36 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
a few paragraphs descriptive of such very imperfect experiments as I have hitherto 
been enabled to make on this subject. 
95. A paper (No. 646.) prepared with acetate of lead, hydrobromate of potash and 
nitrate of silver, when exposed, without any further preparation, to the spectrum, ex- 
hibited an instant action over the whole spectrum and down to the extreme red, be- 
yond which appeared (the paper having been somewhat discoloured by keeping) a 
white circular spot to the extent of a full diameter of the sun, which though not 
strongly marked was unequivocal. A specimen of the same paper previously dark- 
ened to a considerable degree by exposure to sunshine, was then subjected to the 
action of the spectrum, and while so subjected was washed with a dilute solution of 
hydriodate of potash. It speedily began to whiten at the more refrangible end of the 
spectrum, the whitening commencing in the violet rays ; but so far from their action 
being continued downwards into the less refrangible region, the paper in that region 
continued to blacken perceptibly, and the blackening extended even below the extreme 
red*. A second wash of the iodic solution being now added till the paper was drenched 
with it, the negative or darkening action was found to be arrested in the part of the 
paper already acted on, while on presenting a fresh portion of the surface so drenched 
to the spectrum, no such action took place. 
96. Similar experiments were made with various other preparations of paper, both 
with and without the use of lead ; such as with borax, muriates of baryta, strontia 
and lime, sulphate and phosphate of soda, &c. It will only be necessary to detail 
particulars in a few cases offering peculiarities. The general mode of action seemed 
to be as follows. 1st. When the darkening by previous exposure to sun has not gone 
too far, a weak wash of the liquid hydriodate brings the paper into a sort of inter- 
mediate state, in which it continues to be further blackened, or, according to our no- 
menclature, has a negative character, under the action of the less refrangible rays ; 
and that, quite down to the red end of the spectrum, and even below it in cases 
where, without such addition, the darkening would have terminated at the yellow ray. 
On the other hand, under the more refrangible rays it merely ceases to be darkened, 
losing its negative without acquiring a positive character. When the hydriodic ap- 
plication is somewhat more copious, the paper becomes under these rays distinctly po- 
sitive, whitening slowly and gradually under their action, while between the darkened 
and whitened portions a neutral interval occurs. If a further dose of the hydriodate be 
applied, the neutral line descends in the spectrum, the darkening in the less refran- 
gible portion is enfeebled, and the whitening in the more refrangible reinforced, but 
by no means equally. On the contrary, it not unfrequently happens that a tolerably 
definite portion of the spectrum, viz. just where the blue and violet join, is marked by 
a much more intense whitening action than that beyond it, so as to terminate the 
whitened portion by a conspicuous white spot. If still more of the hydriodic solution 
be added, or if it be applied stronger, all blackening ceases in every part of the spec- 
* See Art. 85. 
