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finished — boiling nitric acid would have no effect upon it. But 
it must be remembered that the paper was sensitised by ferric 
oxalate, and it is desirable that it be removed. To effect this 
the print is immersed in a weak solution of oxalic acid, by 
which the whites are rendered very pure. Kinsing in plain 
water completes the operation. There is a great charm about 
these pictures, which are made on plain paper, the tones being 
like that of a warm engraving. Added to their beauty and the 
rapidity of their production, they resist all the usual destruc- 
tive tests. 
The developing action of the potassic oxalate will readily be 
comprehended from the following considerations : — Bearing in 
mind the axiom that no chemical action can take place unless 
one of the substances be in a liquid form, observe that when 
the picture is removed from the printing frame the two sub- 
stances — the ferrous oxalate and the platinum salt — are both 
solid, and hence the former body has not had an opportunity 
of acting upon the latter ; both are side by side, and in the 
most favourable position for one to act upon the other, but it 
is held in check. The question now arises, What will release 
it, so that it may reduce the platinum, so conveniently situated 
for this purpose ? It is not soluble in water, but is so in a 
solution of potassic oxalate. No sooner, therefore, is the picture 
placed in contact with such a fluid than the exposed parts — 
consisting of ferrous oxalate — are liquefied and instanta- 
neously exert their reducing action on the neighbouring par- 
ticles of platinum, which thus are made to form the picture. 
