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 was 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. 



\^. 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 examinhig the 

 effects produced by green media (2, 7, 16.) a peculiar order 

 of interference will be remarked (19.). 



Germination and the groiioth 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 diflPerence 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 tiie 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 oiFwith dry 

 cotton. In five minutes by this process the most perfect lustre may be 

 given to the silver, and it has the advantage of I'endering the plate more 

 susceptible to the influence of the iodine vapour. 



