a vertical gas stove. If a pneumatic holder be used the whole of the plate can be coated, but if not, the bottom left hand corner is held by the ball of the thumb, and a small corner left uncoated.
Several methods of varnishing have been recently introduced, with which no heat is required.
Although a coating of varnish protects the sensitive gelative film from the influences of moisture, yet it is no proof against rain spots or drops of water, which, if not quickly re moved, will soon work their way through the varnish and destroy the film.