CH. VII.] THE FRUIT AND SEED. 225 



Despite all the contrivances for rencleiing more 

 effectual the natural sources of temperature offered 

 by oui' climate, these can never obtain duiing the 

 twelve months, by night as well as by day, a heat 

 sufficient for the successful cultivation of most tro- 

 pical plants. Hence arises the necessity for employ- 

 ing hothouses and other shelters of that description. 

 In these, fuel has to be employed to elevate the 

 temperatm'e, and some transparent medium as a 

 covering, to prevent the radiation of the heat thus 

 obtained, as well as to shut out the colder atmo- 

 sphere, without excluding the hght. But few words 

 ^Yill suffice relative to the fuel employed, this being 

 so generally coal ; yet there are some facts ascer- 

 tained by the chemist, which afford guides to the 

 gardener in the selection of his fuel, as well as tests 

 to enable him to judge whether he employs it eco- 

 nomically. 



The heating quality of the different coals known 

 in Great Britain are in the following proportions : 



Scotch Cannel 199 



Lancashire Wigan 196 



Yorkshire Cannel 188 



Newcastle (best Wallsend) . . . 169 



Gloucestershire (Forest of Dean) . 108 



Welsh (common) 25 



Hence, if the Scotch Cannel coal cost 19s., when the 



