VENTILATION. 



273 



air is stagnant, the absorption of carbonic 

 acid may be much slower." 



" Perspiration is another vegetable 

 function which must be maintained in 

 healthy action. The quantity of water 

 that flies off from the surface of a plant 

 will, cceteris paribus, be determined by the 

 rapidity of the motion of air passing over 

 its surface. In an absolutely still air, 

 perspiration will be reduced to its mini- 

 mum, and it will increase within certain 

 limits, in proportion to the quickness 

 with which the air sweeps over it." " If 

 the motion of air is thus favourable to 

 the two great operations of feeding and 

 perspiration, we shall find that it is 

 equally needed day and night ; for per- 

 spiration goes on principally during day- 

 light, and feeding in the hours of dark- 

 ness. A good system of aeration must 

 then be constantly in action. How to se- 

 cure that is the great horticultural pro- 

 blem which now remains to be solved." 



Mechanical inventions of considerable 

 ingenuity have been applied for the pur- 

 pose of ventilating hothouses, in some of 

 which the motive power employed to put 

 them in action has been derived from 

 increase of temperature within the houses. 

 This regulates temperature only ; but it 

 has little power over the purification of 

 the air, the escape of impure air, or ad- 

 mission of fresh supplies of this indispen- 

 sable element. It is true that whilst the 

 temperature within is high, these ma- 

 chines operate, and, in doing so, allow a 

 considerable quantity of vitiated air to 

 escape from the top of the house, and a 

 corresponding supply of fresh air to be 

 taken in at front to make up the loss ; 

 but when the internal temperature is low, 

 they cease to act, and all change of air 

 is arrested. So it is too often, even where 

 this is accomplished by manual labour, 

 the operator being almost invariably 

 guided by the state of the thermometer, 

 and not by the state of the air in which 

 the plants breathe, and from which they 

 derive so much of their health and vigour. 

 Again, ventilation, as usually applied, is 

 confined to the daytime only, and can 

 only be serviceable to the plants so far as 

 perspiration is concerned, which goes on 

 principally during the day ; while feed- 

 ing, the other of the two great operations 

 in plants, goes on during the night, when 

 the house is shut up as close as if the ad- 

 vol. i, 



mission of air were to be instantly fatal 

 to them. Were it not that hothouses are 

 not so well jointed as a cabinetmaker 

 would joint a piece of furniture, or a 

 cooper a tub, and that glass is fortunately 

 liable to be broken, plants could not exist 

 in these structures ; but, fortunately for 

 them, a constant process of ventilation or 

 aeration is going on night and day almost 

 unknown to man, through the various 

 chinks in the framework and fractures 

 in the glass. Under the old system of 

 ventilation, plants prospered better in 

 houses glazed with the overlaps left open, 

 than in those that were cross-puttied — 

 and still more so where the glass was 

 small than where it was of the largest 

 size, because there were many more open- 

 ings for the admission of air from with- 

 out, these openings being small, and very 

 equally distributed over the roof. There 

 is, however, no possible necessity for re- 

 turning to that mode of glazing, which is 

 so destructive to the glass during frosts, 

 as the same means may be attained by a 

 proper and equally diffused mode of 

 ventilation. 



Calculations have been made as to the 

 amount of ventilation necessary for pub- 

 lic buildings, and where many people are 

 congregated together ; but we are not 

 aware if such have been made regarding 

 plant or hot houses. The calculation for 

 these would be much more difficult, as 

 every plant or tree would require a sup- 

 ply of air in proportion to its size, the 

 number of its leaves, &c. 800 cubic 

 inches of air per minute is a sufficient 

 pulmonary supply for a man, exclusive 

 of that required to carry off the insensible 

 perspiration, which is calculated to be 

 about ten grains per minute, when no 

 particular muscular exertion is making. 



Having such data before us, it is not 

 difficult to calculate the size of the open- 

 ings necessary to ventilate these public 

 buildings ; but having no such guide in 

 regard to hothouses, the difficulty is ob- 

 viously great. Were it possible to calcu- 

 late accurately the quantity of air required 

 to be changed per minute in a hothouse, 

 the next thing would be to estimate the 

 proper size and position of the openings 

 for the escape of the foul or exhausted 

 air, and also for the entrance of fresh air 

 to supply its place. 

 Hood, in " Remarks on Ventilation as ap- 



2 M 



