LIGHT AND ITS ARTIFICIAL PRODUCTION. 283 



it as a small work of art. This involves a proper adjustment of the 

 rate at which the material melts, the rate at which the molten material 

 is vaporized, and the rate at which the vapor thus developed is 

 consumed. 



The proper burning of a luminous flame depends principally on the 

 rate at which air is supplied. If the wick is too large or too small the 

 candle does not burn regularly; it either smokes or gives an insuffi- 

 cient quantity of light. You all understand what is meant by saying 

 that a lamp " smokes." Under those circumstances the wick is too 

 large, the amount of gas developed is too great for the air supply, and 

 clouds of smoke are formed. If the flame is reduced by screwing down 

 the wick the smoking ceases, but even then the carbon is not entirely 

 consumed. With an insufficient air supply all the carbon particles 

 can not be satisfied with oxygen. This is fortunate for us, for the un- 

 combined particles of carbon are heated to incandescence by the hot 

 carbon dioxide. 



A flame can not emit light unless it contains uncombined solid parti- 

 cles in an incandescent state. If I mix illuminating gas with air 

 before combustion and ignite the mixture the flame no longer shines 

 brightly, but gives forth a weak blue-violet light. [Experiment.] The 

 explanation is that in such a mixture the oxygen can combine with all 

 the carbon particles so that the flame contains no solid particles at 

 all, for to these alone a flame owes its luminosity. Transparent gases 

 heated ever so high hardly emit any light and hence the weak luminos- 

 ity of a Bunsen flame. 



One consequence of the combustion of all the carbon contained in 

 illuminating gas is the increase of the temperature of the flame, since 

 there are no foreign particles to be heated as in the ordinary flame. In 

 the Bunsen flame all the carbon contributes to the heat developed, 

 while in a light-giving flame less carbon is burned in the first place, 

 and this uncombined carbon abstracts heat from the hot products of 

 combustion in the second place. The Bunsen flame is therefore hotter 

 than the ordinary gas flame. But on this account it emits less light. 



In order to make it luminous it is necessary to introduce in it incom- 

 bustible materials. You see how brilliantly a piece of sheet platinum 

 shines on being placed in a Bunsen flame. [Experiment.] If it is 

 introduced into the still hotter oxyhydrogen flame it is heated to white- 

 ness and then melts. [Experiment.] By substituting for the platinum 

 an infusible substance, such as lime, chalk, or magnesia, a dazzling 

 light is produced which illumines the whole hall. [Experiment.] 

 Brighter still than this so-called Drummond's lime light is the zirco- 

 nium light, in which zirconium is burned in the oxyhydrogen flame. 

 The intensity of ordinary gas flame can not be compared with that pro- 

 duced by solid substances in an incandescent state. It was therefore 

 a great step in advance in the art of illumination when Mr. Auer von 

 Welsbach succeeded in producing greater luminosity in a gas flame in 



