508 Sir J. F. W. Herschel on certain improvements 
viously sunned half, while it produced no perceptible change 
in the other. I found this experiment to succeed on many 
different varieties of paper, and with very considerable latitude 
in the dosage of the ingredients. It was most successful in 
the case of a paper prepared with a cream, formed by mixing 
one measure ammonio-éartrate of iron (strength ;4,*) and two 
saturated protonitrate of mercury, leaving out the free tar- 
taric acid altogether, which, among many other doses of these 
two ingredients, proved also, generally, the most sensitive to 
light. 
“956. Led by these indications I prepared a paper by washing, 
first with a weak solution of ammonio-citrate of iron (strength 
25), and when dry, with saturated protonitrate of mercury. 
It was exposed when barely dry enough, not to feei damp, with 
an engraving ina frame to a hazy and declining sun. In 
about twenty minutes a very pale and feeble photograph was 
produced. Excited as above, by water, it gained but little in 
intensity (for it deserves remark that the increase of apparent 
intensity produced by either water or the nitrate, is in direct 
proportion to the force of the original impression, which, as 
observed, was in this case very faint). It was then held for 
about five minutes in the sun (near setting), and by degrees, 
and with the utmost regularity of gradation over every part 
of the picture, each line assumed an inky blackness, the lights 
and shades being exquisitely preserved in their due propor- 
tions, and the ground being hardly perceptibly discoloured. 
The result was a very beautiful and perfect negative photo- 
graph. 
237. This singular power of water to excite the dormant 
impression, strongly recalls the analogous power of moisture 
to deepen the tints photographically impressed on auriferous 
papers, of which an instance is given in Art. 45, and of which 
a still more striking example is shown as follows. Let a 
paper be washed first with ammonio-citrate of iron, and when 
dry with neutralized chloride of gold, and thoroughly dried 
in the dark. It is then, apparently, almost insensible to light; 
a slip of it half exposed to sun being hardly impressed in any 
perceptible degree in many minutes; yet if breathed on, the 
impression comes out very strong and full, deepening by de- 
grees to an extraordinary strength. ‘Treated in the same 
manner, silver also exhibits a similar property}+. Nor, in- 
* By this I understand one part (by weight) salt + 11 water. 
+ Note added Dec. 21.—The excitement is produced on such paper by 
the ordinary moisture of the atmosphere, and goes on slowly working its 
effect in the dark, apparently without other limit than is afforded by the 
supply of ingredients present. In the case of silver, it ultimately produced 
