PRINCIPLES OF HEATING HOT-HOUSES. 157 



carbon, which, in the state in which they exist, are highly inim- 

 ical to vegetable life. * 



The quantity of hydrogen which is eliminated by the decom- 

 position of water contained in the air is one thousand three hun- 

 dred and twenty-five cubic inches for every cubic inch of water 

 that is decomposed ; and if the dew point of the air be 45 at an 

 average, this quantity will be given out from every seventy-two 

 cubic feet of air wliich passes over the heated surface. It is, 

 therefore, not difficult to account for the effects produced on 

 vegetation by hot-air stoves, in consequence of the air, when 

 thus artifically dried, abstracting too much moisture from their 

 leaves. It is also clear that the injury must increase in propor- 

 tion to the length of time the apparatus continues in use, by the 

 plants being surrounded by, and compelled to inhale, these extra- 

 neous gases, which are evolved from the decomposition of the 

 constituents of the atmosphere. 



The extreme dryness of the air, after it has been deprived of 



* I am unable to ascertain the exact nature and extent of the change 

 which atmospheric air undergoes by being passed over intensely heated 

 metallic bodies j but whatever be the chemical alteration which occurs, 

 a physical change undoubtedly takes place, by which its electrical con- 

 dition is altered. 



From some experiments recorded in the Philosophical Transactions of 

 the Royal Society, made with a view of ascertaining the effect produced 

 on the animal economy by breathing air which has passed through 

 heated media, it appears that the air which has been heated by metallic 

 surfaces of a high temperature must needs be exceedingly unwholesome. 

 A curious circumstance is related, in reference to these experiments, 

 which is illustrative of this fact. 



" A quantity of air, which had been made to pass through red-hot 

 iron and brass tubes, was collected in a glass receiver, and allowed to 

 cool. A large cat was then plunged into this air, and immediately she 

 fell into convulsions, which, in a minute, appeared to have left her 

 without any signs of life ; she was, however, quickly taken out and 

 placed in the fresh air, when, after some time, she began to move her 

 eyes, and, after giving two or three hideous squalls, appeared slowly to 

 recover. But on any person approaching her, she made the most violent 

 efforts her exhausted strength would allow to fly at them ; insomuch, 

 that, in a short time, no one could approach her. In about half an hour 

 she recovered, and became as tame as before." 



