OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
185 
the yellow state, than the assumption of a certain temperature dependent on its state 
of dryness, and varying according to that state between the limits of 180° and 280°. 
Nevertheless, if we consider that the same change is produced by rays of the spec- 
trum which are very far from being the hottest, while yet the extra-spectral thermic 
rays, under precisely the same circumstances of exposure, produce no such effect, 
though far surpassing in mere calorific power those which do, we shall see reason to 
doubt the sufficiency of this view of the matter. The following experiments were 
therefore instituted with a view to its further elucidation. 
158. A slip of the paper No. 1 168 was moistened and subjected in clear sunshine 
to the action of the spectrum. The colour was discharged from the region occupied 
by the less refrangible luminous rays, as described in Art. 155. At the same time, 
the more distant thermic rays beyond the spectrum produced their proper effect, in 
evaporating the moisture from those portions on which they fell ; so that in due time 
the heat-spots 'h and y became apparent (see Art. 136), the former very distinctly, 
the latter perceptibly. The spot (3 (which is remarkable) was scarcely if at all formed. 
So long then as the paper continued moist and remained under the influence of the 
thermic rays, the appearances were those of a diminution of colour (Art. 131.), ope- 
rated by the thermic rays S and y. But the discoloration in these points was only 
apparent, for as the paper dried these heat-spots disappeared, leaving its colour quite 
unchanged at those points ; while the photographic impression really produced 
within the visible spectrum, remained and went on increasing in intensity. The 
non-lu minous thermic rays, therefore, though clearly shown to have been active as 
heat, were yet incapable of effecting that peculiar chemical change which other rays 
much less copiously endowed with heating power, were all the while producing. 
159. It may be objected to this, that no proof is afforded in the above-related ex- 
periment, that any part of the paper actually attained a temperature of 180° or more ; 
that in consequence no discoloration due to the action of heat ( quoad heat ) was 
produced ; and that the discoloration which did take place was sui generis, and ori- 
ginated with the light and not the heat of that part of the spectrum to which it 
corresponded. A slip of the same paper (1168.) was therefore exposed dry to the spec- 
trum in such a way as to leave its back accessible; and an iron heated below redness 
was then approached to it so as just not to discolour the paper. Under such circum- 
stances it might be expected that the additional heat thrown on the paper in the 
region of the thermic rays would turn the scale in their favour at their points of 
greatest intensity, and give ocular proof of their action by a decided discharge of 
colour at those points. But no such result was obtained, nor could I succeed in ren- 
dering visible any of the heat-spots a, j3, y, c$, even when the heated iron was brought 
so near as to produce a commencement of discoloration over the whole of that region 
of the paper where they ought to have shown themselves. 
160. On the other hand, a remarkable, but by no means an unexpected, influence 
was exercised by the heat so thrown on that part of the paper where the less refran- 
2 B 
MDCCCXL1I. 
