620 Mr. James Sivinhurne [April '2Q, 



the flame of the heat it radiates. In order to give the flame every 

 chance of supplying the heat, the threads of the mantle have to be 

 made very fine, so that the flame can rush through the meshes, and 

 the hot gas should be in brisk movement through the interstices of 

 the mantle. By using a special draught arrangement, known as the 

 intensive system, about twice the light per cubic foot of gas can be 

 obtained. In order to get the highest temperature the emissivity 

 should be low, that is to say, the mantle should be very white ; but 

 then, though it would get to a high temperature, it would give very 

 little light. On increasing the emissivity the light will first increase, 

 but this means a lower temperature, so that as the emissivity is 

 increased from white to black, the total radiation increases, but as 

 that means a greater abstraction of heat from the flame, the mantle 

 is cooler, and therefore radiates a larger proportion of the energy as 

 heat and a smaller proportion as light, so the mantle gets redder and 

 gives less light. This is just what happens in practice, whether ceria 

 or any other coloured oxide is used. 



It has been urged that as pure ceria is white, adding it cannot 

 make the mantle blacker ; but ceria is white only when cold. A 

 mantle may look quite white cold, and be darker in colour when hot. 

 Rubens has devised an experiment to show this. The mantle is 

 strongly illuminated by an arc and condenser, and its image is thrown 

 on the screen. It looks quite white, of course. On lighting the 

 gas, the mantle, instead of becoming still brighter, at once becomes 

 dull. Again, alumina, which is white, gives little light. Chromium 

 oxide is so dark that it gives only a dull red glow. But on adding a 

 little chromium oxide to the alumina, a dark red light is first given, 

 because the chromium oxide is too dark ; but as soon as it combines 

 with the alumina to make a light pink mantle, a good light is 

 obtained. 



The incandescent mantle is now applied not only to the ordinary 

 Bunsen burner, but to an inverted form, which lends itself to 

 decoration, and to the petroleum lamp. It is now also applied to air 

 caiTying a little hydrocarbon gas, and this application is said to 

 provide an extraordinarily cheap light, which is especially useful for 

 country houses. 



One of the drawbacks of gas, compared with electric lighting, is 

 that merely turning on does not light gas. This difliculty has been 

 largely overcome by the use of the by-pass, but further advances 

 have been made. Welsbach has discovered that an alloy of cerium 

 and iron gives off sparks on being scraped or filed, and a burner has 

 been designed in which the act of turning on the gas scrapes a little 

 wheel of this alloy, causing a spark, wliich lights the gas. This 

 overcomes the drawback of having a little jet always burning. 

 Another invention allows the gas to be lighted from a main tap. 

 Each burner has an attachment, which lets the gas straight through 

 to the burner when the pressure is on ; but on turning the main 



