90 



GENERAL SCIENCE 



useful, and if the insulating material were thick enough (say f inch 

 or more), it would save heat even on bright tin pipes. 



A bright nickel or aluminum kettle will cool very much more 

 slowly than a black kettle. On a coal or wood stove, or directly 



over a coal or wood fire, a 

 kettle is heated largely by 

 heat radiated from the stove 

 or fire; therefore if the bot- 

 tom is black, the kettle will 

 heat more rapidly than if it 

 is bright. Over a gas, gaso- 

 line, or similar blue flame, 



FIG. 69. Why do we have a sea breeze 

 during the day and a land breeze at night? 



the condition of the bot- 

 tom will not make so much 

 difference, since here most of 

 the heat is received by contact with the hot gases. The best kettte 

 for general use is, therefore, one with the bottom black and the 

 remainder polished, but for use on a gas stove it makes little 

 difference whether the bottom is black or not. 



Effects of Painting Radiating Surfaces. The amount of heat 

 given off by radiation from a radiator is small, probably not exceed- 

 ing on an average 40 per cent of the total amount emitted. If we 

 consider that a new surface of cast iron will give off 40 per cent of 

 the total heat by radiation, the following table will give some 

 idea of the results obtained by radiation from various surfaces. 



Per cent. 



Cast iron, new 40 



Wrought iron 37 



Dull lampblack 42 



Rusty surfaces 41 



Bright iron surfaces 29 



White lead, dull , . 42 



It is found that two coats of black asphaltum paint increase the 

 amount of radiation 6 per cent, two coats of white lead 9 per cent, 

 rough bronzing about 6 per cent. A coat of glossy white paint 

 reduces the heat emitted by radiation about 10 per cent. 



