THE ATMOSPHERE OP PLANT-HOUSES. 227 



under any other treatment. The end of this is obviously to preserve a 

 saturated atmosphere ; and it affords a parallel case to that of Dr. 

 Wells, of the anticipation of theory by practice. 



The effect of keeping the floor of the hothouse continually wet has 

 been already tried, and the plants have grown with unprecedented 

 vigour ; indeed their luxuriance must strike the most superficial ob- 

 server in such houses in the Royal Gardens of Kew. 



To the human feelings the impression of an atmosphere so saturated 

 with moisture is very different from one heated to the same degree 

 without this precaution ; and any one coming out of a house heated in 

 the common way into one well charged with vapour cannot fail to be 

 struck with the difference. Those who are used to hot climates have 

 declared that the feel and smell of the latter exactly assimilate to those 

 of the tropical regions. 



But there is a danger attending the very success of this experiment, 

 which cannot be too carefully guarded against. The trial has been 

 made in the summer months, when the temperature of the external air 

 has not been low, nor the change from day to night very great. In 

 proportion to the luxuriance of the vegetation will be the danger of 

 any sudden check ; and it is much to be feared that, unless proper pre- 

 cautions are adopted, the cold long nights of winter may produce irre- 

 parable mischief. 



I am aware that an objection attaches to the plan of double glass on 

 account of the increased expense ; but I think that this may appear 

 greater at first sight than it may afterwards be found to be in practice, 

 especially now that glass is so very cheap. 



The principles which I have been endeavouring to illustrate should 

 be, doubtless, extended to the pinery and melon-frame ; in the latter 

 of which a saturated atmosphere might be maintained by shallow pans 

 of water. An increase in the size of the fruit might be anticipated 

 from this treatment, without that loss of flavour which would attend 

 the communication of water to the roots of the plants. 



I have but few additional observations to offer upon the artificial 

 climate of a greenhouse. The remarks which have been made upon 

 the atmosphere of the hothouse are applicable to it, though not to the 

 same extent. The plants which are subject to this culture seldom 

 require an artificial temperature greater than 45° or 50°, and few of 

 them would receive injury from a temperature so low as 35 . When 

 in the house they are effectually sheltered from the effects of direct 

 radiation, which cannot take place through glass; but the glass itself 

 radiates very freely, and thus communicates a chill to the air, which 

 might effectually be prevented by rolling mats. With this precaution, 

 fire would be but rarely wanted in a good situation to communicate 

 warmth ; but in this damp climate it may be required to dissipate 

 moisture. The state of the air should be as carefully watched with 

 this view as where a high temperature is necessary to guard against the 

 contrary extreme. Free transpiration, as I have before remarked, is 

 necessary to the healthy progress of vegetation ; and when anj r mouldi- 

 ness or damp appears upon the plants, the temperature of the air 

 should be moderately raised, and free ventilation allowed. When the 



