126 Inquiry concerning the Nature of Heat, 



practical uses that may be derived from a knowledge of 

 the facts which have been established by the results of 

 the foregoing experiments. 



In all cases where it is designed to -preserve the heat 

 of any substance which is confined in a metallic vessel, 

 it will greatly contribute to that end if the external sur- 

 face of the vessel be very clean and bright ; but if the 

 object be to cool anything quickly in a metallic vessel, 

 the external surface of the vessel should be painted, or 

 covered with some of those substances which have been 

 found to emit calorific rays in great abundance. 



Polished tea-urns may be kept boiling hot with a 

 much less expense of spirit of wine (burnt in a lamp 

 under them) than such as are varnished ; and the cleaner 

 and brighter the dishes and covers for dishes are made, 

 which are used for bringing victuals on the table, and 

 for keeping it hot, the more effectually will they answer 

 that purpose. 



Saucepans and other kitchen utensils which are very 

 clean and bright on the outside may be kept hot with a 

 smaller fire than such as are black and dirty ; but the 

 bottom of a saucepan or boiler should be blackened, in 

 order that its contents may be made to boil quickly, and 

 with a small expense of fuel. 



When kitchen utensils are used over a fire of sea-coal 

 or of wood, there will be. no necessity for blackening 

 their bottoms, for they will soon be made black by the 

 smoke ; but when they are used over a clear fire made 

 with charcoal, it will be advisable to blacken them, 

 which may be done in a few moments by holding them 

 over a wood or coal fire, or over the flame of a lamp or 

 candle. 



Proposals have often been made for constructing the 



