436 Polmaise Method of Heating Gree7ihoiises, Sfc. 



Art. II. Polmaise Method of Heating Greenhouses and 

 Hothouses, compared with Hotwater, scientifically and 

 practically considered. By R. B. Leuchars. 



{Continued, from p. 389.) 



We will now proceed to take a view of the influence of 

 the atmospheres warmed by the two methods of heating 

 aheady mentioned. 



It has been shown that animals cannot subsist in an at- 

 mosphere which has been warmed in its passage through 

 metallic tubes, and it has also been satisfactorily ascertained 

 that plants will not thrive in an atmosphere which has been 

 heated to a certain temperature, even though its due equiva- 

 lents of aqueous vapor be restored to it. 



It is scarcely necessary to prove facts with which every 

 gardener is acquainted, regarding the extreme susceptibility 

 of many plants to the presence of deleterious gases in the 

 air which they respire. Indeed they are, in many instances, 

 more susceptible than animals ; we have often seen plants 

 injured by tobacco smoke, when myriads of aphides and 

 coccus remained alive. The facts are even more strikingly 

 sensible under the influence of sulphurous acid gas, as well 

 as chlorine, muriatic, hydrogen, and other gases ; by these, 

 many plants will be destroyed when insects remain uninjur- 

 ed ; and we have proved that to apply these gases in quan- 

 tity sufficient to exterminate insects, vegetation, if present, 

 must also suff"er. Sulphurous acid gas has been found to de- 

 stroy leaves in forty-eight hours, even when present only to the 

 amount of xoW of its volume, and the vapor arising from a 

 solution of corrosive sublimate has proved destructive to 

 vegetable life, even when its presence was inappreciable to 

 the senses. 



Now, I am not aware that these gases are neutralized by 

 the evaporation of water in the house. We know that 

 when perfectly free from vapor, they expand ^{-^ or .002083 

 •for each degree of Fahrenheit, but then air is 20 times 

 .heavier at 100° than at 20°, i. e. it contains 20 times more 



