CONSIDERED WITH REFERENCE TO HORTICULTURE. 



87 



with the moisture deposited on the glass ; at No. 4 it is in the same state ; at 

 No. 5 it has ceased to lose heat or moisture ; at No. 6 and 7 the same ; at 

 No, 8 it again comes within the influence of the pipes, and is heated, becoming 

 again very dry. Now the air which descends to the floor (8) in the first 

 place, is a small and feeble current, and secondly, is nearly saturated, so 

 that it can take up little moisture ; and what little it does get is because the 

 floor, being slightly warmed by the radiation of the pipes, warms, and at 

 the same time moistens, the air ; but, nevertheless, the air at No. 1, in which 

 air a visitor walks, is anything rather than saturated. My belief is, that air 

 nearly saturated is always agreeable to the feelings. Dry air, which is ab- 

 sorbing moisture, is anything but agreeable ; hence the unpleasant sen- 

 sation in orchidaceous houses. Now it is unnecessary to show how Mr. 

 Penn's plan obviates all these defects, and produces a uniformly saturated 

 atmosphere which must be wholesome alike to plants and men." (Gard. 

 Mag. vol. xvi. p. 273.) Corbett's mode of heating, by circulating water in 

 open gutters (which can be closed at pleasure), is said to keep the air of 

 those houses in which it is employed more effectually saturated with mois- 

 ture than any other mode. {See Gard. Mag. 1841, p. 57, and Gard. Gaz. 

 184], Jan. 23.) 



272. Though too much moisture can scarcely be admitted into the at- 

 mosphere of plant structures kept at a high temperature, yet this is not the 

 case with houses in which the degree of heat is not much greater than that 

 of the open air ; for example, Greenhouses. In these houses the object of 

 the gardener is frequently more to exclude frost than to increase the heat 

 already there ; and consequently, when the thermometer in the open air 

 ranges between 40° and 50°, no fire heat is required. In this case, however, 

 if the air is not agitated by some artificial process, it becomes surcharged with 

 moisture or damp, not in a state of elastic vapour, but as steam or fog. This 

 excess is favourable to the growth of mould or fungi on the surface of the 

 soil in the pots ; and being, from the excess of water, unfavourable for the 

 respiration of the leaves, it occasions them to decay and drop off. In cases 

 of this kind, it is more desirable to introduce dry air than moist air ; but as 

 the air of the external atmosphere is generally not drier than that of the 

 house, it is found desirable to employ heat so as to raise the temperature of 

 the house, and this raised temperature having an increased capacity for 

 heat, the water which was before in a state of mixture with the air is now 

 changed into elastic vapour ; the consequence is, that the air of the house 

 becomes dried, the growth of fungi checked, and the leaves of the plants no 

 longer decay and drop off. Some persons are of opinion that Mr. Penn's system 

 of circulating the air is only applicable to houses where fire heat is constantly 

 used, and that for greenhouses and conservatories it is nearly useless. An 

 experienced and scientific gardener, however, is of a directly contrary opinion. 

 " In addition to its use in forcing-houses, where it may be deemed indispen- 

 sable," he says, " I would adopt it in the greenhouse in preference to all 

 other modes of heating. Greenhouse plants invariably do well while w^e can 

 admit plenty of air, or while we can maintain a current to counteract the 

 effects of damp. But there are sometimes months together that we cannot 

 open a sash to effect this, without admitting air injuriously cold, or saturated 

 with moisture ; it is then we are doomed to see many of our tender favourites 

 pine, droop, and die ; and then that the advantage of an independent atmo- 



