146 
CHANGES PRODUCED BY THE SOLAR RAYS. 
were placed aside in a dark cupboard to serve for compari- 
son with those exposed to light ; other tubes of powder 
were now exposed, day after day, to the action of the rays 
of a prismatic spectrum, which was maintained in a fixed 
position, by means of a good heliostat. The tubes were so 
placed across the spectrum, that each tube was kept under 
the influence of a particular ray. Although I frequently 
placed seven tubes in the seven rays of the spectrum I gene- 
rally used only three, for the following reasons. Sir David 
Brewster considers that there are but three primitive colours 
in the solar beam — red, yellow, and blue; and that all the 
other colours are but compounds of those rays with each 
other. My researches have proved to me that the three 
great phenomena of the solar radiations — light, heat, and 
chemical action — have their points of maximum power in 
three distinct parts of the spectrum ; (he red rays giving the 
largest amount of calorific power, the maximum intensity of 
light residing in the yellow rays, and the chemical power 
being greatest in the blue rays. It will, therefore, now be 
seen that by using three tubes only, the vegetable powder 
under examination was exposed to the influence of the heat 
of the suns ray's — to their light — and to their chemical, or, 
as I have proposed to term it, actinic power. By this 
arrangement I soon ascertained that the decolorization of 
green vegetable powders was not due to the influence of 
light, as the powders exposed to the full intensity of yellow 
light, in which we have the maximum of luminous influence, 
underwent no change — the alteration of the colour was in 
nearly all cases, but not in all, first set up under the influence 
of the red rays. No great difference could, however, be 
detected between the action of the red and blue rays. 
Hence we learn, that the change in question is due to the 
action of the heating and chemical principles of the sun's 
rays. By using two spectra, and so arranging them that 
the blue ray of the one fell on the space occupied by the 
red ray of the other, we are enabled to examine the com- 
