FERROTYPES AND PLATINOTYPES. 375 



a sensitive paper is obtained, which only requires washing with 

 water to develop the print after exposure. Or the paper may be 

 soaked in ferric ammonium citrate solution alone, and developed 

 after exposure with a solution of ferricyanide of potassium. In 

 this way ferrotype copies may be readily obtained of all objects 

 capable of being printed from in silver, as in Expt. 392. 



Expt. 397. Platinotypes. The colour of the iron prints 

 described in the last experiment renders them inapplicable for many 

 kinds of photographic purposes, especially portraiture and views. 

 On the other hand, the want of permanence frequently exhibited 

 by silver prints renders it extremely desirable that some more 

 stable material should be obtained. One means of overcoming 

 this difficulty is by the use of chromatised gelatin (Expt. 398) ; 

 another consists in treating paper with materials such as will 

 cause the deposition of metallic platinum instead of silver in the 

 print. One of the most successful of these platinotype processes 

 consists of impregnating suitable paper with a mixed solution of 

 ferric oxalate and platinous chloride and drying thoroughly ; addi- 

 tion of a little potassium chlorate to the fluid improves the result. 

 On exposing behind a negative of the ordinary kind the ferric 

 oxalate becomes reduced to the ferrous state, as in the last experi- 

 ment ; when the exposed picture is developed by treating it with 

 a hot solution of potassium oxalate, the ferrous oxalate formed 

 reacts on the platinous chloride somewhat as ferrous sulphate 

 on gold chloride in Expt. 135, setting free particles of metallic 

 platinum. The iron compounds are partly removed from the 

 paper by the solvent action of the hot developing fluid, partly 

 by subsequent treatment of the print with dilute hydrochloric 

 acid, so that finally the paper retains the print simply as platinum 

 particles. Prints thus prepared are said to be as imperishable 

 as the paper, and not in any way liable to the fading observed 

 with silver prints. By suitable modification of the process, varia- 

 tions in colour and tone can be produced within certain limits. 

 By printing a platinotype picture on to the surface of a glass or 

 porcelain plate, and then applying heat in such a fashion as to 

 burn away all organic matter from the film supporting the particles 

 of reduced metal and vitrify or fuse the surface of the plate, a 

 photo enamel picture is obtainable of imperishable nature, so far as 

 fading is concerned, the kind of action which produces fading of 

 an ordinary silver print being here impossible. 



Expt. 398. To take Photographs by the Chromatised Gelatin 

 Process. Besides compounds of silver and iron, many other kinds 

 of metallic derivatives are capable of being similarly affected by 

 radiant energy in such fashion that chemical changes are brought 



