CHEMICAL ACTION OF THE SOLAR RADIATIONS, 149 
terrestrial heat is shown to be capable of assisting 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 beyond the 
spectrum, acting under precisely similar circumstances, and in an equal state of 
condensation.” ‘The action of the solar rays is positive, that is to say, vege- 
table colour is destroyed ; but in most cases it is susceptible of restoration by 
chemical agents. When vegetable colours have been removed—bleached— 
by th@action of bleaching agents, they may be restored by the action of the 
sun’s rays. If exposed to the action of the prismatic spectrum, it will be 
found that the restoration of colour is operated by rays complementary to those 
which destroy it in the natural state of the paper; “the violet rays being the 
most active, the blue almost equally so; the green little, and theyellow, orange 
and most refrangible red not at all*.” 
' Although the restoration of vegetable colours is occasioned by rays within 
the limits of the luminous spectrum, it must be remembered that the green, 
yellow and orange rays—those having the most illuminating power,—are, in 
nearly all cases, inactive. The effects would appear to be due to the com- 
bined influences of the light and of the chemical agency, whatever it may be. 
But even under this view a peculiar difficulty presents itself; we find for ex- 
ample the blue rays, or the actinic power associated with that colour, destroy- 
ing a vegetable colour; and then, having used a chemical agent,—as sul- 
phurous acid—to destroy that colour, it can be restored by the action of the 
orange or red rays. The peculiar variations in the scale of action which we 
find in almost every different substance exposed to solar influence, presents 
the greatest difficulty to any theoretical view of the physical constitution of the 
sunbeam. 
Mrs. Somerville has pointed out some very remarkable actions of the 
spectrum on vegetable juicest. The colouring matters examined by this. 
lady were derived from the 
Pomegranate. Scarlet Geranium. 
Globe amaranthus. Scarlet Balsam. 
Plumbago auriculata. Dahlia. 
Beet root. Scarlet Zinnia. 
Rose Verbena. Walnut. 
Nasturtium. Fig. 
These were sometimes employed pure, in other cases they were united with 
common salt, some acid, or carbonate of soda. The differences produced were 
singular, presenting, as in the case of the silver salts, a variation in the scale of 
action in every case. The maximum amount of action was observed, however, 
to lie between the yellow and the green rays, and seldom extending beyond the 
blue; showing that those radiations which exert the most energetic action on 
metallic compounds have little or no influence on the products of the vegetable 
world. In nearly all cases a peculiar effect was observed at the least refran- 
gible end of the spectrum. Coloured spots were produced, which appear to 
correspond to the rays named by Sir John Herschel, the Parathermic rays ; 
and at the same time, as the evidences of heat were clear from the drying of 
the paper, it became apparent that some peculiar chemical change was being 
induced. At present, however, there is nothing determined as to the real 
agency producing this set of phenomena. Allusion having been made more 
* Herschel, Philosophical Transactions, Part 2 for 1842, p. 192. 
+ On the Action of the Rays of the Spectrum on Vegetable Juices (Philosophical Trans- 
_ actions, 1846, p. 111). 
