of the Solar Spectrum on Vegetable Colours. 15 
2nd. The action of the spectrum is confined, or nearly so, 
to the region of it occupied by the luminous rays, as contra- 
distinguished both from the so-called chemical rays, beyond 
the violet, which act with the chief energy on argentine com- 
pounds, but are here for the most part ineffective, on the one 
hand, and on the other, from the thermic rays beyond the 
red, which appear to be totally so. Indeed, I have hitherto 
observed no instance of the extension of this description of 
photographic action on vegetable colours beyond, or even 
quite up to, the extreme red. 
170. Besides these, it may also be observed that the rays 
effective in destroying a given tint, are, in a great many cases, 
those whose union produces a colour complementary to the 
tint destroyed, or at least one belonging to that class of colours 
to which such complementary tint may be referred. For 
example, yellows tending towards orange are destroyed with 
more energy by the blue rays; blues by the red, orange, and 
yellow rays; purples and pinks by yellow and green rays, 
171. These are certainly remarkable and characteristic pe- 
culiarities, and must indeed be regarded as separating the 
luminous rays by a pretty broad line of chemical distinction 
from the non-luminous; though whether they act as such, or 
in virtue of some peculiar chemical quality of the heat which 
accompanies them as heat, is a point which the experiments 
on guaiacum, above described, seem to leave rather equivocal. 
In the latter alternative, chemists must henceforward recog- 
nize differences not simply of intensity, but of quality in heat 
from different sources; of quality, that is to say, not merely 
as regards degree of refrangibility or transcalescence, but as 
regards the strictly chemical changes it is capable of effecting 
in ingredients subjected to its influence. 
172. As above stated, these peculiarities, at least the first 
two, obtain almost universally. Exceptions, however, though 
very rare, do occur, as will be more particularly mentioned 
hereafter. The third rule is much less general, and is to be 
interpreted with considerable latitude; but among its excep- 
tions I have been unable to detect any common principle ca- 
pable of being distinctly enunciated. 
173. Lastly, it requires to be expressly mentioned, that the 
habitudes of the colours, both of the flowers and leaves of 
plants, with relation either to white light or to the prismatic 
rays, vary materially with the advance of the season, and per- 
haps also with the hour of the day at which they are gathered, 
Generally speaking, so far as 1 have been able to observe, 
the earlier flowers of any given species reared in the open air 
(provided they are well ripened, i.e. the colour fully deve- 
