VENTILATION OF EARLY FORCING-HOUSES. J7B 
apparatus of greater power than has'hitherto been attained 
should not some day be contrived, yet in adapting those at 
present in use, and in erecting them so as to have a perfect 
control over them there is much to be accomplished. One 
half of the heating apparatuses at present erected perform their 
work without such a thing as control over them being known 
between the two extremes; and the greater part of the other 
half are very far from being perfect in this respect. Here, then, 
is the point to which improvement should be directed : provide 
an apparatus which in a few minutes will produce a greater or 
less amount of heat, as it may be required, almost with mathe¬ 
matical exactitude, and then there can be no serious risk in 
not maintaining an uniformly unnecessary degree of heat. 
The other consideration which refers to the escape of the 
deteriorated or confined air, and the consequent necessity of 
admitting a fresh volume in its place, does not appear to offer 
insurmountable difficulties in the way of a belief that the ad¬ 
mission of air is very frequently carried to an injurious excess. 
Admitting that plants in the process of growth, and in the dis¬ 
charge of their vital functions, do abstract certain matters from 
the atmosphere about them, there is nothing, even in this, to 
render the admission of fresh air, in volumes of any considerable > 
extent, at all necessary. The elastic and all-pervading pro¬ 
perties of the atmosphere must not be lost sight of. Under 
any circumstances, and with the mode of construction at present 
adopted in hothouse architecture, a considerable interchange 
will be found to take place between the internal and external 
volume; and, with evidence of the successful growth of plants, 
in situations so far closed up as the Wardian cases, we cannot 
do otherwise than believe that this interchange, which takes 
place as it were of necessity, is sufficient to secure the health 
and vigour of the plants, so far as the admission of air alone is 
concerned. If it is argued that deterioration will take place by 
reason of evaporation from flues or pipes or any substances 
confined within the structure, or from the decomposition of any 
organic matter, the same fact of an interchange continually 
going on is sufficient to meet the case, so far as to show that on 
this ground at least the admission of a large bulk of external 
air is not at all essential. Besides, with a proper routine of 
management, the gases which arise by means of evaporation 
from ordinary surfaces, or the decomposition of ordinary sub- 
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