CHAPTER IX. 
HEATING BY HOT WATER. 
This being undoubtedly the most satisfactory and generally 
‘useful method of heating horticultural structures of all 
-kinds, will be treated first. 
The principle or basis of all hot water heating consists 
in the well-known fact that water, when heated, expands 
-considerably in volume, and its specific gravity being reduced 
thereby, it rises ; also that as it cools again it contracts, 
and, if allowed to do so, again falls. This may be easily 
demonstrated by applying the flame of a spirit lamp to 
one side of a test-tube or the like filled with coloured 
water. 
Following out this principle, the boiler and pipes in 
all heating apparatus of this kind are so arranged that 
as the water in the former becomes heated it rises into 
one set of pipes leading out of the top (of the boiler), and 
through these to a point which may be some considerable 
distance (often some hundreds of feet) away from the 
boiler and fire. By the time this farthest point has been 
reached it will have parted with a good deal of its caloric, 
or in other words will have begun to cool somewhat ; it 
