242 



GENERAL SCIENCE 



and the burning of heavy oils either in open dishes or in 



the form of candles. Lard, olive oil, and whale oil were 



the oils commonly used (Figure 

 217). 



Candles were originally made 

 by dipping a wick into melted fat 

 and allowing it to cool, repeating 

 the operation until the candle was 

 of the desired thickness. Candles 

 are now made of fats and different 

 waxes. The wick is set in a mold 



FIG. 217. A whaie-oii Lamp. an d the melted material is poured 



around it. .When it has solidified, it is removed from 



the mold and is ready for use. 



The Kerosene Lamp. - - The discovery of petroleum 



gave a distinct impetus to lighting problems. Many 



forms of the kerosene lamp were 



placed on the market. The success 



of the kerosene lamp requires that 



the kerosene be burned in a good 



supply of air ; otherwise the lamp 



will smoke (Figure 218). 



Gases for Lighting. - - There are 



a number of gases which may be 



used for lighting -purposes with 



proper burners. When soft coal is 



heated in a retort, illuminating gas 



is formed. However, most of the 



gas which is sold as illuminating gas 



is formed by passing steam through a 



hot bed of coke or hard coal. This water gas has a high 



fuel value but burns with an almost colorless flame, giv- 

 ing little or no light. In order that it may be used for 



A 



FIG. 



218. A Kerosene 

 Lamp. 



