SCORCHING IN DAMP HOUSES. 207 



"becomes indispensably necessary. There is no doubt that bedewing 

 by day does more harm than good. Experience shows that when plants 

 are so treated they lose their verdure and become yellow, especially if 

 the sun shines on them immediately after the operation. It cannot be 

 useful, for it dries up before it can be absorbed. It must do harm ; 

 because the rapid evaporation from the surface of a wet leaf under a 

 bright sun is attended by a degree of cold which paralyses the vitality 

 of the leaf itself. When rain falls by day the sky is overcast ; or if we 

 have a rainstorm in sunshine the occurrence is casual and the effect is 

 transient, but in a hothouse so mismanaged a gardener has to encounter 

 the consequence of a continually recurring bad effect. 



But there are some circumstances, easily overlooked, which 

 interfere very seriously with this power, and which, it will he 

 conceived, may reduce it very much helow the expectations of 

 the cultivator. The most imsuspected of these is the destruc- 

 tion of aqueous vapour hy the hot, dry, ahsorhent surface of 

 flues. The advantages of hot-water pipes over brick flues 

 arises in great measure from the former not drying the air. 

 Gardeners explain the difference in the action of the two, hy 

 saying that the dry heat produced by hot- water pipes is sweeter 

 than that given off by flues ; which is not a very infelHgible 

 expression. The fact is, that, in houses heated by flues, the 

 soft burnt clay of the brick flues robs the air of its moisture, 

 while the unabsorbent surface of iron pipes abstracts nothing. 



Another source of dryness is the coldness of the glass roof, 

 especially in cold weather, when its temperature is lowered by 

 the external air, in consequence of which the moisture of the 

 artificial atmosphere is precipitated upon the inside of the 

 glass, whence it runs down in the form of " drip." Daniell 

 observes that the glass of a hot-house, at night, cannot exceed 

 the mean of the external and internal air ; and, taking them at 

 80° and 40°, 20° of dryness are kept up in the interior, or a 

 degree of saturation not exceeding "538. To this, in a clear 

 night, we may add at least 6° for the effects of radiation, to 

 which the glass is particularly exposed, which will reduce the 

 saturation to '424 ; and this is a degree of drought which must 

 be destructive. It wiU be allowed that this is by no means an 

 extreme case, but one that must frequently occur during the 

 winter season. Some idea, he adds, may be formed of the 



