directions for making the same : - Powder the ordinary com-mercial iodo-cyanin; place in a porcelain or platinum dish, and cover with some water and hydrochloric acid; evaporate to dryness with constant stirring ; the mass thus obtained is again wetted with hydrochloric acid, and again evaporated, when hydriodic acid is given off and chloro-cyanin is left behind. By weighing the capsule containing the cyanin before and after this treatment one may dissolve the mass at once in alcohol and keep in the dark for use.
Cyanogen, Cyanogen Soap. Cyanogen is a compound radical, CN or (CN)2, entering into the composition of the cyanides. Cyanogen soap, a soap containing potassium cyanide, sold for removing silver stains from the hands.
Cyanotype. Synonyms: Negative Cyanotype, Ferroprussiate, or Blue Process. A process discovered in 1842 by Sir John Herschel. It is called negative cyanotype, because it produces copies of engineers' or architects' plans with white lines on a blue ground, the action being the reduction of a ferric salt by light to the ferrous state, and the precipitation of Prussian blue by the action of ferridcyanide of potassium. It is a very useful printing process for amateurs on tour as very little need be taken, and smooth writing-paper may be used as a basis. Herschel's formula was
Potassium ferridcyanide ......... 8 parts.
Water ............... 50 ..
Ammonio-citrate of iron ...... ... 10 parts.
Water............... 50 ..
The solutions are mixed in equal parts, filtered, and kept in the dark, and will keep for some little time. The ferridcyanide should be just rinsed with water to clean it from any adherent powder, and the solution should be of a yellowish or orange-red colour, and not greenish blue ; if the latter colour, it should be thrown away and fresh mixed up, or heated and a few drops of bromine water added. Latimer Clark suggested the following improved formula : -