260 PRINCIPLES OF VENTILATION. 



vapor ; and suppose, further, that the temperature of the earth's 

 surface were uniformly 212, and its weight equal to 30 inches 

 of mercury. Now, at the elevation of about six miles, the 

 weight would be fifteen inches, or one half of that below ; at 

 twelve miles, it would be 7 inches, or one fourth of that at the 

 surface, and the temperature would probably diminish 25 at 

 each of these intervals. It could not diminish more ; for the 

 diminution of temperature 25 reduces the force of vapor one 

 half. If, therefore, a greater reduction of temperature were to 

 take place, the weight of the incumbent atmosphere would in- 

 crease, being converted into water, and the general equilibrium 

 would thus be disturbed by condensation in the upper regions." 

 It has been observed, that if the ventilators of hot-houses be 

 kept close during the day, the internal temperature will rise, 

 although no artificial heat be applied ; from which it has been 

 supposed that glass freely admits the calorific rays to pass 

 through it, in their descent, but arrests it in their upward progress. 

 Professor Robinson has proved that glass freely transmits the 

 luminous rays, but stops the calorific rays, till it becomes satu- 

 rated with heat to a certain degree ; which proves, also, that 

 light and heat are not identical, although both obey the same 

 laws of reflection, refraction, and radiation. Although heat may 

 arise from the same source as light, and possess a great affinity 

 to it, yet caloric possesses properties peculiar to itself, and differs 

 in its degree of affinity for other bodies ; for, although it has a 

 tendency to come to an equilibrium, when bodies differing in 

 quality are exposed to its influence, it has been found thit 

 these bodies do not all come into an equal temperature at the 

 same time. Caloric readily enters into some bodies, and freely 

 combines with them, whereby their temperature becomes in- 

 creased, and their properties sometimes changed. [See Part I., 

 Construction, sec. Glass, p. 106.] 



From the foregoing remarks, it will easily be perceived that 

 many of our operations, in the management of hot-houses, are 

 not only theoretically wrong, but diametrically opposed to the 

 laws of nature. Our methods of ventilation are wrong in prac- 

 tice, because our notions are wrong in principle. We raise the 



