112 MH, THOMAS MOOEE ON TUE 



the cold air which has been admitted becomes warmed by con- 

 tact with the heated air of a plant-house, its capacity for moisture 

 is increased, and it will draw the moisture from the surrounding 

 volume, until the whole is brought to a state of equality : if this 

 goes on long enough the moisture of the atmosphere becomes dis- 

 sipated, and that contained in tlie tissues of the plants becomes 

 acted on in a similar way. 



A considerably higher temperature may be indulged in, when 

 accompanied by a corresponding degree of moisture, than would 

 be safe were less moisture employed ; so that the application of 

 moisture in the form of impalpable vapour may often be the 

 raeaus of avoiding the necessity of admitting air when the tem- 

 perature becomes suddenly raised by sun heat. 



Although the admission of large volumes of cold air is in- 

 jurious to forced plants in the winter and early spring, yet as 

 some degree of ventilation is required, it is of importance that 

 what air is admitted may be so far under control as not to affect 

 injuriously the tender plants, for whose benefit it is intended. 

 One means of effecting this is to have the cold air vvarnied 

 before it comes in contact with the plants, but this must be by a 

 process which will not deprive it of its moisture, nor render it 

 in any way impure. 



Motion of the intei'nal volume is another condition of im- 

 portance to the plants, and though distinct from ventilation, is 

 nevertheless a very valuable auxiliary and substitute for it when 

 the latter cannot be ventured on. This motion may be secured 

 in a variety of ways by modification, and tlie principle of the 

 plan adopted by the late Mr. Penn of Lewisham for warming 

 buildings, and to which the Polmaise plan of heating is very 

 )iearly allied, differing more in detail than in principle. 



In a little book * on the culture of the Cucumber, published 

 in 1844, I recommended the warming of the external air before 

 admitting it to the plants, by a plan which will be explained by 

 the annexed diagram (A). 



The main point which this plan was intended to secure was this ; 

 that the cold air should pass directly over the surface of the 

 heated water in a tank provided for supplying bottom heat to 

 the cucumber plants ; and by passing over this surface, it was 

 supposed that it would not only be warmed, but so far charged 

 with moisture as not to abstract any from the succulent foliage 

 and stems of the plants, but rather to furnish them with a source 

 whence they themselves might draw part of their supply. This 

 plan was entirely unconnected with any scheme for securing 



* Theory and Practice applied to the cultivation of the Cucumber in the 

 ■winter season. 



