Warmth and Moisture in Plant Structures. 107 



pipe, heated to the same degree as an equal surface of bricks, 

 will extract the moisture from any substance (a wet cloth, for 

 example) as readily, which must be admitted ; and the two 

 bodies will act upon the moisture contained in air in the same 

 manner. But it is said that the moisture extracted by the heated 

 pipe is driven into the body of the house, while the brick surface 

 absorbs, and carries it out of the house. I think not, as the 

 moisture extracted in the process of drying the piece of cloth 

 instanced is turned into palpable vapour, and also palpably 

 driven back into the house ; or, rather, being rarefied, it flies 

 upwards until it comes in contact with the radiating and cooling 

 surface of the glass roof, where it is condensed, and falls into 

 the house, or is carried into the gutter outside, as the case may 

 be; but in either case it is driven back by the heated or ex- 

 tracting surface, with which, unless confined, it cannot possibly 

 be brought in contact. Therefore, unless the invisible part of 

 such moisture is governed by laws different from those which 

 govern the grosser, or visible, part, which is improbable, perhaps 

 impossible, the above theory falls to the ground ; that is, 

 while the flues are hotter than the air, or substances that sur- 

 round them, as, if I conclude aright, moisture is repelled (and 

 cannot be absorbed) by any thing hotter than itself. When 

 flues, after being heated and all the moisture they contain driven 

 off, become cold, then, it is true, they will greedily absorb mois- 

 ture until they are equally saturated with the air, or substances 

 that surround them ; but, when heat is again applied, the inside 

 of the flue is first acted upon, and consequently the greatest 

 part of even this moisture is driven back, as may be observed, 

 by the steam created upon the application of heat to a flue in 

 this condition. Thus, as regards moisture, I do not see much 

 to prefer in pipes, to flues heated to the same degree : but the 

 greater objection to which flues must ever be liable, is their 

 aptness to become heated to a destructive extent, while pipes can 

 never be so heated. 



Having thus stated why I think the quantity of moisture 

 absorbed of no moment, I will state why I consider that the 

 aridity said to prevail in plant-houses has also been overrated, 

 and mention, in support of this, some simple facts that are every 

 day occurring before the eyes of the observant. Suppose a 

 day of strong sunshine (during hard weather) when air has 

 been freely admitted; then draw up your sashes, and fasten the 

 ends of the cords, or sash-lines, that hold them up, to holdfasts 

 in the wall ; then go at midnight, or any other time after strong 

 fires and radiations have (as is held forth) reduced the in- 

 ternal air to a very cinder of its former self. Under such cir- 

 cumstances, aggravate them as you may, examine your sashes, 



