53 



the internal volume, by contracting the vessels, impeding 

 the circulation of the juices, and thereby checking the 

 regular course of the growth of the plant. If these rea- 

 sons fail to stamp it as a practice which ought not largely 

 to be indulged in, it is further objectionable, as being 

 productive of a prodigal expenditure of fuel : there can 

 be little doubt but that generally speaking, a far greater 

 quantity of fuel than is requisite, is expended in main- 

 taining the temperature of forcing houses, solely from 

 this cause ; for the cold air when admitted, continues to 

 abstract a portion of heat from the warmed air, until 

 the temperature of both becomes equal, and consequently 

 an increased application of fuel is requisite, in order to 

 raise the newly admitted air to the same temperature as 

 that which has been suffered to escape ; and as the buoy- 

 ancy of heated air is so great, an immense volume must 

 necessarily rush out through a very small aperture, and thus 

 there must also of necessity be an immense waste both 

 of heat, and of fuel. A given portion of fuel, in its com- 

 bustion, can give off but a certain proportionate ratio of 

 heat, and if this is allowed unnecessarily to escape, the 

 prodigality is self-evident. It is but a weak argument, 

 which would seek to give to the admission of cold air, 

 the office of regulating the temperature of plant houses ; 

 this ought to be effected by limiting the degree of heat 

 applied, and not by attending to the abstraction of that 

 which had been previously administered with two lavish 

 an hand. Besides the extravagance of such a course, the 

 constitutional vigour and energy of the plants is at the 

 same time sacrificed by undue excitement. The admis- 

 sion of cold air in large quantities, therefore, brings con- 

 demnation in its train, since it is unnecessary, and extra- 

 vagant, as well as directly injurious. 



There are nevertheless some considerations which ren- 

 der the admission of air, when regulated and applied with 

 discretion, an operation of importance to the health of 



