158 PRINCIPLES OF HEATING HOT-HOUSES. 



its hygrometric vapor by passing over a hot-air stove, such as 

 polmaise, is productive of the worst consequences to growing 

 plants. To remedy this evil, a trough of water is laid over the 

 heating surface, which in some degree mitigates this evil. The 

 evil, however, cannot be entirely got rid of by this means ; for 

 even if the proper quantity of moisture can be again restored to 

 the air, the effects which result from the use of extraneous 

 gases are in no way removed. When the surface of radiation is 

 an iron plate, these injurious effects are much greater. 



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

 similar to the effects produced by hot-air stoves, but only when 

 the flues are heated to a high temperature, which is unneces- 

 sary. In the latter case, an unwholesome smell is also produced, 

 by the decomposition of the organic matter in the atmosphere, 

 and in some cases, probably, by a small portion of sublimed 

 sulphur from the bricks, as well as by the escape of various 

 gases through the joints or accidental fissures of the flues. 

 These contingent causes may, however, be in a great measure 

 avoided. The hygrometric vapors of the atmosphere are not 

 decomposed by this system of heating, as by a hot-air stove, 

 because when the flues are warmed to a common temperature, 

 the heat is perfectly pure, and the materials of which the flues 

 are built having but little affinity for oxygen, they are conse- 

 quently more healthy than hot-air stoves. 



Air passing over a highly heated surface of iron is, therefore, 

 more injurious than when passed over any other body, as stone, 

 or brick, as the power of iron to decompose water increases with 

 the temperature to which it is heated. The limit to which the 

 temperature of any metallic surface ought to be raised, for warm- 

 ing horticultural buildings, (or indeed any other buildings,) is 

 212, if a healthy, uncontaminated atmosphere be desired. The 

 importance of this rule cannot be too strongly insisted on, for 

 upon it entirely depends the healthiness of every system of 

 artificial heat. 



2. Laws of Heat. Heated bodies give off their caloric by 

 two distinct methods radiation and conduction. These are 

 governed by different laws ; but the rate of cooling or parting 



