8IL VERMERCUR Y. 333 



brought in contact with animal matter, it is readily decomposed into 

 free nitric acid and metallic silver, which produces the characteristic 

 black stain ; it is this decomposition, and the action of the free nitric 

 acid, to which the strongly caustic properties of silver nitrate are 

 due. 



Silver nitrate is used for various kinds of indelible inks and hair- 

 dyes, and very largely in the manufacture of those silver compounds 

 employed for photographic purposes. 



Photography is the art of obtaining images of objects by means 

 of chemical changes produced in certain substances (chiefly com- 

 pounds of silver or platinum) by the action of light. Three separate 

 operations are required to obtain the image ; they are exposure, devel- 

 oping and fixation. 



The exposure of a light-sensitive surface to a projected image of the object to 

 be photographed is made in the camera, which is so arranged that the image 

 can be thrown upon the surface by means of a lens. The surface used is gen- 

 erally that of a glass plate or a gelatine film, sensitized with the bromide or 

 iodide of silver. 



On the exposed plate a chemical change has taken place in the silver salt 

 wherever light has acted on it. The exact nature of this change is not under- 

 stood, and nothing can be detected on the plate with the eye after exposure. 

 But when the exposed plate is treated with certain solutions, called developers, 

 that portion of the silver salt which has been acted upon by light is decomposed 

 with the formation of a deposit of metallic silver, forming the visible image. 

 The acting constituent of developers are deoxidizing agents, such as pyrogallol, 

 hydroquinone, ferrous sulphate, etc. 



After the silver image has appeared there yet remains on the plate that por- 

 tion of the silver salt which has not been acted on by light, and consequently 

 not by the developer. This portion of the undecomposed silver salt must be 

 removed before the plate can be taken to the light, and this removal, called 

 fixation, is accomplished by immersion of the plate in a fixing solution of 

 sodium thiosulphate, Na 2 S 2 3 (hyposulphite), which dissolves the salt in con- 

 sequence of the formation of a double salt of the composition Ag 2 S 2 3 . 2Na 2 S 2 3 ; 

 this is eliminated by washing in water. 



The image thus obtained is called a negative, because it shows dark what 

 ought to be light, and vice versa. By placing this negative upon sensitized 

 material (generally paper) and permitting light to pass through the negative 

 to the underlying paper the light-sensitive material is chemically affected most 

 where the silver deposit in the negative is the thinnest, and vice versa. By 

 developing and fixing the exposed paper the positive picture is obtained. 



Silver oxide, Arg-enti oxidum, Ag 2 O = 23O.12. Made by the 

 addition of an alkali hydroxide to silver nitrate : 



2AgN0 3 + 2KOH = 2KNO 3 + H 2 O + Ag 2 O 



