LIGHT AND ITS ARTIFICIAL PRODUCTION. 285 



with increasing- brightness, while the cylinder P P sinks with the 

 basket K into the water; that is, with diminishing air content of the 

 gas. Finally only pnre acetylene gas escapes, and you are no doubt 

 surprised at the fullness of the white light which fills the dark hall. 

 But in this flame, as in the others, the luminosity is due to uncombined 

 carbon particles, though judging from the color of the light, the 

 temperature is far in excess of that of the ordinary gas flame. 



The question may be asked, why does not the mixture of air and 

 acetylene first issuing from the burner explode? The answer is, that 

 the heat from the match can not penetrate through the narrow aper- 

 ture of the burner and through the long tube R into the interior of the 

 cylinder PP, otherwise a violent explosion would surely have taken 

 place; for all combustible vapors mixed with air are extremely explo- 

 sive and are similar in their action to gunpowder. If a spark comes in 

 contact with them the ignition spreads rapidly from the given point 

 through the whole mixture. The enormous increase in volume thereby 

 produced exerts a sudden pressure correspondingly large on the inclo- 

 sure which, therefore, bursts if it is not able to withstand the pressure 

 to which it is thus subjected. [Experiment.] 



The apparently empty space in this bottle, containing a small quan- 

 tity of petroleum, similar to the reservoir of a nearly empty petroleum 

 lamp, is filled with petroleum vapors mixed with air. I ignite the 

 mixture by means of an electric spark and you hear the report pro- 

 duced by the blowing out of the cork stopper. In blowing out a lamp 

 care must be taken not to turn down the wick too far, so that no open- 

 ing into the interior of the petroleum reservoir is produced, through 

 which the flame can enter when being extinguished. It is best to cut 

 off the air supply by covering the wick with a cap which can be manip- 

 ulated from without. At all events the flame should not be turned 

 down too low before it is blown out. 



That which makes acetylene gas so dangerous is its property of 

 spontaneously exploding at a pressure of only two atmospheres. By 

 exerting a greater pressure on the reservoir PP, as I now do (experi- 

 ment), this alone will suflice to produce an explosion. 



NATURE OF ELECTRIC LIGHTS. 



In electric lights the light is produced by incandescent carbon just 

 as it is in freely burning flames. In the electric incandescent light a 

 carbon filament (for instance, of carbonized bamboo fiber with chem- 

 ically precipitated carbon) serves to carry an electric current, and in 

 consequence of the resistance which it offers to the current it becomes 

 heated. In the electric arc light the current passes between two rods 

 of retort carbon through an air space, under which conditions an 

 electric arc is formed in this space and the ends of the carbon rods are 

 heated to the temperature of volatilization of carbon. In this respect 

 electric lights are similar to gas flames, for in both highly heated carbon 



