126 SCIENTIFIC APPARATUS. 



compounds, such as gelatine or gum, dried, and be then exposed 

 to light, a singular change takes place ; the chromic salt gives up a 

 certain proportion of its oxygen to the organic bodies, and the 

 new compound thus formed becomes insoluble in water. If paper 

 be coated with a mixture of gelatine, pigment of any kind, and 

 potassium dichromate, and be exposed under an ordinary photo- 

 graphic negative, the gelatine will become insoluble to a depth 

 dependent on the intensity of the light passing through the va- 

 rious portions of the negative, nearly all the surface of the gelatine 

 next the glass becoming insoluble. If the paper thus prepared 

 and exposed were placed in hot water the soluble gelatine impri- 

 soned between the paper and the exterior insoluble surface of the 

 film could not be washed away, an artifice is therefore necessary 

 to enable the picture to be developed. This consists in caus- 

 ing the film to adhere to a temporary support, placing it in warm 

 water, and then peeling away the paper. All the gelatine not in- 

 soluble can now be washed away, and the picture is fully developed. 

 When thus finished it can again be transferred to paper, or can be 

 left on the support ; in the latter case, in order to get a picture in 

 proper position, the negative must be what is called " reversed." 

 In this class of printing the image is formed of gelatine, and the 

 intensity is mostly dependent on the character of the pigment it 

 contains. The autotype process is an example of this kind of 

 photographic printing. If an image formed of gelatine be sup- 

 ported on a thin homogeneous film, an impression of it can be 

 taken on a soft metal plate, as in " nature printing." When 

 coloured liquid gelatine is run into the mould thus formed, and 

 paper backed by a perfectly flat hard plate is brought down on its 

 surface, the excess of coloured material is squeezed out. The 

 paper can be removed, bearing the "set" gelatine image with it. 

 Such is a bare outline of the Woodbury Process, specimens of 

 which, with a gelatine film and mould, are shown. 



There has been a further extension of this action of the dichro- 

 mates of the alkalies on gelatine, during the last few years. 

 Not only does the gelatine become insoluble when acted upon by 



