142 APPLICATION- OF PRINCIPLES. 



ance already produced by its expulsion from the 

 proper air cavities ; while, on the other hand, when 

 the thaw is gradual, the air may retreat by degrees 

 from its new situation without producing additional 

 derangement of the tissue. It is also possible that 

 leaves from which their natural air has been expelled 

 by the act of freezing, may, from that circumstance, 

 have their tissue too little protected from the evapo- 

 rating force of the solar rays, which, we know, pro- 

 duce a speciiic stimulus of a powerful kind upon those 

 organs." {Hort. Trails.^ n. s., ii. 305.) 



In our glazed houses, we have full control over the 

 state of the atmosphere, as regards both its moisture 

 and temperature, by means familiar to every gar- 

 dener ; but the manner of applying those means, and 

 the causes that oppose their action, deserve to be the 

 subject of inquiry. 



It will have been seen, from what has been already 

 stated upon that subject, that in general, in warm 

 countries, the air is occasionally at least, if not per- 

 manently, filled with vapour to a much greater extent 

 than in northern latitudes,* and, as in our glazed 

 houses we cultivate exclusively the natives of warm 

 countries, it is also obvious that, as a general rule, the 

 air of such houses requires, at certain periods, to be 

 damper than that of the external air. Those periods 



* " Captain Sabine, in his meteorological researches between the 

 tropics, rarely found at the hottest period of the day so great a dif- 

 ference as 10° between the temperature of the air and the dew- 

 point: making the degree of saturation about -VSO, but most fre- 

 quently 5° or '860 ; and the mean saturation of the air could not 

 have exceeded -910." [Daniell.) 



