388 Polmaise Method of Heating Greenhouses, Sec. 



consequence of the air, when thus artificially dried, abstract- 

 ing too much moisture from the leaves. It is also clear that 

 the injury must increase in proportion to the length of time 

 the apparatus continues in use, {i. e. without intermission 

 and change of the internal atmospheric volume by the plants 

 being compelled to inhale these extraneous gases,) and in no 

 other way can the purity of the internal volume be restored. 



The extreme dryness of the atmosphere, after its hygro- 

 metric condition has been changed, is, in the experience of 

 every one, productive of the very worst consequences to 

 growing plants. To remedy this condition of things troughs 

 of water are placed over the heating surface, so as to moisten 

 the atmosphere by evaporation. The evil is in some de- 

 gree mitigated, so far as mere moisture goes, but only in the 

 upper regions of the house, for the rarefied air immediately 

 carries the water it holds in suspension directly upwards, so 

 that the lower portions of the atmosphere are not in the 

 slightest degree affected by it. 



Let us suppose, however, that the whole atmosphere has 

 its equivalent of water restored, (which it has not,) we have 

 not yet got rid of the evil, for we are just supplying the heated 

 air with more moisture to decompose, and hence the quantity 

 of extraneous gases must continue to increase, and also their 

 effects, so long as the evaporation and decomposition con- 

 tinue. Unless, therefore, some method be devised for recom- 

 bining these gases with other bodies, or neutralizing them 

 by the same means, the effects of the decomposition of 

 water by the heated air still remain, notwithstanding the 

 evaporation of the same element by the same agent of de- 

 composition. 



The heating by means of brick flues, is, in some respects, 

 similar to the effects produced by currents of hot air ; but 

 this is only when the flues are overheated, which is or ought 

 to be quite unnecessary. In the latter case the unwholsome 

 smell is also produced by the decomposition of the organic 

 matter in the atmosphere, and probably, in some instances, 

 of a small portion of sublimated sulphur from the bricks as 

 well as by the escape of various gases through the joints or 



