58 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



doubtedly absorbs a very large percentage of the sulphurous acid, 

 and this can only have a beneficial result. The adaptation of the 

 system to old plant-houses does not involve any very serious 

 reconstruction. The charcoal-boxes and the exhaust-caps are 

 easily fixed ; whilst it is only very old and leaky houses that 

 cannot be rendered reasonably airtight. In this way the toxic 

 action of fog will be mitigated to an appreciable extent. 



As regards cultural precautions to be observed in foggy 

 weather, experience indicates that a low temperature and a moist 

 atmosphere are conducive to the well-being of the plants, though' 

 they, of course, afford no absolute protection. This aspect of 

 the question has been clearly put in a note to the Gardeners'- 

 Chronicle by Mr. Thiselton-Dyer, which I quote in extenso : — 



" The Kew practice of keeping the winter temperature of the 

 houses as low as we dare is based on the result of practical ex- 

 perience. I do not dogmatise for other people who want to solve 

 their own problems, and find out what is best for their particular 

 requirements for themselves. But as Mr. Henslow has pointed 

 out, the theory of the subject has been stated clearly by Lindley ; 

 and it may not be amiss to quote a few words from his classical 

 ' Theory and Practice of Horticulture ' on the subject. 



" The point of the whole matter is that in winter, with a low 

 external temperature and nocturnal radiation, it is practically 

 impossible, in a large glasshouse, to keep the internal atmosphere 

 humid with a high temperature. I quote from Lindley, p. 207 : — 



" 1 Another source of dryness is the coldness of the glass roof, 

 especially in cold weather, when its temperature is lowered by 

 the external air, in consequence of which the moisture of the 

 artificial atmosphere is precipitated upon the inside of the glass, 

 whence it runs down in the form of " drip." 1 



" Again, 1 It is evident that the mode of preventing this dry- 

 ing of the air by the cold surface of a glass roof will be either 

 by raising the temperature of the glass, which can only be 

 effected by drawing a covering of some kind over our houses at 

 night, so as to intercept radiation, or by double glass sashes ; or 

 else by keeping the temperature of the air as low as possible, 

 consistently with the safety of the plants, and so diminishing the 

 difference between the temperature of the external and internal 

 air.' 



u In large glasshouses it is obviously impracticable to adopt 



