218 GREENHOUSES 



an opening and lessening the danger of an 

 explosion. 



The boiler and all pipes, except those in 

 the greenhouse itself, should be insulated as 

 much as possible to prevent loss of heat. The 

 best known material for this purpose is as- 

 bestos. For coating boilers it may be had 

 in a granular form, which is mixed with 

 water and applied with a trowel or the bare 

 hands. For covering pipes molded casings 

 may be had to fit all sizes of pipe. 



Fig. 117. Asbestos pipe covering 



FUELS 



Coal is used almost universally for fuel in 

 greenhouse heating, except in sections where 

 natural gas or oil are cheap and abundant. 

 Gas is an ideal fuel, but somewhat treach- 

 erous inasmuch as the pressure is likely to be 

 lowest in the coldest weather. Care should 

 be taken to see that there are no leaks, as it 

 is very explosive, and it is also poisonous to 

 vegetable as well as animal life. 



Broadly speaking, coal is of two kinds, 



