SECTION II. 



PRINCIPLES OF HEATING HOT-HOUSES. 



1. Effects of artificial heat. — The effects that are produced 

 upon the functions of vegetables, by atmospheric air that has 

 passed over intensely heated surfaces, are perceptible to the 

 most casual observer. The changes, therefore, that are produced 

 upon atmospheric air by subjecting it to a high temperature, are 

 of the utmost importance to the horticulturist, and consequently 

 demand our particular attention. 



When common air passes over highly heated surfaces, the 

 small particles of animal and vegetable matter, (organic mat- 

 ter,) which are always held in suspension by it, are decomposed 

 by the heat, and resolved into various elementary gases. This 

 is one of the causes of the unpleasant smell which results from 

 this method of heating, as in common stoves, Polmaise furnaces, 

 &c. But, in addition to this, the aqueous vapors of the atmos- 

 phere are almost entirely decomposed, the oxygen entering into 

 combination with the iron, and the hydrogen mixing with the 

 air. The changes which have thus taken place, render the 

 atmosphere extremely deleterious to both animal and vegetable 

 life. 



The mixture of the hydrogen thus disengaged is even more 

 injurious to the plants than the alteration which bas taken place 

 in its hygrometric state, as this will be partly supplied by the 

 moisture contained in their tissue, until it be restored to the 

 atmosphere by evaporation, which is easily effected. 



The particles of animal and vegetable matter — as we have 

 said — are decomposed by the heat; and they then produce 

 extraneous gases, consisting of sulphuretted, phosphuretted, and 

 carbi\retted hydrogen, with various compounds of nitrogen and 



