M. TECHNOLOGY. 479 



quired to entirely supplant the silver process, as it was hoped 

 it would. A process recently proposed by Mr. Willis, how- 

 ever, is full of promise in this respect. In it the salts of pla- 

 tinum are employed, and the results are said to be equal in 

 all respects to those obtained by the silver process, while a 

 far shorter exposure to light is required. It rests upon the 

 fact that ferric oxalate is reduced to ferrous oxalate by the 

 action of light, and upon another fact, discovered by him, 

 that ferrous oxalate is soluble in neutral potassium oxalate, 

 and in this condition instantly precipitates metallic platinum 

 from a solution of its chloride. When, therefore, paper that 

 has been washed with a solution of ferric oxalate and pla- 

 tinic chloride is exposed under a negative to the light, fer- 

 rous oxalate is formed in proportion to the action of the 

 light, producing a very feeble image. By drawing the paper 

 then over a solution of potassium oxalate, the ferrous oxalate 

 passes into solution, and reduces the platinum salt in contact 

 with it to a degree corresponding to the action of the light, 

 giving rise to a strong, rich, velvety-black picture, which is 

 perfectly permanent under ordinary influence. The un- 

 changed ferric oxalate is then removed by immersing the 

 print in a Aveak solution of oxalic acid, and subsequently 

 rinsing it well in pure water. 



ETCHING PHOTOGEAPHS ON ZINC, IN RELIEF, FOE OEDINAEY 



PEINTING. 



Zinc plates that may be used with the ordinary printing- 

 press may be prepared from ordinary photographic negatives, 

 according to the following process given by Fichtner: A var- 

 nish, sensitive to light, is first made by dissolving five parts 

 of Syrian asphaltum in ninety parts of benzol, and adding ten 

 parts of oil of lavender, and then filtering. Much depends 

 on the quality of the asphaltum, and only such pieces as do 

 not melt at 234, and dissolve with difficulty in oil of turpen- 

 tine, should be selected. The benzol must be entirely freed 

 by distillation from substances sensitive to light, and must 

 also be entirely free from Avater. The oils also must be pure, 

 and contain no water. A pure zinc plate coated with this 

 varnish, and allowed to become perfectly dry in a dark room 

 on a level surface, is exposed from twenty-five to thirty min- 

 utes in the sunliofht under a neojative Avhich has been trans- 



