SENSITISING MIXTURE.
The following formula is as good as any, but it should be understood that between the multitude of formula available there is very little to choose. White sugar, 200 grs. ; gum ("rabic, 250 grs. ; ammonium bichromate, 200 grs. ; methylated spirit, 1 oz ; water, 20 or.. Every formula must contain a gum, a hygroscopic substance, and a sensitiser, and beyond this it does not appear to matter much. A hygro meter (see Fig. 303) in the room in which the work is done is an assistance but not es sential. The proportion of hygroscopic sub stance in the mixture may then be varied according to the atmospheric conditions indicated by the hygrometer. The gum and sugar, which must be pure and cf good quality, should be covered by the water and dissolved by heat. When thoroughly dissolved and cooled down, add the ammonium bichromate and the methylated spirit. The quantity
of spirit is not arbitrary, but as it is used to allow the solution to flow more evenly, it may be increased or re duoed as found necessary. The sensitive solution thus made will last only a few days, but without the addition of the bichromate it will keep indefinitely, es pecially if 2 grains of mercuric chloride are added. Any quantity may be kept made up, and then, to render the solution sensitive, it is merely necessary to add the ammonium bichromate. Other formulae are : Dextrine, 110 grs. ; sugar, 110 grs. ; bichromate of potash, 120 grs. ; water, 5 oz. Albumen, 30 grs. ; honey, 30 grs. ; ammonium bichromate, 50 grs. ; water, 5 oz. Honey, 30 grs. ; glucose, 60 grs. ; albumen, 50 grs. ; dextrine, 20 grs. ; am monium bichromate, 60 grs. ; water, 5 oz.