268 EFFECTS OF VENTILATION. 



temperature, but rather to the hygrometrical state of the atmos- 

 phere, we ought to look. We ought to regulate the admission 

 of air, not solely by the thermometer, but also by the hygrome- 

 ter ; for, upon the latter condition, the health of the plants, and 

 the perfection of their flowers and fruit, very much depend ; 

 and, consequently, it is a matter which ought to be studiously 

 considered. Nothing is more injurious than the admission of 

 currents of air when the external temperature is lower than the 

 internal one ; and more especially so to plants that have been 

 for a considerable time subjected to a high temperature by arti- 

 ficial heat. 



The causes which operate in rendering the atmosphere of 

 hot-houses unnaturally arid may be said to be two-fold. The 

 first is the condensation of moisture upon the glass, arising 

 from the action of the external cold upon its upper surface. The 

 second is the escape of heated air through the laps and crev- 

 ices of the glass, and otherwise. This heated air escaping, car- 

 ries along with it a large quantity of contained moisture, the 

 loss of air being supplied with cold, dry air, which finds access 

 by the same means. The loss of heat and moisture sustained 

 by these means is far more than would be supposed by those 

 who have not calculated the amount. 



7. We have seen that the quantity of moisture a cubic foot 

 of air will hold in invisible suspension depends on its tem- 

 perature ; and as the temperature is increased, so is its capacity 

 for moisture. Suppose, then, that this capacity is doubled 

 between the temperature of 40 and 60 ; that is to say, every 

 cubic foot of air that enters the house at 40, and escapes at 60, 

 carries with it just double the quantity of moisture it brought in. 

 Now, every one must be sensible that these circumstances, con- 

 tinued for any length of time, must render the atmosphere of 

 the house too arid for healthy vegetation ; and, consequently, if 

 the deficiency of moisture so occasioned be not supplied by 

 artificial evaporation, then the plants must part with their secre- 

 tions to supply the atmospheric demand, and the soil and other 

 materials in the house will also be drained of their moisture, to 

 make up the deficiency. The .greater the difference between 



