OF THE SPECTRUM ON VEGETABLE JUICES. 
117 
Various other plants might be mentioned to show the peculiarly strong influence 
of the rays of mean refrangibility in darkening vegetable juices, and in this respect 
there seems to be a difference in the action of the spectrum on vegetable matter and 
metallic salts ; it would however require a more extensive series of experiments to 
establish this point. 
The very frequent recurrence of two insulated dark spots, or of some indication of 
them, is another peculiarity in the action of the spectrum on by far the greater 
number of vegetable juices which I have used. If these be identical with the heat 
spots discovered by you, there surely must be a difference in the nature of the calo- 
rific rays in the visible and invisible part of the spectrum, since these spots, or a 
darkish disc, often appeared when the red rays had little or no effect, or even possessed 
a powerful bleaching energy, as on the juice of the violet-coloured Globe amaranthus. 
Since the simultaneous appearance of the insulated spots and the maximum inten- 
sity of darkening under the rays of mean refrangibility occurred so often, I am led 
to suspect a similarity, possibly identity, in the nature of the agent producing these 
phenomena. The greatest heat does not lie in the mean rays of the spectrum, nor 
does the greatest chemical action ; and as you have shown that the parathermic rays 
are situate there, may not they be the cause of the phenomena in question, being also 
mixed with those rays of caloric which form the heat spots, and which I have no 
doubt are identical with the insulated spots in these experiments ? 
Some cases have occurred, and there may be many more, in which the maximum 
intensity was produced by those rays in which the chemical energy is most active, 
as for instance in the juice of Coreopsis tinctoria , which was darkened from the red 
to the end of the violet rays with almost equal intensity ; the maximum, if there was 
any, lay under the blue and indigo; also the juice of the red and yellow variegated 
Marvel of Peru, which was darkened under the whole of the visible spectrum, the 
lavender rays excepted. But by far the greater number of vegetable juices which I 
have examined, were most affected by the most luminous rays. 
The point of maximum intensity was sometimes altered by the addition of an acid 
or alkali, and sometimes by using alcohol instead of water ; when a drop of acid was 
added to the juice of the crimson marvel of Peru, the greatest intensity of colour ap- 
peared under the indigo and blue, and a little carbonate of soda changed the point 
of greatest intensity from the yellow and green to the blue rays in the juice of Plum- 
bago auriculata. A still greater change was produced on the juice of the scarlet 
geranium by the addition of carbonate of soda. The colour of the liquid was violet, 
but it stained the paper pale bluish-green. There was no action under the red rays, 
but a circular spot of a decided yellow colour was formed under the orange and yel- 
low rays. A brownish tint was impressed by the green and blue, from the end of 
which up to the termination of the lavender rays, the ground assumed an inky colour 
of considerable intensity, with a strong maximum in the violet. At the usual distance 
below the red, two green spots appeared (fig. 16 .). 
