Polmaise Method of Heating Greenhouses, Sfc. 387 



rapid at high temperatures, than those bodies which radiate 

 most, is one of the most remarkable of the laws of cooling, 

 and has led many practical men into serious mistakes, in 

 estimating the merits of hot-air heating. 



The atomic particles of atmospheric air, being separated 

 and rarefied by the application of a high temperature, it is 

 plain that this air must be changed in its properties, and this 

 is practically correct. 



Atmospheric air in passing into a house at a high temper- 

 ature, over a highly heated surface, not only loses its water ; 

 but the small particles of organic matter, which it holds in 

 suspension, are decomposed by the heat, and resolved into 

 various elementary gases. This is one of the causes of the 

 unpleasant odor which invariably results from this method 

 of heating, as in common stoves, Polmaise furnaces, &c. ; 

 but, in addition to this, the aqueous vapor of the atmos- 

 phere is almost entirely decomposed, the oxygen entering 

 into combmation with the iron wherever it can act upon it, 

 and the contained hydrogen, now set free, mixes with the 

 air. The changes which have thus taken place renders the 

 atmosphere deleterious in an extraordinary degree, both to 

 animal and vegetable life. 



The mixture of the hydrogen, thus disengaged, is even 

 more injurious to the plants than the alteration which has 

 taken place in the hygrometric condition of the atmosphere, 

 as this will be for some time supplied by the water contained 

 in their tissue, until it be restored by absorption or evapora- 

 tion, which is easily effected. 



The particles of animal and vegetable matter, as I have 

 said, are rapidly decomposed by the heat, and they then 

 produce extraneous gases, consisting of sulphuretted, phos- 

 phoretted and carburetted hydrogen, which, in the state in 

 which they then exist, are highly inimical to every species 

 of vegetable life. 



The quantity of hydrogen eliminated by the decomposi- 

 tion of water is 1325 cubic inches for every cubic inch of 

 water that is decomposed. It is, therefore, easy to account 

 for the effects produced on vegetation by hot-air currents, in 



