Sir yohn Snell 253 



so hard that it can only be fused in an electric furnace, yet 

 it can be drawn into exceedingly fine wire which gives a 

 beautiful and economical light. 



The gas-filled tungsten lamp introduced just after the 

 War is still more economical. The gas used in these bulbs 

 is argon or nitrogen, or some other inert gas which allows 

 the tungsten to be heated very highly without melting. 

 These wire-drawn lamps have a much longer life than the 

 older carbon lamps, and the saving is indicated by the 

 fact that a big London shop, whose bill for electricity 

 used to be four thousand two hundred pounds a year, 

 now lights for about twelve hundred pounds. 



Very large lamps can be made for street lighting on 

 the wire-drawn plan. Some of these are of as much as 

 two thousand candle-power. By the way, we all talk of 

 candle-power, but very few of us know exactly what it 

 means. This standard was laid down as long ago as i860, 

 when it was necessary to fix a standard for gas-lighting. 

 The measure is a candle made of spermaceti and beeswax, 

 weighing six to the pound, and burning at the rate of one 

 hundred and twenty grains of spermaceti to the hour. 



Science has given us lamps without any filament at all. 

 One is the mercury- vapour arc lamp used for night photo- 

 graphy. A small amount of mercury is placed in each end 

 of a vacuum tube. The arc is started by tilting the 

 tube so that a stream of mercury unites the two pools for 

 a moment, then separates. Then an electric discharge 

 continues through the mercury vapour. The light is a 

 brilliant green, and makes people's complexions look so 

 ghastly that it is not suitable for domestic use. Another 

 wireless lamp contains neon gas, which gives a lovely rosy 

 glow. 



Proud though we are of our electric light, our descen- 

 dants will wonder how we could have been satisfied with 

 such a wasteful form of lighting, for even by the best 

 tungsten lamp the amount of light given out is less than 



