42 
Paper, which does not contain any size of an animal 
origin, when coated with a solution of sodium orthovana- 
date, is darkened on exposure to light, the depth of tint 
depending on the length of exposure and on the strength 
of the solution used. The tint, however, never becomes 
darker than a slate colour. 
If the paper thus prepared be immersed, after exposure 
to light, in a solution of silver nitrate, the colour in the ex- 
posed part instantly changes to a deep brown or to a black 
colour, 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 exhibit a very perceptible tint. 
It is evident that paper prepared in this way might be 
employed 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 
hyposulphite. This act of fixing converts the dark brown 
or black part into those of a red colour. This may be pre- 
vented 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 sun light. 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-men- 
tioned slaty tint ; for if an albuminous body be present, 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 
