and on the chemical action of the Solar Spectrum* 271 
Having filled my trough with a saturated solution of the 
bichromate of potassa, I exposed a plate for five minutes to 
its influence in full sunshine. There nsoas not the slightest ac- 
tion. 
12. In one hour on a similar plate, under the same circum- 
stances, I obtained a faint, but still defined outline of a dried 
fern. 
13. I exposed a bare iodidated plate for two hours to the 
same influence. On removing it from the chamber no dif- 
ference was apparent ; but I found it was no longer sensitive 
to light, and the iodide adhered more closely to the metal 
than it did (28.). 
This is a reverse action, for after the exposure of a pre- 
pared daguerreotype plate to light, the sensitive film is most 
easily rubbed ofF^ (28.). 
14'. Red solutions impart a very decided rose hue, or more 
strictly speaking the influence of red light on the iodidated 
plate occasions that peculiar arrangement of the mercurial 
particles, which is necessary to the production of red co- 
lour. 
15. Green solutions act with more or less effect in ob- 
structing the passage of the so-called chemical rays according 
to their depth of colour. But in no instance have I found 
them to produce that close combination, which the yellow 
and sometimes the red fluids do, of the iodide and the un- 
der surface of unattacked silver (28.). By examining the 
effects produced by green media (2, 7? 16.) a peculiar order 
of interference will be remarked (19.). 
Germination and the growth of Plants. 
16. I planted in a box some curled cress seed, and so ar- 
ranged bottles of carmine fluid, chromate of potassa, acetate 
of copper, and the ammonia sulphate, that all but a small 
space of the earth was exposed to light which had permeated 
three-fourths of an inch of these media. 
For some days the only apparent difference was that the earth 
continued damp under the green and blue , fluids, whereas it 
rapidly dried under the red and yellow. The plumula burst 
* On this principle I now polish my silvered plates, by which the trou- 
blesome process with nitric acid and pumice is got rid of. I wash - the sur- 
face of silver over with a solution of the iodide of potassium holding a 
little iodine free, and rub it lightly until all the parts are equally attacked, 
i then expose the plate to light for a few minutes, and polish off with dry 
cotton. In five minutes by this process the most perfect lustre may be 
given to the silver, and it has the advantage of rendering the plate more 
susceptible to the influence of the iodine vapour. 
