INTERNATIONAL BOTANICAL CONGRESS, 187 
fixed or varied, mean or extreme, or the effect of light, it is exceedingly 
difficult, and sometimes impossible, when observations are made in the 
usual manner, to eliminate the effects of the constant variations of heat 
and light. In the laboratory it is possible to operate under more 
exactly defined conditions, but they are rarely sufficiently persistent ; 
and the observer is led into error by growing plants in too contracted 
a space, either in tubes or bell-glasses. This last objection is appa- 
rent when it is wished to ascertain the influence of the gases diffused 
in the atmosphere around plants, or that of the plants themselves upon 
the atmosphere. 
Place plants under a receiver, they are no longer in a natural condi- 
tion; leave them in the open air, and the winds and currents, produced 
at each moment of the day by the temperature, disperse the gaseous 
bodies in the atmosphere. Every one is aware of the numerous dis- 
cussions concerning the more or less pernicious influence of the gases 
given off from certain manufactories. The ruin now of a manufacturer, 
now of a horticulturist, may result from the declaration of an expert ; 
hence, it is incumbent on scientific men not to pronounce on these de- 
licate questions without substantial proof. 
With a view to these researches, of which I merely point out the 
general nature, but which are immensely varied in details, I lately put 
this question :*—* Could not experimental greenhouses be built, in 
which the temperature might be regulated for a prolonged time, and be 
either fixed, constant, or variable, according to the wish of the ob- 
server?" My question passed unnoticed in a voluminous work, where, 
in truth, it was but an accessory. I renew it now in the presence of 
an assembly admirably qualified to solve it. I should like, were it 
possible, to have a greenhouse placed in some large horticultural esta- 
blishment or botanic garden, under the direction of some ingenious 
and accurate physiologist, and adapted to experiments on vegetable 
physiology ; and this is, within a little, my idea of such a co c- 
tion :— 
The building should be sheltered from all external variations of tem- 
perature ; to effect which, I imagine it should be in a great measure 
below the level of the ground. I would have it built of thick brick- 
work in the form of a vault. The upper convexity, which would rise 
above the ground, should have two openings—one exposed to the 
* * Géographie Botanique,” 1855, pp. 49, 1346. 
