96 GREENHOUSE CONSTRUCTION AND HEATING. 
is two-fold : firstly, to provide storage for the rain or other 
water, and secondly, to ensure its being of about the same 
temperature as the interior of the house itself. The use 
of very cold water, as that drawn direct from a well or 
cistern in the open air, is often very detrimental to the 
health and welfare of plants, especially of such as are 
being forced, or cultivated in a high temperature. By 
allowing it to stand in an inside tank for a day or two the 
chill is taken off it, and its temperature becomes nearly the 
same as that of the atmosphere. 
Tanks may be constructed in various ways, and of 
various materials, At one time slate tanks were very 
much in vogue, and wooden ones, constructed of stout 
planks well bolted together, and lined with lead or zine, 
or simply well tarred inside, are frequently employed, 
but it may be as well to state that zinc tanks should be 
avoided, as when rain-water has stood in this material 
for any length of time, it has a very deleterious effect 
upon pot and other plants, and many otherwise 
unaccountable failures have been traced to this cause. 
The rust from ordinary iron tanks is innocuous to plant 
life, unless applied frequently and in _ considerable 
quantities, when it has the effect of stunting the growth 
of most plants, though it intensifies the colour of most 
flowers. Galvanized iron cisterns (plain) are, however, 
perfectly safe, and very convenient, as they can be placed 
anywhere, and are comparatively inexpensive as well. The 
square or oblong forms are the most convenient, but the 
cheapest and strongest as well is the circular form, 
constructed of stout sheets of the corrugated iron, bent 
to the proper form, and rivetted and soldered together. 
