Scientific Lectures. 209 



a solid white product, which is intensely hot, and, consequently, very 

 luminous. Magnesium does not take fire spontaneously, and I must, 

 therefore, first raise it to its kindling point, in order to cause it to 

 combine with the oxygen. The high luminosity of this flame is due 

 to the solid particles of magnesia produced by the combustion. In 

 the combustion of phosphorus in oxygen, which, you see in this globe, 

 the white cloud which rises is the result of the combination : and it 

 is the incandescence of these solid particles which produces this pure 

 white light. 



"We may destroy the luminosity of a gas rich in carbon by simply 

 mingling it with air. We have here a little burner designed for the 

 burning of street gas Without the production of soot. It is called the 

 Bunsen burner, and is used in the chemical laboratory to produce a 

 flame for heating purposes. Air is admitted by these openings, and 

 mingles with the gas before it is burned. If we cut off the air from 

 these openings the flame becomes at once luminous. 



The amount of heat depends on the amount of combustion ; but 

 the amount of light depends upon the production of a high tempera- 

 ture, and upon the solid particles which become incandescent. It is 

 important, therefore, in burning gas for illumination, to produce a 

 large amount of carbon and a high temperature ; but, if we produce 

 too much carbon, our gas smokes. In fact, in order to obtain the 

 greatest amount of light from a given quantity of gas, it is desirable 

 to burn it at the point at which it is on the eve of smoking. If we 

 turn up the gas as high as we can without allowing it to smoke, we 

 shall get the largest amount of light the gas is capable of giving. 



The same is true of oils of various kinds. We adapt our lamps 

 to the oil, in order to produce the largest amount of light without 

 smoking. The chimney that we use upon the lamp is designed to 

 make a draft and- increase the temperature. Kerosene oil cannot be 

 burned satisfactorily without a chimney, simply because the tempera- 

 ture produced by its combustion in the open air is not suflficienc to 

 burn up the carbon. Adding the chimney enables us to produce a 

 more thorough combustion of the carbon. Each oil must have a 

 lamp adapted to its wants, so as to produce a complete combustion 

 of the carbon, and yet so as not to burn it with too much air. If 

 we have too tall a chimney, it makes too strong a draft of air, burns 

 up the carbon too quickly, and diminishes the illuminating power. 



It is, perhaps, not out of place to say a word or two here on the 

 subject ot our street gas. It is a common subject of complaint that 

 " our gas is bad " ; " that it does not give us enough light." 

 [Inst.] 14 



