302 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



covered the glass plate was formed of a liquid composed in the fol- 

 lowing manner : albumen, obtained by beating white of egg to the 

 consistency of snow ; iodide of potassium, 1 per cent. ; water, 25 per 

 cent. The glass, covered with a very even coating, is put to dry in the 

 dark, and this requires nearly a whole day. It is then immersed in 

 a solution of aceto-nitrate of silver, and a plate is ready prepared to 

 the action of the light. From fifteen to thirty seconds are a sufficient 

 exposure. 



The negative proof being thus obtained, we take positive proofs 

 from it, as described above. These proofs being on paper, we again 

 have the inconvenience of the grain, but with this considerable 

 difference, that, the positive proof alone being taken on it, the delicacy 

 of contours, features, and shades is less damaged, the negative picture 

 being perfect in all its details. Again, nothing prevents our avoiding 

 this inconvenience, by taking the positive proofs on albuminized glass. 

 This is done more particularly for stereoscopic pictures, transparency 

 being essential in using the stereoscope with transmitted light. 



III. PHOTOGRAPHY ON COLLODION. 



Schoenbein discovered in 1846 a substance which attracted a 

 large share of scientific and public attention. It was thought for a 

 time that this substance, known as gun-cotton, or pyroxyline, would 

 entirely replace ordinary gunpow T der. Pyroxyline is prepared in a 

 very simple manner, by steeping carded cotton in nitro-sulphuric 

 acid, washing it in water and drying it in the air. It is soluble in a 

 mixture of alcohol and ether. 



This solution, which is used in surgery and medicine, is named 

 collodion. An English photographer, Mr. Archer, suggested, in 1851, 

 the substituting of collodion for albumen in preparing the glass 

 plates for the negative proofs. Albumen and collodion play the 

 same parts ; but the pictures made by the latter process require even 

 less exposure, and the effect may be produced almost instantaneously. 

 Hence the possibility of reproducing views containing animate 

 objects, of seizing the rapidly- vary ing expressions of physiognomy in 

 portraits, of representing bodies in motion clouds scudding before 

 the wind, the waves in a rough sea, and the like. The processes in 



