FROM THE NITRATE OF SILVER. 
329 
action of the sun's light, and the deposition there of its me- 
tallic basis. All persons are not subject to this accident; for 
the influence of the sun's rays ean only be effective in those 
cases where the cutis is more than ordinarily vascular and is 
clothed with a thin transparent cuticle. 
The permanence of the stain is not easily accounted for; 
but it would seem that the metals constitute one class of sub- 
stances for which the absorbents have no attractive affinity, 
as is shown in those instances where bullets have remained 
for years in the body, in the use of metallic ligatures, and in 
the internal exhibition of quicksilver. 
Means of Prevention. — Nitric Jicid. 
Dr. Patterson considers that the contemporaneous adminis- 
tration of nitric acid, with the intention of preventing the de- 
composition of the nitrate of silver, must be entirely useless. 
The nitric acid undergoes decomposition in its passage through 
the circulation, and consequently can hardly reach the sur- 
face of the body to influence the chemical changes there in 
operation: and even if it did, and met with nitrate of silver 
there, its action would be to promote and not to retard the 
formation of the chloride of that metal; for this reason, that 
coming into contact with the soluble muriates, it would de- 
compose the muriatic acid, with the evolution of free chlo- 
rine. 
The conclusion to which Dr. Patterson comes on this sub- 
ject is, that the only way to prevent all risk of discoloration, 
would be to substitute for the nitrate, some preparation of sil- 
ver not liable to be acted on by chlorine, or the sun's light. 
And happening to be employed in some photographic experi- 
ments, his attention was directed to the property displayed 
by solutions of the iodide of potassium in rendering nitrate of 
silver insensible to the influence of the sun's rays. When a 
piece of paper was washed with solution of nitrate of silver, 
and then immediately immersed for a few seconds in a solu- 
