ON THE CLIMATE OF HOT-HOUSES. 
225 
glass with which it is in contact. In a well ventilated hot-house, by watering the 
Hoor in summer, we may bring the dew point within four or five degrees of the tem- 
perature of the air, and the glass will be perfectly free from moisture; by closing 
the ventilators, we shall probably raise the heat ten or fifteen degrees ; but the 
degree of saturation will remain nearly the same, and a copious dew will quickly 
form upon the glass, and will shortly run down in streams. A process of distillation 
is thus established, which prevents the vapour from attaining the full elasticity of 
the temperature. 
This action is beneficial within certain limits, and at particular seasons of the 
year ; but when the external air is very cold, or radiation proceeds very rapidly, it 
may become excessive and prejudicial. It is a well known fact, but one which, I 
believe, has never yet been properly explained, that, by attempting to keep up in a 
hot-house the same degree of heat at night as during the day, the plants become 
scorched. From what has been premised, it will be evident that this is owing to 
the low temperature of the glass, and the consequent low dew point in the house, 
which occasions a degree of dryness, which quickly exhausts the juices. Much of 
this evil might be prevented by such simple and cheap means as an external cover- 
ing of mat or canvass. 
The heat of the glass of a hot-house at night, does not probably exceed the 
mean of the external and internal air ; and, taking these at eighty degrees and forty 
degrees, twenty degrees of dryness are kept up in the interior, or a degree of satu- 
ration not exceeding five hundred and twenty-eight. To this, in a clear night, we 
may add at least six degrees, for the eff"ects of radiation to which the glass is par- 
ticularly exposed, which would reduce the saturation to four hundred and thirty-four 
degrees, and this is a degree of drought which must be nearly destructive. It will 
be allowed, that the case which I have selected is by no means extreme ; and it is 
one which is liable to occur, even in the summer months. Now, by an external 
covering of mats, &c., the effects of radiation would be at once annihilated, and a 
thin stratum of air would be kept in contact with the glass, which would become 
warmed, and consequently tend to prevent the dissipation of the heat. But no 
means would, of course, be so effective as double glass, including a stream of air: 
indeed, such a precaution in winter seems almost essential to any degree of perfec- 
tion in this branch of horticulture. When it is considered that a temperature at 
night of twenty degrees is no very unfrequent occurrence in this country, the satu- 
ration of the air may, upon such occasions, fall to one hundred and twenty degrees ; 
and such an evil can only be guarded against, by diminishing the interior heat in 
proportion. 
By materially lowering the temperature, we communicate a check, which is 
totally inconsistent with the welfare of tropical vegetation. The chill which is in- 
stantaneously communicated to the glass by a fall of rain or snow, and the conse- 
quent evaporation from its surface, must also precipitate the internal vapour, and 
dry the included air to a very considerable amount, and the effect should be closely 
watched. I do not conceive that the diminution of light which would be occasioned 
VOL. III. NO. XXXIV. G G 
t 
