SCIENCE OF COMMON THINGS. 231 



Why all flames are not equally luminous. Cause of the color of a fire. 



at the same time bad conductors of heat, by the 

 absorption of oxygen may become red hot, and finally 

 burst into a flame. 



1544 Is pine charcoal capable of taking fire at an extremely low tem- 

 perature ? 



Porous bodies, like pine charcoal, when perfectly 

 dry, absorb oxygen rapidly from the air, and take fire 

 at a temperature below 212 Fahrenheit, or the boil- 

 ing-point of water. 



This has been proved by actual experiment, a piece of light pine char- 

 coal taking fire on a surface of sheet iron, heated below the boiling-point 

 of water. 



1545 Why are not all flames equally luminous t 



In all flames, the light is mainly emitted from mi- 

 nute particles of matter, intensely heated, and floating 

 in the burning gas or vapor ; and hence a flame con- 

 taining few such particles, will emit but a feeble light, 

 even though its temperature is very great. 



1546 Upon what fact does the production of artificial light depend ? 



Upon the fact that at certain high temperatures all 

 matter becomes luminous. 



3-547 In order that we may profitably use a combustible body for illu- 

 minating purposes, what is required of the products of the combustion f 



That they should be volatile, and freely escape from 

 the immediate vicinity of the illuminating centre. 



1548 The product of all the ordinary forms of combustion is a gas 

 carbonic acid : what would have been the result if the product of every com- 

 bustion had been a permanent solid ? 



The world would have been buried beneath its own 

 ashes. 



1549 Why is the flame of an ordinary fire ydloto f 



Because the heat is not sufficient to render the car- 

 bon white hot. Increase the intensity of combustion, 

 arid the color of the burning bodies or the flames rises 

 from red to yellow, and from yellow to white. 



1550 A candle burns when lighted : explain how this is 1 



The heat of the lighted wick decomposes the tallow 

 into its elementary constituents, hydrogen and carbon. 

 The hydrogen is nrst consumed as a gas by itself with 



