Oy A ape 
THE CHEMICAL ACTION OF LIGHT. 73 
that just as the deprivation of light produces blanching or etiola- 
tion in plant life, so the absence or diminution of light exerts an 
injurious influence on animal life and development, whilst free 
exposure to sunlight helps to produce animal life in its highest 
form: the remarkable absence of deformity among savage races 
being attributed to the constant exposure of the body to strong light. 
Before proceeding, I must remind you that light, as we know 
it, is not homogeneous. When ordinary white light—be it sun- 
light, candle-light, or gas-light—is passed through a prism we find 
that it is split up into its component parts, and in place of white 
light we get a bright band of many colours. This is due to the 
phenomenon of refraction, the component parts of white light 
being bent aside, each to a different extent, during passage 
through the prism, the result being that the band of seven 
colours, called the spectrum, is produced. The colours are, 
beginning with the least refrangible, red, orange, yellow, green, 
blue, indigo-blue, and violet. 
But in addition to the visible rays of the spectrum, there are 
also invisible rays at either end which are known respectively as 
the ultra-red and the ultra-violet. 
Different parts of the spectrum have very different effects; thus, 
in addition to the rays which give the impression of light, there 
are others which produce heating effects, and others, again, the 
special function of which is to promote chemical combinations 
and decompositions. Experiments show that the maximum 
heating effect is found beyond the red end of the visible spec- 
trum, whilst, on the contrary, the maximum chemical action is 
produced by the rays of the violet end, the action extending 
into the ultra-violet on the one side, and into the green on the 
other ; all perceptible action, except in the case of plants, ceasing 
in the yellow or most luminous portion of the visible spectrum. 
Turning now to the action of light upon the vegetable king- 
dom, we find that the entire life of a plant depends on this action 
on the cells which contain the green colouring matter, chloro- 
phyll; for here are constructed the organic substances, which go to 
build up the plant, from the materials of its food. Carbonic acid 
