of the Solar Spectrum on Vegetable Colours. 175 
proved by the photographic impressions described in Arts. 
204 and 209, where no iron is added beyond what exists in the 
ferrocyanic salts themselves. Nevertheless the following ex- 
periments abundantly prove that in several of the changes 
above described, the zmmediate action of the solar rays is not 
exerted on these salts, but.on the iron contained in the ferru- 
ginous solution added to them, which it deoxidizes or other- 
wise alters, thereby presenting it to the ferrocyanic salts in 
such a form as to precipitate the acids in combination with 
the peroxide or protoxide of iron, as the case may be. To 
make this evident, all that is necessary is simply to leave out 
the ferrocyanate in the preparation of the paper, which thus 
becomes reduced to a simple washing over with the ammonio- 
citric solution. Paper so washed is of a bright yellow colour, 
and is apparently little, but in reality highly sensitive to pho- 
tographic action. Exposed to strong sunshine for some time 
indeed, its bright yellow tint is dulled into an ochrey hue, or 
even to gray, but the change altogether amounts to a moderate 
per-centage of the total light reflected, and in short exposures 
is such as would easily escape notice. Nevertheless, if a slip 
of this paper be held for only four or five seconds in the sun 
(the effect of which is quite imperceptible to the eye), and 
when withdrawn into the shade be washed over with the fer- 
rosesquicyanate of potash, a considerable deposit of Prussian 
blue takes place on the part sunned, and none whatever on 
the rest, so that on washing the whole with water, a pretty 
strong blue impression is left, demonstrating the reduction of 
iron in that portion of the paper to the state of protoxide. 
The effect in question is not, it should be observed, peculiar 
to the ammonio-citrate of iron. The ammonio- and potasso- 
tartrate fully possess, and the perchloride exactly neutralized 
partakes of the same property: but the experiment is far more 
neatly made and succeeds better with the other salts. 
211. Ifa long strip of paper, prepared as in the last article, 
be marked off into compartments and subjected to graduated 
exposure to sunshine, so that the times of exposure in each 
succession shall form an arithmetical progression of 1™, 2™, &c., 
and when withdrawn washed over as aforesaid with the ferro- 
sesquicyanuret and rinsed in water, the blue deposit is found 
to increase with the time of exposure up to a very deep and 
full colour, after which its total intensity, so far from increa- 
sing, diminishes, and at length almost vanishes. Again, if a 
slip of the same paper be exposed a long while to the spectrum, 
the whole impression consists in a feeble ochrey-brown streak, 
extending over the region of the blue, violet and lavender rays 
as far as about + 55, But on the application of the cyanic¢ 
