310 PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 



condition more conducive to their health, than if their safety from 

 excess of cold had involved their submission to a higher degree 

 of artificial heat during the night. 



Night coverings, moreover, seem to afford facilities for night 

 ventilation, a time when ventilation, of all others, appears to be 

 most necessary; for then, deleterious gases are generated in 

 the greatest abundance, and the agitation and circulation of 

 the atmosphere is most required. We have seen that motion 

 and interchange of atmospheric particles are, to a certain extent, 

 beneficial to the health of plants ; and as their functions are in a 

 state of activity during the night, motion and circulation are as 

 necessary during that time as at midday. If a close confined 

 atmosphere be injurious to plants in the daytime, it must be 

 more so during the night, especially when artificial heat is in 

 process of generation. This fact is now beginning to be recog- 

 nized by the sounds, which are echoing in our ears though as 

 yet but faintly, the injunction, to keep a little air on all night; 

 and which is responded to by the practice of the best cultivators 

 of the present day. 



Under ordinary circumstances, where artificial heat is neces- 

 sary, there is some risk in following these recommendations. 

 A chilly blast, which cannot be refused admission when the bar- 

 rier to ingress is removed, would deal death and desolation 

 around ; and if this would be liable to happen in the daytime, 

 when attendants are at hand, the risk would be still greater at 

 night, when none were present to guard*igainst it ; and, under 

 the most favorable circumstances, night ventilation, if carried to 

 any extent, would involve a great loss of heat. It becomes, 

 therefore, a question, if the motion and circulation of the inter- 

 nal atmosphere during the night could not be so far facilitated 

 by other means, as to secure the chief advantage of an actual 

 interchange of air, without the internal heat being carried off by 

 the cause that produced it ? 



Whatever prevents the radiation of heat from the interior to 

 the exterior atmosphere through the conducting agency of glass, 

 decreases in the same ratio the amount of required heat, and 

 hence, saves the plants from being subjected to unnecessary 

 excitement. The principle upon which a covering acts effi- 



