INTERNATIONAL BOTANICAL CONGRESS. 189 
deed, a question more complicated might be investigated, towards the 
solution of which science has already made some advances, namely, 
that of the action of variable temperatures; and it might be seen, if, 
as appears to be the case, these temperatures are sometimes beneficial, 
at other times injurious, according to the species, the function investi- 
gated, and the range of temperature. The action of light on vegeta- 
tion has given rise to the most ingenious experiments. Unfortunately, 
these experiments have sometimes ended in contradictory and uncertain 
results. The best ascertained facts are, the importance of sunlight for 
green colouring, the decomposition of carbonic acid gas by the foliage, 
and certain phenomena relating to the direction or position of stems 
and leaves. There remains much yet to learn upon the effect of diffused 
light, the combination of time and light, and the relative importance 
of light and heat. Does a prolonged light of several days or weeks, 
such as occurs in the Polar regions, produce in exhalation of oxygen, 
and in the fixing of green matter, as much effect as the light distributed 
during daily interrupted periods of twelve hours, as at the Equator ? 
No one knows. In this case, as for temperature, curves should be 
constructed, showing the increasing or diminishing action of light on 
the performance of each function; and as the electric light resembles 
that of the sun, we could in our experimental hothouse submit vege- 
tation to a continued light.* 
building such as I propose would allow of light being passed 
through coloured glasses or coloured solutions, and so prove the effect 
of the different visible or invisible rays which enter into the composi- 
tion of sunlight. For the sake of exactness nothing is superior to the 
decomposition of the luminous rays by a prism, and the fixing the rays 
by means of a heliostat. Nevertheless, a judicious selection of colour- 
ing matters, and a logical method of performing our experiments, will - 
lead to good results. I will give as proof, that the recent most careful 
ees concerning the action of various rays upon the production 
apparatus which produces the most persistent and vivid light is the 
magnets decre m achine, based on the development of induction by magnet- 
sm, doe Meer dide 2 the illustrious Faraday. The ape ae pile is Mi nated 
low 
magnets (i am Univ. de Genèv ve, Archives Scientif. 1861, xe x: i 160). 
The w this machine is inexpensive, but, unfortunately, the magnets 
senden s system n applied to two lighthouses— 
that at'the South Foreland, and to that ofa ei =i Souisté Sociol Havre— 
in consequence of the experiments of MM. E. Becquerel and Tresca. 
