of the Solar Spectrum on Vegetable Colours. il 
proached to it so as just not to discolour the paper. Under 
such circumstances 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 rendering visible any of the 
heat-spots a, 8, y, 2, 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 refrangible rays 
fell, and where the discoloration was in progress under their 
agency. For it was observed that, under these circumstances, 
the discoloration in question went on with much greater ra- 
pidity, so much so indeed, that the same amount of it, which 
without extraneous heat would have required twenty minutes 
or half an hour’s exposure to the spectrum to produce, was 
now produced in two or three minutes. Obscure terrestrial 
heat, therefore, is shown to be capable of asststing and being 
assisted in operating this peculiar change, by those rays of 
the spectrum, whether luminous or thermic, which occupy its 
red, yellow, and green regions; while on the other hand it 
receives no such assistance from the purely thermic rays be- 
yond the spectrum, acting under precisely similar circum- 
stances, and in an equal state of condensation. 
161. When heat was similarly applied by radiation from 
behind, and from a non-luminous source, over the more re- 
frangible region of a spectrum thrown on paper simply washed 
with tincture of guaiacum and not previously blued either by 
chlorine or by light, the blue colour induced in the more re- 
frangible rays was still produced, and of the same tint in the 
same points as if no heat had acted. This effect, the con- 
trary to what the previous experiment would have led to ex- 
pect, shows how little any reasonings on these points enable 
us at present to anticipate experience. 
162. The discharge of colour from blued guaiacum by mere 
heat, has been shown above (Art. 156.) to take place at a 
much lower temperature in the presence of moisture than 
when dry; and a similar destruction of colour, under similar 
circumstances, takes place with many other vegetable prepara- 
tions. Paper, for instance, coloured with the juice of the 
Viola tricolor (Art. 90.), is speedily whitened in the dark, 
while wet, by the heat of boiling water, though dry heat’ does 
