VENTILATION. d15 
plants do not thrive, and that the skins of grapes 
are thick, and other fruits without flavour, in 
crowded forcing-houses; but in these it is pro- 
bably light, rather than a more rapid change of 
air, that is wanting; for, in a forcing-house 
which I have long devoted almost exclusively to 
experiments, I employ very little fire heat, and 
never give air till my grapes are nearly ripe, in 
the hottest and brightest weather, further than 
is just necessary to prevent the leaves being 
destroyed by excess of heat. Yet this mode of 
treatment does not at all lessen the flavour of 
the fruit, nor render the skins of the grapes 
thick ; on the contrary, their skins are always 
most remarkably thin, and very similar to those 
of grapes which have ripened in the open air.’ 
While, however, the natural atmosphere cannot 
be supposed to require changing in order to 
adapt it to the respiration of plants, it is to be 
borne in mind that the air of houses artificially 
heated may be rendered impure by the means 
employed to produce heat. Sulphurous acid gas 
escapes from brick flues, ammoniacal vapour 
from fermenting manure; and there may be 
many unsuspected sources of the introduction 
of vaporous impurities ; an inconceivably minute 
quantity of which is enough to deteriorate the air, 
so far as vegetation is concerned. Drs. Turner 
and Christison found that 1-10,000th of sulphur- 
ous acid gas destroyed leaves in 48 hours; and si- 
milar effects were obtained from hydrochloric or 
muriatic acid gas, chlorine, ammonia, and other 
agents, the presence of which was perfectly un- 
discoverable by the smell. We also know that 
the destructive properties of air poisoned by cor- 
rosive sublimate, perhaps by its being dissolved 
in the vapour of a hothouse, are not at all ap- 
preciable by the senses. Ventilation is neces- 
sary, then, not to enable plants to exercise their 
respiratory functions, provided the atmosphere 
is unmixed with accidental impurities, but to 
carry off noxious vapours generated in the arti- 
ficial atmosphere of a glazed house, and to pro- 
duce dryness or cold or both. 
“ When the external air is admitted into a 
glazed house containing a moist atmosphere, it, 
under ordinary circumstances, is much colder 
than that with which it mixes; the heated damp 
air rushes out at the upper ventilators, and the 
drier cold air takes its place; the latter rapidly 
abstracts from the plants and the earth, or the 
vessels in which they grow, a part of their mois- 
ture, and thus gives a sudden shock to their 
constitution, which cannot fail to be injurious. 
This abstraction of moisture is in proportion to 
the rapidity of the motion of the air. But it is 
not merely dryness that is thus produced, or 
such a lowering of temperature as the thermo- 
meter suspended in the interior of the house 
may indicate; the rapid evaporation that takes 
place upon the admission of dry air produces a 
degree of cold upon the surface of leaves, and of 
which our instruments give no indication. To 
counteract these mischievous effects many con- 
trivances have been proposed, in order to insure 
the introduction of fresh air warm and loaded 
with moisture,—such as compelling the fresh 
air to enter a house after passing through pipes 
moderately heated, or over hot water pipes sur- 
rounded by a damp atmosphere, and so on, the 
advantages of which of course depend upon the 
objects to be attained. If ventilation is merely 
employed for the purpose of purifying the air, as 
is often the case in hothouses and in dung-pits, 
it should be effected by the introduction of fresh 
air damp and heated. If it is only for the pur- 
pose of lowering the temperature, as in green- 
houses, or in the midst of summer, the external 
air may be admitted without any precautions. 
But it is very commonly required in the winter, 
for the purpose of drying the air in houses kept 
at that season at a low temperature; such, for 
instance, as those built for the protection of 
heaths, and many other Cape and New Holland 
plants: in these cases it should be brought into 
the house as near the temperature of the house 
as possible, but on no account loaded with mois- 
ture. One of the principal reasons for drying 
the air of such houses is, to prevent the growth 
of parasitical fungi, which, in the form of moul- 
diness, constitute what gardeners technically call 
‘damp.’ These productions flourish in damp air 
at a low temperature, but will not exist either 
in dry cold air or in hot damp air. If the air of 
cool greenhouses is allowed to become damp, the 
fungi immediately spring up on the surface of 
any decayed leaves, or other matter which may 
be present, when they spread rapidly to the 
young and tender parts of living plants; and 
when this happens they consume the juices, 
choke the respiratory organs, and speedily de- 
stroy the object they attack. [See the article 
Movup.| Ventilation is also required in the 
winter in such places as dung-pits or frames, 
especially those in which salad, cucumbers, and 
similar plants are grown. In those cases, the 
object is to dry the air, in order that the plants 
may not absorb more aqueous particles than 
they can decompose and assimilate. Although 
plants of this kind will bear a high degree of at- 
mospherical moisture in summer, when the days 
are long and the sun bright, and when, conse- 
quently, all their digestive energies are in full 
activity, yet they are by no means able to en- 
dure the same amount in the short dark days of 
winter, when, from the want of light, their pow- 
ers of decomposition or digestion are compara- 
tively feeble. Hence, no doubt, the advantage 
of growing winter cucumbers in forcing-houses, 
instead of dung-frames. One of the causes of 
success in the Dutch method of winter forcing 
is, undoubtedly, their avoiding the necessity of 
winter ventilation, by intercepting the excessive 
vapour that rises from the soil, and which would 
otherwise mix with the air. For this purpose 
the porous earthen pots in which plants grow, of 
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