Ventilation

The accumulation of gaseous matters, such as sulphurous acid and ammonia, and the consumption of carbonic acid, render ventilation essential to the health of plants in forcing-pits and hut-houses. They cannot inhale air overloaded with these contaminations, without being speedily injured, and the proportions of those gases which rapidly cause disease, or even death, are much less than the gardener usually suspects; for if the sulphurous acid amounts to no more than one cubic foot in ten thousand of the air in a hot-house, it will destroy most of its inhabitants in two days. To avoid such destruction, for the comfort of visitors, and above all for the sake of the plants' vigour, air should be admitted as freely as the temperature will permit. The foul warm air can be easily allowed to escape through ventilators in the most elevated parts of the roof, and fresh warm atr can be as readily supplied through pipes made to enter near the flooring of the house after passing through hot water, or other source of heat.

I am quite aware that Mr. Knight has stated that he paid little attention to ventilation, and that plants will be vigorous for a time in Wardian cases; but this does not prove that their Creator made a mistake when he placed vegetables in the open air.

Plants confined in houses or other close structures may be made to grow in spite of such confinement; but all experience proves that other favourable circumstances, such as heat, light, and moisture being equal, those plants are most vigorous and healthy which have the most liberal supply of air.

There have been many modes suggested for self-acting ventilators, descriptions of which may be found in Loudon's Encyclopedia of Gardening, and the Transactions of the London Horticultural Society; but there are none that can supersede the gardener's personal care, directed by the thermometer and experience.

The practice of all ventilation is founded on the principle that the hottest air rises to the highest part of the house, and if there allowed to escape, colder air will come in below, if allowed, to supply its place. To prevent the hot air escaping too rapidly, the ventilators should be fitted with doors or caps, capable of regulating the size of the orifice; and the openings admitting fresh and colder from without, should have similar regulators, and be made by means of pipes passing through the bark-bed, tank of hot water, or other source of heat, so that the reduction of temperature be not too rapid.

Some guide in constructing ventilators proportioned to the size of the house to be ventilated, will be found in Mr. Hood's following table of the quantity of air, in cubic feet, discharged per minute, through a ventilator, of which the area is one square foot.

The foregoing table shows the discharge, through a ventilator of any height, and for any difference of temperature. Thus, suppose the height of the ventilator from the floor of the room to the extreme point of discharge to be thirty feet, and the difference between the temperature of the room and of the external air to be 15°, then the discharge through a ventilator one foot square, will be 347 cubic feet per minute. If the height be forty feet, and the difference of temperature 20°, then the discharge will be 465 cubic feet per minute.