144 APPLICATION OF PRINCIPLES. 



is, that the air of houses heated by flues is, under 

 equal circumstances,; much drier than that where hot- 

 water pipes are employed ; because 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 tem- 

 perature is lowered by the external air, in conse- 

 quence of which the moisture of the artificial atmo- 

 sphere is precipitated upon the inside of the glass, 

 whence it runs down in the form of " drip." Mr. 

 Darnell observes that the glass of a hot-house, at 

 night, cannot exceed th% mean of the external and 

 internal air ; and, taking them at 80° and 40°, 20 

 degrees of dryness are kept up in the interior, or a 

 degree of saturation not exceeding *528. 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 

 very prejudicial. It will be allowed that this is not 

 an extreme case, and much more favourable than 

 must frequently occur during the winter season. 

 Some idea, he adds, may be formed of the prodi- 

 giously increased drain upon the functions of a plant, 

 arising from an increase of dryness in the air, from 

 the following consideration: — If we suppose the 

 amount of its perspiration, in a given time, to be 57 

 grains, the temperature of the air being 75° and the 

 dew-point 70°, or the saturation of the air being "849, 



