16 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
most energetic in the redaction of argentine compounds are those which have been 
hitherto regarded as extra-spectral, or as lying beyond all visible illumination on the 
more refrangible side, is a proposition familiar to photologists. How far this opinion 
is strictly correct I shall take occasion presently to examine. It has also, on the 
ground of Dr. Wollaston’s experiments on guaiacum, been received, that the less re- 
frangible end of the spectrum produces chemical changes of a certain kind ; but as 
that eminent philosopher found changes apparently of the same kind, in that sub- 
stance, to be operated by a gentle heat, unaccompanied with light, and as those 
changes were, in consequence, referred by him to the calorific rays, which we know 
to accompany that end of the spectrum, rather than to any peculiar chemical action, 
this point must be considered as hitherto undecided. The facts about to be stated, 
however, leave no further doubt on the subject. 
50. The experiments described in my first communication on this subject, in which 
coloured and other media were used to analyse the incident light by absorption, are 
quite sufficient to show a high probability at least that the chemical energy is distri- 
buted throughout the spectrum in such a way as to be, by no means, a mere function 
of the refrangibility, but to stand in relation to other physical qualities, both of the ray 
and of the analysing medium, and that relation by no means the same as that which 
determines the absorptive action of the latter on the colorific rays. The experiments I 
am about to describe will show that there is also a third set of relations concerned in 
this action, and most materially influencing both the amount and character of the 
chemical action at each point of the spectrum, viz. those depending on the physical 
qualities of the substance on which the rays are received, and whose changes indicate 
and measure their action. This indeed might be expected, or rather it would seem 
unlikely, on a general view, that the contrary should be true, seeing that some sub- 
stances are, others are not, affected by light. But no such general and a priori 
reasonings would, I think, lead us to many of the conclusions actually arrived at ; 
as, for instance, that one and the same spectrum thrown on papers differently pre- 
pared, shall indicate by the impressions photographically left, on those papers, the 
most capricious differences in the scale of action, whether estimated by the extent of 
the discoloration in the direction of the length of the spectrum, or by its intensity at 
different points of that length. We could hardly have predicted, a priori, for example, 
that, acting on one description of paper, the chemical spectrum (as indicated merely 
by the length of its photographic impression,) should include within its limits the 
whole luminous spectrum, extending much beyond the extremest visible red rays on 
the one hand, and on the other to a surprising distance beyond the violet ; while, if 
another paper be used, all action should appear definitively cut off at the orange ; if 
another, at the commencement of the green ; and if another, at that of the blue rays: 
that, in the case of one description of paper, the maximum of apparent action should 
lie beyond, in that of another within the visible violet ; in a third far in upon the blue; 
while in a fourth several successive maxima should appear. Still less would any 
