160 Introduction to the Study of Science 



be enriched water gas (page 162), coal gas (page 163), acetylene 

 or gasoline gas. The clean, convenient, and comparatively 

 cool electric light is rapidly taking the place of all others. It 

 has shown already (as illustrated in Fig. 44 A and B) an in- 

 teresting decrease in cost to the consumer. Electricity bids 

 fair to come into more general use for heating and cooking as 

 well as for lighting, as the cost of production and transmission 

 over long distances is reduced. Recent improvements in the 

 incandescent lamp make electric lighting very efficient and of 

 relatively low cost when convenience is considered. In this 

 way it has come to pass that in contrast with the old-time 

 means of lighting, the electric arc and the tungsten incandescent 

 turn night into day, making possible the uninterrupted con- 

 tinuance of the world's work and play. 



I. ILLUMINATING GAS 



61. The Welsbach mantle. Certain interesting improve- 

 ments have been made in the use of artificial gas for lighting. 

 The most important of these is the Welsbach gas mantle, which 

 gives a satisfactory incandescent light and makes illuminating 

 gas not only less offensive, but also less expensive to the con- 

 sumer. It has also made gas an active competitor with elec- 

 tricity in localities where the apparent cost of production or 

 other conditions have prevented the general use of electricity 

 for lighting. The graphic illustration (Fig. 45) shows the 

 number of hours of twenty candle power light that each of the 

 principal illuminants will give for ten cents. From this it is 

 easily seen that the gas mantle occupies an important place 

 among the modern means of illumination. 



How the gas mantle is made. The gas mantle is an interest- 

 ing achievement from the point of view of science. It illus- 

 trates what may be accomplished through a knowledge of 

 chemistry. The mantle is made of two rare metals or " rare 

 earths," as they are generally called. One of these is the metal 

 thorium, which is found mostly in North and South Carolina 



