32 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
85. The evidence obtained in this experiment of the existence of a darkening or 
deoxidizing power in the invisible chemical rays beyond the extreme red, or (as perhaps 
we ought to express it) of the reversal, by the iodic salts, of the ordinary whitening or 
oxidizing power exerted by those rays (Art. 60.), is confirmed by measurement of the 
dark spectrum impressed on the paper (No. 644.) prepared with acetate of lead and 
chloride of platina when washed, under the action of the light, with hydriodate of 
potash, as described in Art. 47, as the following statement will show. 
Negative sensible termination of the spectrum — 21*5 
Subtract sun’s semidiameter — 3‘6 
Last efficient ray in a linear spectrum 17*9 
Last visible red ray in ditto — 13'3 
Extent of chemical action in ditto beyond the extreme red . — 4'6 
Positive sensible termination of the spectrum + yO'O 
Total extent of sensible chemical action 91 '5 
A power of precisely the same nature (i. e. of continuing the action up to and beyond 
the red rays,) is exercised by the hydriodate of potash also, on many other argentine 
compounds, as the oxalate, sulphate, chloride, See., and probably on all. 
86. The phenomenon of a triple maximum of chemical action observed in papers 
prepared with a tartaric salt, is also not peculiar to that class of bodies. In a paper 
(No. 578.) prepared with oxalate of ammonia, two very evident occur in the blue and 
at the extremity of the violet rays, and one, much less so, about the point where the 
green passes into blue. The spectrum is of a brown colour, of a dull and smoky cast 
at its less refracted end, but becoming livelier and more verging to a yellow at the 
other. Its whole extent lies clearly within the positive region of the solar spectrum, 
the sensible termination being at + 0-0, so that the place of the least refracted active 
ray in a linear spectrum would be + 3’6. The following are the measured coordi- 
nates. 
Least refracted sensible termination ....... + 0‘0 
Most refracted sensible termination + 88’0 
1st. Maximum, barely traceable + 14 
2nd. Ditto, pretty conspicuous -f- 25 
3rd. Ditto, very conspicuous, and pretty abrupt . . . +41 
In the spectrum exhibited on paper (No. 579.) prepared with citrate of ammonia, 
a triple maximum is also traceable. 
Least refracted sensible termination 0 0 
Most refracted sensible termination + 74'0 
1st. Maximum + 24*7 
2nd. Ditto + 39'4 
3rd. Ditto + 569 
