412 Dynamic Theory. 



solves the salts not affected by the light but leaves the parts that have 

 been. Observe the mechanical difference effected by the sunlight in 

 the molecular arrangement of this compound. 



Gelatine is an organic product soluble in water, as is well known. 

 But if it be dissolved with chromate of potash a compound is formed 

 which when dried and exposed to sunlight is tanned and rendered in- 

 soluble in water. To take advantage of this property the chromate of 

 potash and gelatine or glue must be dissolved together in the dark. 

 Paper or other objects coated with a film of this compound and allowed 

 to dry, still in the dark, may be used in taking pictures or silhouettes. 

 The parts of the film which receive the rays of the light are rendered in- 

 soluble by the union in a new compound, of the gelatine and the chro- 

 mate of potash, which union does not take place till the latter is torn 

 apart by the action of the light. As long as they remain in the dark, 

 although so near each other, they will not unite. The parts of the film 

 not thus affected by the light are still soluble in water and are washed 

 away in order to produce a picture. A mixture of acetic acid and nitric 

 acid, which will eat steel, does not affect this tanned film, so that a pic- 

 ture made on a steel plate by this process can be eaten into the steel for 

 engraving purposes. 



The susceptibility of chromate of potash to light enables it to be used 

 with aniline, under the influence of various acids, to produce a variety of 

 colors. The aniline printing, invented by Willis, depends on the prop- 

 erties of chromate of potash. A curious use of the insoluble property 

 of the tanned solution of chromate of potash and glue, was made by the 

 Germans in 1870. Their army was largely fed on " pea-sausage, " an 

 article that is stuffed into pigs' intestines for safe keeping. The large 

 demand for the food caused the supply of the natural cases to give out, 

 and they resorted to artificial ones. These were made of blotting paper 

 dipped for a second into sulphuric acid, then washed and dried, a pro- 

 cess which makes it tough and impervious to water. This paper was 

 cut into sheets and folded into cylinders, and the edges pasted together 

 with a paste made from gelatine prepared with chromate of potash, as 

 mentioned above. After being pasted, when exposed to the action of 

 light the cement becomes insoluble, and the artificial tube, with its con- 

 tents, can be boiled without coming to pieces. 



One of the processes of photo-lithography, is to cover the stone ( or 

 zinc plate, in zincography ) by sprinkling with a solution of asphalt in 

 ether, allowing it to dry in the dark, and then placing a negative over 

 it and allowing it to be exposed to light. The asphalt becomes insolu- 

 ble on the exposed places, and is retained upon treating the stone with 

 ether or benzine. If the stone is then damped, the moisture only pene- 

 trates where no asphalt covers the stone. On rolling it, after this, with 



