VENTILATION OF EAKLY FORCING-HOUSES. Ill 



of any forcing-house, provided it is kept pure and properly ba- 

 lanced with regard to moisture. When smoke flues were used 

 as the medium of distributing artificial heat, the admission of 

 air for this purpose might have been required on account of the 

 sulphurous acid gas transmitted through the bricks ; but the present 

 almost universal use of hot water for this purpose, by doing away 

 with the source from wlience the air became impregnated with 

 impurities, has also removed the necessity of admitting air for 

 the purpose of purification. 



If this be correct, then the regulation of temperature may 

 be considered as the only legitimate reason for opening forcing- 

 houses during the period under consideration. An excess of 

 heat is injurious to all plants, and to forced plants, growing at an 

 unnatural period of the year, even more than to tliose whose 

 growth is affected during their ordinary and natural period of 

 action. But it is necessary to distinguish between natural and ar- 

 tificial heat — between the heat of the sun and tliat of a hot-water 

 apparatus. In the former case, when the temperature of the con- 

 firmed atmosphere has been raised to what is judged to be the maxi- 

 mum point which may be allowed without injury to the plants, yet 

 tlie heat being inevitable, and its source uncontrollable, the act of 

 ventilation becomes justifiable ; but, in the latter case, when the 

 excess of heat is altogether artificial, while ventilation is equally 

 necessary as in the former case to correct the evil, it is by no 

 means so justifiable, inasmuch as a little management of the 

 source whence the supply was derived would have altogether pre- 

 vented its necessity ; and hence it is (the action of cold air on 

 the tender tissues of forced vegetables being excessively injuri- 

 ous) that the proper course, so far as concerns the health of the 

 plants, is to avoid the application of so much heat as renders the 

 admission of air for lowering the temperature an act of necessity. 

 There does not appear to be good reason for the admission of 

 cold air simply for the purpose of lowering a temperature that 

 has been artificially raised too high : how much more consistent 

 as well as economical to apply only as much heat as is necessary ! 

 besides, such a course would preserve the plants from risk of 

 injury by exposure to cold. 



But there is another evil attending this practice. Cold air 

 drains the moisture, first from the warmed atmosphere and then 

 from the plants themselves ; so that the constant admission of 

 cold air and the maintenance of that degree of humidity which 

 is judged necessary for excitement and support of vegetation (the 

 growing shoots of a vine, for example) are circumstances quite 

 incompatible with each other. Air, at a given temperature, can 

 hold in suspension but a certain degree of moisture, and as it 

 becomes heated its capacity for moisture is increased ; thus, when 



