Taking a small test tube we half filled it with a ten per cent. solution of gelatine. When this had solidified we poured into the top a small quantity of formalin solution. In a few hours this had penetrated to the depth of nearly half an inch, its action being apparent by its clarifying effect which is peculiarly remarkable and worthy of more attention.
This penetrating power gives us the reason why its action upon the film is so rapid, and why a very dilute solution can be employed.
Gelatine acted upon by formalin and rendered insoluble is termed "formalin gelatine." In a recent number of the Therapist is an article in which the substance is recommended for micro scopic preparations of bacteria. The bacteria colonies are developed in the gelatine and fixed by the application of formalin in solution or by the vapor. By this method the cultures maintain their natural appearance, and staining can be carried out as usual, as the formalin does not attack the coloring solution. The gelatine plate—laid in a dilute, aqueous fuschin solution for twenty four hours, acquires a delicate rose tint, while the bacteria are stained a deep red.
As a substance of service to the photographer. formic aldehyd, says the British Journal of Photography, is likely to attract further notice. It has already been shown what a powerful tanning it exerts upon gelatine, and a printing process has now been devised and patented by the Chemische Fabrik auf Actien, of Berlin, in which this property of formic aldehyd is utilized, in conjunction with another well-known property, and one common to all the alde hyds, viz., that of readily taking up oxygen from any substance capable of employing it, such substances, for instance, as ferric salts. The process consists in treating gelatine films, which have been rendered insoluble by means of formic aldehyd, with a solution of a ferric salt. On
exposure to light under a negative, the ferric salt is reduced to the ferrous state, and the oxygen disengaged in the process is transferred to the formic aldehyd, converting it into formic acid, with the result that, where this oxidization of the aldehyd has taken place, the gelatine film is rendered soluble again. By this means, therefore, a positive image is obtained from a positive on development by washing.
A modification of the process is also patented, whereby a positive print may be obtained from a negative. To this end the formic aldehyd is first converted into its so-called " sulphite " a compound which does not render gelatine insoluble. A gelatine film previously treated with this sulphite, and impregnated also with a ferric salt, is exposed to light, the result being that, in the parts where light is transmitted, the sulphite is oxidized through the agency of the reduction of the ferric salt, and the formic aldehyd is thus liberated with the subsequent reduction of in soluble gelatine in these same portions.
According to Mr. Roy negative films, after being hardened with formalin, can be readily stripped from the glass without unequal contraction or enlargement. The plate is first treated with a rather strong formalin bath, and the film is cut through at the edges, when the plate is immersed in a 2o per cent. solution of sodium carbonate, and then, without rinsing, in 5 to to per cent. hydrochloric acid. Carbonic acid is now disengaged under the film and it floats off. It is convenient to receive it on collodionized glass and then to collodionize the other side, and when removed from this glass the pellicular negative is quite flat and convenient for handling.*