652 



FRUIT GARDEN. 



the drain becomes greater also : air entering at 

 44°, and escaping at 80°, carries off three times 

 as much as it brought in ; escaping at 90°, four 

 times. Now the escape of air from our best 

 glazed buildings is considerable at all times, 

 even when the lights are closed; and if the 

 glazing be defective, and the laps be not puttied, 

 it is very great indeed. The amount of moisture 

 thus abstracted cannot be very easily estimated, 

 varying exceedingly according to the height 

 and construction of the building heated. There 

 exists, however, another drain of moisture, con- 

 stantly affecting all hothouses, however per- 

 fectly constructed, and however cautiously ven- 

 tilated — viz., the condensation on the glass. In 

 this case the expenditure is capable of pretty 

 accurate calculation. It has been ascertained 

 by experiment that each square foot of glass 

 will cool lj cubic feet of air as many degrees 

 per minute as the temperature of the inner air 

 exceeds that of the outer air — that is to say, if the 

 temperature of the outer air be 44°, and of the 

 house 66°, for every square foot of glass 1| cubic 

 feet of air will be cooled 22° per minute ; and 

 the moisture which this air held in solution, in 

 virtue of its 22° of heat, will be deposited on 

 the glass, and will either drain away out of the 

 house or fall in drip. The greater the difference 

 between the temperature of the internal and ex- 

 ternal air, the greater will be the amount of 

 condensation ; and be it observed, that the ca- 

 pacity of air for moisture does not increase 

 simply in the arithmetical ratio of its tempera- 

 ture, but by a scale considerably more rapid, so 

 that the expenditure of moisture at high tem- 

 peratures is much greater than at low tempera- 

 tures, for equal differences between internal and 

 external air." 



We stated at the beginning of this subject, 

 that in the olden time heat and light only were 

 thought of as necessary elements in hothouse 

 management, and hence the miserable appear- 

 ance of the productions in them, compared with 

 those met with in our improved state of cultiva- 

 tion. The skilful balancing of the temperature 

 and moisture of the air, as Dr Lindley has so 

 correctly designated it, is the grand secret. 

 Unless that balance be maintained, every other 

 part of cultural skill goes for nothing. The 

 means of effecting this deserve inquiry. The 

 water required for the actual support of the 

 plants at their roots is quite insufficient for this 

 other purpose ; recourse is therefore had to the 

 application of water, so disposed of throughout 

 the house as to create moisture in the atmos- 

 phere of it. This is, however, too hap-hazard a 

 way : by pouring water on the floor until it be- 

 comes nearly sodden, if of earth ; on the flues or 

 hot- water pipes, when probably at their greatest 

 heat, until the interior is reeking with half-boil- 

 ing vapour as thick as a London fog ; or if steam 

 be employed, the steaming- valve is opened, and 

 the house is suddenly charged with scalding 

 vapour, to an extent to which an Indian jungle 

 is purity itself, while a few minutes before all 

 was nearly as dry as an Arabian harmattan ; — 

 while we want only an increase of humidity 

 analogous to what occurs in nature, a humidity 

 which there is governed by the temperature — 



an atmosphere possessing great elasticity. There 

 is a danger, however, of overdoing this artificial 

 endeavour to establish the balance above alluded 

 to; and when this extreme is fallen into, the 

 remedy is as bad as the disease, and the process 

 of transpiration is prevented from proceeding in 

 its proper course. We have shown, vol. i. p. 

 248, and elsewhere, various means for effecting 

 this increase of moisture; but no certain results 

 can ever be expected in practice, until the hy- 

 grometer becomes as permanent a piece of fur- 

 niture in every hothouse as the thermometer is 

 at present. The neglect of this instrument is 

 notorious; we do not recollect of having seen half- 

 a-dozen in use in hothouses during our horticul- 

 tural existence. The one instrument is as use- 

 ful as the other, and until both be equally con- 

 sulted, that balance on which so much of our 

 success depends can never be accurately at- 

 tained. Intimately connected with this subject 

 is another, which has of late (by some means or 

 other, probably imaginary, or not previously in- 

 quired into) caused great alarm amongst some 

 cultivators, who have attributed their disasters 

 to a scorching influence of the glass during the 

 night. When we first heard of this, we began 

 to quake for sheet and all other glass of modern 

 introduction, as the evil appeared to be directed 

 against that material. Day-scorching we have 

 had some experience of, but the idea of noctur- 

 nal scorching was rather astounding. A refer- 

 ence to the excellent papers of Mr Daniell, which 

 we have already, and we hope with some degree 

 of usefulness, quoted, set our mind at rest upon 

 this rather alarming subject. " In a well venti- 

 lated hothouse," says that authority, " by water- 

 ing the floor in summer we may bring the dew- 

 point within four or five degrees of the tem- 

 perature of the air, and the glass will be per- 

 fectly free from moisture ; by closing the venti- 

 lators, we shall probably raise the heat ten or 

 fifteen degrees, but the degree of saturation will 

 remain nearly the same, and a copious dew will 

 quickly form upon the glass, and will shortly 

 run down in streams. A process of distillation 

 is thus established, which prevents the vapour 

 from attaining the full elasticity of the tempera- 

 ture. This action is beneficial within certain 

 limits, and at particular seasons of the year; but 

 when the external air is very cold, or radiation 

 proceeds very rapidly, it may become excessive 

 and prejudicial. It is a well-known fact, but 

 one which, I believe, has never yet been pro- 

 perly explained, that by attempting to keep up 

 in a hothouse the same degree of heat at night 

 as during the day, the plants become scorched. 

 From what has been premised, it will be evident 

 that this is owing to the low temperature of the 

 glass, and the consequent low dew-point in the 

 house, which occasions a degree of dryness that 

 quickly exhausts the juices." 



A great amount of evaporation, as we have 

 already stated, takes place during night, and is 

 greatly increased when a cold frosty night is 

 succeeded by bright sunshine during the early 

 part of the following day. This must be pro- 

 vided against, particularly during winter -forc- 

 ing; at other times its effects are less palpable. 

 How this is to be counteracted is deserving of 



