10 LECTURES TO SCIENCE TEACHERS. 



We will now take from the water our developed print and 

 fix it in a solution of sodium hyposulphite ; after washing 

 it will be permanent, or nearly so. 



Ten years after the patenting of the calotype by Fox Talbot, 

 a new era arrived in photography. In 1851 was published 

 the collodion process a process which we use to the present 

 day, and one which there seems to be no chance of super- 

 seding, at all events for ordinary work. In the calotype 

 pictures the surface of the paper was found to be too rough 

 to render fine details, and at an early period of experimental 

 photography Sir J. Herschel had suggested the employment 

 of glass as a substitute, and in fact himself had produced 

 pictures on it, for in our Exhibition we find such a picture 

 taken as early as 1839. The method he adopted was to 

 obtain a fine precipitate of silver chloride in water, and at the 

 bottom of the containing vessel to place a glass plate. 

 After a lapse of some time the chloride was deposited with 

 sufficient solidity to render it practicable to remove the 

 glass from the vessel. After flowing over the crust of 

 silver chloride a little silver nitrate he allowed it to dry, and 

 exposed the plate in the camera. The picture I hand round 

 was produced in this manner. 



At a later date Niepce de St. Victor went a step further 

 and employed a film of albumen for holding the sensitive salts 

 of silver in situ on glass an example of an early picture so pro- 

 duced is in the Exhibition ; but to Le Gray belongs the honour 

 of suggesting collodion as a vehicle to attain the same end. 

 Archer, with whom was associated Dr. Hugh Diamond, 

 however, practically introduced it. 



Collodion is a solution of gun-cotton in ether and alcohol, 

 and when properly prepared should leave a transparent film 

 when the solvents evaporate. The iodides, bromides, and chlo- 

 rides of the alkalies, and also of many of the metals, are soluble 

 in alcohol, and can therefore be introduced into the collodion, 

 and be left in a film of this viscid body when poured over 

 a glass plate. A mere outline of the collodion process is as 

 follows : A plate is coated with collodion containing an iodide 

 and bromide, as I do this one [shows], and when the ether 

 has evaporated and the film is " set " or become gelatinous, 

 I place it in a dish containing a seven per cent, solution of 

 silver nitrate. On looking at the plate I see the silver 

 compound gradually forming, and after the lapse of about a 



