270 EFFECTS OF VENTILATION. 



bination, and by raising the humidity in the atmosphere of the 

 house, to compensate for that carried away by the egress of 

 heated air, the plants will breathe an atmosphere more con- 

 ducive to their healthy development, and will be benefited by 

 the change. 



8. Every gardener has observed the water on the under sur- 

 face of the glass, in the morning, before the sun has risen, 

 warmed the glass, and driven it off again, in the form of aqueous 

 vapor. This affords us a good illustration of the immense quan- 

 tity of moisture carried upwards by the heated air, and depos- 

 ited upon the glass, by condensation. This moisture is, of 

 course, taken away from the plants, and other bodies capable 

 of giving it off, and is demanded by the air as it becomes warm, 

 and capable of carrying a larger quantity than when no fire was 

 applied, or rather, when the temperature of the house and 

 the temperature of the external air were alike, for in such case 

 no condensation on the glass would take place ; and, as I have 

 remarked, the proportion of water deposited will be in exact 

 ratio to the intensity of the external cold ; thus, the greater 

 the difference, the greater the deposition ; for then the action 

 of the external cold upon the upper surface of the glass being 

 greater, and the two atmospheres being brought into more rapid 

 proximity, the particles of heated air are cooled as quickly as 

 they ascend to the under surface of the glass ; they then fall to 

 supply the place of others, leaving the contained moisture upon 

 the cooling surface, in the form of dew, the same process 

 being repeated through the whole night, or until an equality of 

 temperature is established ; the quantity thus deposited amounts 

 to immense volumes of water. 



9. Experiments have proved that each square foot of glass 

 contained in the roof of a hot-house will cool down 1J cubic 

 feet of heated air per minute as many degrees as the temper- 

 ature of the internal exceeds that of the external atmosphere. 

 Suppose, for instance, that the external air stands at 40, 

 and that of the house 60 ; then, for every square foot of 

 glass contained in the house, one and one fourth cubic feet of 



