Polmaise Method of Heating Greenhouses, 6^0. 439 



We have already stated, in a previous paper, that the heat 

 radiated from hotwater pipes and smoke flues is precisely- 

 similar as regards purity. This statement, however, must 

 be read with a reservation, and though the statement is theo- 

 retically and absolutely correct, it is open to misconstruction. 

 Caloric radiated from the one body is just as pure as the 

 other, — their increments of moisture being just in proportion 

 to their increments of heat. Thus, if heat given off from 

 hotwater pipes be S0°, and heat from flues 80°, then both 

 have the same capacity for moisture, and both take the same 

 quantity of moisture from the house. This seems a paradox 

 to some gardeners but it is correct to the smallest decimal, 

 and it is difficult to get an intelligible reason of the common 

 expression used by gardeners, " that heat from hotwater pipes 

 is sweeter and purer than by any other means of heating." 

 This is an undeniable fact, but let it be attributed to the 

 right cause, and then, after all, we will find that the old 

 smoke flue does not deserve the character for impurity that 

 some would ascribe to it. 



The unsuspected cause of the dryness of the atmosphere 

 in hothouses heated by smoke flues, is caused by the destruc- 

 tion of the aqueous vapor by absorbent surfaces of the ma- 

 terial of which they are built ; too often of very soft bricks. 

 Their power of absorbing the moisture of the air increases 

 rapidly with the softness of the material, and the tempera- 

 ture to which they are heated ; while the unabsorbent sur- 

 faces of hotwater pipes attracts no moisture at all. 



This state of things is not, however, an inevitable condi- 

 tion of this method of heating, which serves to show how 

 comparatively little attention has been directed to the con- 

 struction of smoke flues. Notwithstanding the many sys- 

 tems of heating now brought into notice, we find the com- 

 mon flue, with all its faults and imperfections, just the self- 

 same thing it was a hundred years ago ; and instead of any 

 attempt to improve it, our apparatus improvers have intro- 

 duced various abortive methods which approach it in merit 

 just in proportion as they approach to it in construction. It 

 is true that the vapor is destroyed and gases eliminated by 



