ON CERTAIN VANADIUM COMPOUNDS. 277 



black, varying according to the amount of exposure. A 

 tint of the decomposed vanadate which is of so slight an 

 amount as to be with difficulty distinguished from the 

 whiteness of the paper will, by immersion in the silver 

 nitrate, be toned so as to become very perceptible. It is 

 evident that paper prepared in this way might be em- 

 ployed for the purposes of photographic printing. 



The unexposed parts are converted, by treatment in the 

 silver bath, into yellow silver vanadate. This substance 

 may be dissolved out either by ammonia or by sodium hypo- 

 sulphate. This act of fixing changes the dark brown or 

 black parts to red ; this may be prevented to some extent 

 by using a bath of ammonio-silver nitrate with an excess 

 of ammonia, instead of the simple silver nitrate bath. 

 The developed print can afterwards be toned with gold 

 chloride. 



The length of exposure required to produce a deep black 

 is about one hour to a strong sunlight — this by using a 

 solution of the sodium orthovanadate containing about 

 1 1 per cent, of the salt. 



Some ligneous substance only must be present with the 

 sodium orthovanadate for the production of the above- 

 mentioned slaty tint ; for if an albuminous body be pre- 

 sent, a faint brown tint is produced after exposure to light, 

 and the silver nitrate is not afterwards reduced to any very 

 great extent. 



The slate- colour of the reduced salt appears to be due to 

 the formation of vanadium trioxide. 



If the exposed paper be kept for some weeks, its colour 

 changes to that of a yellowish brown, free vanadic acid 

 appearing to be produced. 



Gelatine impregnated with sodium orthovanadate, ex- 

 posed to light and afterwards dipped into a solution of 

 silver nitrate, becomes insoluble in hot water. 



Silver orthovanadate is capable of forming a photo- 



