2 7 8 



ARTIFICIAL LIGHTING 



solution of thorium and cerium until it became saturated with 

 the chemicals. The mantle thus impregnated with thorium 

 and cerium is placed on the gas jet and sup- 

 ported by a Welsbach fixture, but before the gas 

 is turned on, a lighted match is held to the man- 

 tle in order to burn away the thin fabric. After 

 the fabric has been burned away, there remains 

 a coarse gauze mantle of the desired chemicals. 

 If now the gas cock is opened, and the escaping 

 gas is ignited, the heat of the flame raises the 

 mantle to incandescence and produces a bril- 

 liant light. A very small amount of burning 

 gas is sufficient to raise the mantle to incandes- 

 cence, and hence, by this method, intense light is FIG. 142. A gas 



i 1*1 mantle. 



secured at little cost. 



When a Welsbach burner is fastened to the gas jet, the pressure 

 of the gas is lessened by a mechanical device and 

 less gas escapes and burns. By actual exper- 

 iment, it has been found that an ordinary bur- 

 ner consumes about five times 

 as much gas per candle power 

 as the best incandescent burner 

 and hence is about five times as 

 expensive. One objection to the 

 mantles is their tendency to 

 break. But if they are carefully 

 adjusted to the burner, and are 

 not roughly jarred in use, they 

 last many months. Since the best quality can be purchased 

 at twenty-five cents, the expense of renewing the mantles is 

 only slight. 



Natural gas. Animal and vegetable matter buried in the 

 depth of the earth sometimes undergoes natural distillation, 



FIG. 143. Inverted gas burner and 

 mantle. 



