i88 5 .J 
Sanitary Agency of Light. 
709 
fungus of another class— creeps away during the day and 
hides itself in the shade, as it is well known to do. In faft, 
there is a large number of phenomena in the vegetable 
world, isolated before, which group themselves together 
under this law, and acquire a significance which they pre- 
viously wanted. . „ , 
It is impossible, however, to contemplate this aspect 01 
the aftion of light in the natural world without haying 
another and a very different one forced upon our attention. 
If we continue to expose our long white bean shoot to light 
we find that it becomes green. This result, as is well known, 
is due to the production of chlorophyll, and is the com- 
mencement of true growth in the sense of increase, for 
before the shoot contained nothing but what it obtained 
from the parent seed and the water in which it was placed, 
now it is beginning to “ shift for itself,” and provide its own 
nutriment from the surrounding air. Now, it seems veiy 
strange at first sight that the same white light should pro- 
mote growth and increase on one hand while it checks and 
stunts it on the other, but the explanation lies in the natuie 
of white light, which is not one and indivisible, but is made 
up of rays of many refrangibilities. We have seen that the 
power of injuring white protoplasm lies chiefly among the 
more refrangible rays represented by the blue and violet, 
but it is the yellow and red rays at the other end of the 
spectrum which produce chlorophyll. And here we come 
upon another remarkable contrast; the rays at the violet 
end of the speftrum check vegetable growth by promoting 
oxidation, those at the red extremity afford nutrition to 
green plants by promoting de-oxidation, at least in the 
special case of carbon dioxide, and very probably of water 
It is not meant to imply that there is a sharp line of 
demarcation in the speftrum, at either side of which oxidis- 
ing or de-oxidising influences exist, but that such influences 
predominate at opposite extremities, the peak or maximum 
of the curves representing them being thus situated, the 
declining lines of the curves crossing each other at a neutral 
zone Here again we come upon a pregnant faft ; the 
neutral zone of the speftrum, where the curves interseft, 
and where oxidising and de-oxidising forces both disappear, 
is situated at or near the green. . 
In the paper alluded to at the commencement of this 
article, the authors mention “ Special colouiing matters 
which filter out the more injurious rays ” as one means of 
protection afforded to protoplasm in the oiganism, it is 
