360 SCIENTIFIC AMUSEMENTS. 



readily transmitted, producing coloured emergent light ; and just 

 as a similar peculiarity is observed with certain substances in 

 regard to rays of low refrangibility, producing the phenomena of 

 heat colour (Expt. 382), so are various substances capable of 

 undergoing chemical change under the influence of light more 

 sensitive to rays of particular refrangibilities than to others, the 

 energy of these particular rays being absorbed in doing the chemical 

 work. 



Of the numerous substances sensitive to this kind of radiant 

 chemical action, certain silver compounds are, for a variety of 

 reasons, the most convenient for the ordinary purposes of photo- 

 graphic reproduction of likenesses, views of landscapes, &c., &c. 

 Accordingly, the most widely employed processes in photography 

 are those involving the use of these materials. " Chromatised gela- 

 tin " and certain iron and platinum compounds, however, are also 

 employed for special kinds of work to some considerable extent ; 

 whilst many other substances are known capable of use which have 

 not hitherto come into extensive practical employment, on account 

 either of their greater cost or of their requiring more troublesome 

 manipulation, &c., as compared with silver compounds. 



Expt. 387. To take a Photograph by the Collodion Process. 

 One of the most extensively used photographic processes consists in 

 covering the flat surface of a sheet of glass or other supporting 

 material with a film of "collodion," which is subsequently im- 

 pregnated with silver compounds so as to obtain a flat surface 

 sensitive to light. Collodion consists of a solution in a mixture 

 of ether and alcohol of a variety of gun cotton (pyroxylin) obtained 

 by treating cotton with sulphuric and nitric acids ; the fluid thus 

 obtained, when poured out on a flat surface and allowed to 

 evaporate, leaves a film of collodion, somewhat resembling thin 

 parchment when dry, or the white membrane lining an egg. A 

 film thus obtained is often employed as a coating or protection for 

 the skin when this has been severely injured by burning or scald- 

 ing, an artificial protecting membrane being thus produced. In 

 order to impregnate the film with silver compounds, iodide of 

 cadmium, bromide of ammonium., or other corresponding salts 

 (preferably those soluble in alcohol or ether) are also dissolved 

 in the collodion, forming a compound fluid sold under the name 

 of iodised collodion. A glass plate is made scrupulously clean and 

 highly polished, care being taken not to touch the flat surface 

 with the fingers, otherwise spots and smears will ultimately be 

 produced; iodised collodion is then poured over the plate held 

 horizontally, and allowed to flow all over the surface by dexterously 

 inclining it, the surplus fluid being poured back into the bottle ; 



