A good sample of gelatine will absorb a sufficient quantity of water to dissolve it when heated to 90 degs, Fahr., the solution setting to a jelly when cold. By the continued application of heat, gelatine loses its setting properties, a new compound, termed meta-gelatine, being said to be formed. It keeps indefinitely in a dry state, but when water is present it putrifies rapidly, the liquid becoming first acid and then alkaline, giving off ammoniacal vapors. To prevent speedy decomposition many antiseptics may be used. The principal are alum, alcohol, carbolic, salicylic, and boracic acid, thymol, hydrate of chloral, fuchsine, and zinc salts. Glycerine (in large quantities) will also act as an antiseptic. Alcohol will precipitate gelatine from its aqueous solution in white flakes. It is also precipitated by tannin, forming an insoluble, non-putrescible compound. By dry distillation gelatine yields a quantity of ammonium carbonate, and a vile smelling brown, oily substance, containing carbonate, sulphide, and cyanide of ammonium, aniline, methylamine, picaline, and a number of pyridine bases. When boiled with strong alkalies, or with diluted sulphuric acid, it yields succine and glycocine (sugar of gelatine).
The uses of gelatine in photography are innumerable. It is used in the manufacture of emulsions, its duty being to hold the silver salts in suspension, and enable them to be spread over plates, paper, etc. For emulsion work a good quality of gelatine is essential. There are many ways of testing it. Dr. Eder recommends that the gelatine be incinerated, and the ash weighed. This should vary from per cent, in good qualities, to 5 per cent. in inferior gelatine. When adulterated with alum the ash will sometimes reach io per cent. One of the best tests for gelatine is the amount of water it will absorb. This varies with good gelatine between 5 and io ten times its own weight. In the following table will be seen the percentage of ash obtained, and the absorbing powers of several of the best known gelatines : It will be seen by this that the amount of ash left after incineration does not exceed 2 per cent. ; also that the amount of water absorbed is in every case less than ten times its own weight. Another test is its solubility, as a gelatine which becomes soluble at too low a temperature would be quite unsuitable for emulsion making. When thoroughly soaked in cold water the tempera ture of the latter is gradually raised, and the degree of the thermometer noted when solution takes place. A further test must be made by noting the expansive power. This is important when plates or other substances are to be coated with it, as upon the expansive properties of the gelatine depends to a very considerable extent the frilling or non frilling of the film. The greater the expansion, the greater the possibility of frilling or the film leaving the glass.
Abney, in his work on emulsion processes, gives the result of his experiments in this di rection. He coated plates with the different gelatines, and dried them under the same conditions under which plates are dried in the drying box. The films were then stripped from off the plates, and different portions cut away, and accurately measured while dry. The films were
then allowed to swell in water, ammoniacal water, or a solution of mono-carbonates of soda and potash, and again measured. Results : The next important test is for acidity or alkalinity. This is easily done by test papers. For emulsion work a gelatine which is slightly acid is to be preferred.
Gelatine for photographic purposes is usually classed under three headings, i. e., soft, medium, and hard. This term refers to the strength of a solution of given quantities, as tested by the power required to crush the jelly formed when cold. This hardness generally varies in versely as the weight of water which the sample will afford. Soft gelatine usually absorbs water easily and quickly, requiring a low temperature to dissolve it, while a hard gelatine absorbs water but slowly and requires a greater temperature to cause its solution.
The quality of gelatine required for photographic purposes will depend considerably on the use to which it is to be put. For some purposes the commoner kinds can be used; while for others it will be necessary to employ only the fine sorts.
The property of alkaline dichromates to render gelatine insoluble and incapable of absorb ing water when exposed to the light is the basis of the carbon process, and also of a large number of photo-mechanical printing processes. This peculiar property was first observed by Mungo Ponton about the year 1839, and was utilized by Poitevin in 1855.
Gelatine is also a sensitizer. Silver bromide mixed in collodion does not possess nearly so much sensitiveness to light as if mixed in gelatine. Knopp, a German chemist, showed how gelatine is capable of combining with bromine, thus assisting the action of light in decomposing the silver bromide by attracting and uniting with the bromine given off from the silver salt under the action of light.
Other uses to which gelatine is put are based upon the fact that it is rendered insoluble by sub stances such as chrome alum without the action of light.
Gelatine dissolved in acetic acid forms a very useful substance, i.e., liquid glue, retaining its adhesive properties, but losing its setting powers.
It also dissolves cold in a solution of barium chloride, and also, but to a lesser degree, in strontium chloride. This discovery was made in 1890 by MM. Lumiere, who state* that with a solution of 15 per cent. of barium chloride the solubility is so great that sufficient gelatine may be dissolved to render the solution syrupy. The liquid keeps well and does not decompose. This property may be made useful in recovering residues from emulsion. The gelatine is dis solved in the chloride and diluted, when the haloid salts fall to the bottom of the vessel, and the liquid can be decanted. Bolton observed that I ounce of crystallized barium chloride dissolved in 6 ounces of water will readily dissolve 13o grains of Coignet's gelatine, the time occupied being about two hours, at a temperature of 65 degrees Fahr.
Corrosive sublimate, platinic chloride, cadmium bromide, chrome alum, formalin, and chlorine gas will cause gelatine to become insoluble.