Polmaise Method of Heating Greenhouses, S^c. 437 



vapor at the temperature of 100° than at 20°, and therefore 

 the expansive power of these gases must be decreased by 

 saturation, and consequently their escape from the house pre- 

 vented. The following Table, showing the quantity of 

 vapor contained in atmospheric air, at different temperatures, 

 computed from Dalton's experiments on the elastic force of 

 vapor, will be useful to those who are interested in this im- 

 portant subject : — 



By the foregoing Table we find that a cubic foot of air at 

 the temperature of 20°, contains only 1.52 grains of moist- 

 ure, while the cubic foot of air at 100° contains 19.39 grains, 

 or nearly twenty times the amount. 



Now, if the amount of moisture abstracted by the rarefied 

 air, the absorbing surfaces of flues or other bodies, must be 

 restored to the atmosphere by evaporation of water into the 

 house, then it appears evident that the internal atmosphere 

 must be kept continually at the point of saturation, through- 

 out the whole of the winter months, and this is precisely 

 how it is with many, the effects of which are no less injuri- 

 ous than excessive aridity ; for we all know very well, that 

 when not in a state of active growth, an excess of moisture 

 in the air, either with a high or low temperature, is prejudi- 

 cial to plants ; and in winter, plants are rarely in a condition 

 to bear an excess of moisture for any length of time. 



