CHAPTER VIII. 
GENERAL REMARKS ON HEATING. 
The chief objects of growing plants under glass are 
(1) to enable more or less tender exotic (7.¢., foreign) subjects 
to be successfully cultivated in our comparatively cold and 
always changeable climate, and (2) to ‘‘force” various 
fruits, flowers, and vegetables into an abnormally early 
bloom, etc., and thus induce them to afford their produce 
more or less in advance of their natural seasons. With very 
few exceptions, neither of these objects can be attained 
without the aid of artificial warmth, obtained by some 
means, and though the unheated greenhouse (which will be 
considered more fully presently) is certainly valuable for 
some purposes, yet its capabilities are limited, and it is of 
comparatively little value for either the culture of really 
tender plants (i.e., such as are destroyed by any appreciable 
amount of frost), or for the forcing or forwarding of garden 
produce of any description, that is, beyond a certain very 
limited extent. 
A well-constructed glass-house will protect its occupants 
from several degrees of frost—often as much as 6 or 8 
