92 



NA TURE 



[May 23, 1907 



iijob, and Mr. Johnstone's results agree with his previous 

 ones in so far as they show an off-shore migration in the 

 summer and an along-shore movement in the winter. A 

 number of other movements were observed, just as in the 

 North Sea, where individual fish moved long distances, 

 e.g. to the east and south coasts of Ireland, but such 

 movements appear to be irregular according to present 

 knowledge. 



The same author also gives a valuable contribution on 

 sewage pollution of shell-fish, and in this connection we 

 regret that Prof. Herdman has not been able to give us 

 a report upon the action of copper in connection with the 

 purification of infected shell-fish, of which a preliminary 

 statement appeared last year. 



As usual, there is no lack of charts, tables, and illus- 

 trations, and the volume certainly shows very well Prof. 

 Herdman 's idea as to the association of scientific research 

 and fishery problems. 



Frank Balfour BroxVne. 



INCANDESCENT ILLUMINANTS.'^ 

 A LITTLE more than twenty years ago Auer von 

 Welsbach, who was engaged on researches on the 

 rare earths, invented the modern incandescent mantle. 

 His first mantles were made of zirconia and yttrite earth 

 in the proportion to make a normal zirconate. Shortly 

 afterwards he found that the best material has a basis of 

 thoria. Pure thoria, which requires care in its prepar- 

 ation, gives very little light, but if a small percentage 

 of a coloured and permanent oxide, such as ceria, is 

 added, it gives good illumination. 



There has been much discussion about the theory of 

 the incandescent mantle. It has been generally assumed 

 that the temperature of a Bunsen burner is too low for a 

 mantle to give the light it does by simple radiation unless 

 it is much hotter than the flame. Unfortunately, the 

 temperature of the tlame is generally taken with a thermo- 

 couple, and this gives far too low a reading, as th- 

 thermocouple never reaches the real temperature of the 

 fiame ; but, admitting that the temperature of the flame 

 is high, it is still urged that the light given by the thoria 

 with a small percentage of ceria is so great that there 

 is something more than mere thermal radiation. It is said 

 that the ceria acts as a catalytic agent, and that it 

 oscillates between two states of oxidation. Ceria does 

 act somewhat in the same way as platinum ; for Instance, 

 if a ceria mantle is put on a lighted burner, the burner 

 turned out, and the gas turned on again, the ceria 

 mantle will glow and finally light the gas. It is odd 

 that this is not brought forward by the advocates of the 

 catalysis theory ; but the opponents might urge that 

 zirconia will do the same thing, and the zirconia mantle 

 gives very little light. This does not prove that ceria 

 does not increase the rate of combustion, however. 



According to the simple radiation theory, the light 

 depends only on the emissivity, or blackness of the mantle, 

 and its temperature. Its temperature must be lower than that 

 of the flame, as it must be robblrrg 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 pro- 

 portion of the energy as heat and a smaller proportion 

 as light, so the mantle gets redder and gives less 



1 Abstract of a Iliscou'se 

 April 26, by Mr. J, Swinburne, F.R.S, 



d at the Royal Institution on Friday, 



NO. i960, VOL. 76] 



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 inantle may look quite white cold, and 

 be darker in colouf when hot. Rubens has devised an 

 experiinent to show this. The mantle is strongly illu- 

 minated 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 carrying 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 to gas, compared with electric 

 lighting, is that merely turning on does not light gas. 

 This difficulty has been largely overcome by the use of 

 the bye-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 which 

 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 

 supply off, and alknving a little gas to pass at the con- 

 trolling tap, the attachment to each burner turns off the 

 burner and lights a little pilot jet, which keeps alight 

 until light is wanted again. On turning on the main tap 

 the pilot jets light the various burners and go out them- 

 selves. By this moans burners can be fully lighted up by 

 turning one tap at the door of the room. 



The electric incandescent light is undergoing a great 

 change. Carbon is being replaced by metal wires. It has 

 been found possible to make wires of high enough resist- 

 arrce of tungsten, osmium, tantalum, and a few other 

 metals and compounds. The osmium lamp was the first 

 of these, but there was difliculty in making it of high 

 enough resistance. The tantalum lamp is now in great 

 demand. It is made for too volts to 130 volts, and is 

 much more efficient than the carbon lamp. It will not 

 last long on alternating currents, however. The wires 

 of a lamp that have been run for some time on a direct 

 current show a curious notched or crinkled appearance 

 under the microscope ; but a wire that has been run on 

 an alternating circuit looks as if the metal had been 

 melted into short cylinders with round ends, and these 

 cylinders had stuck together end to end without their 

 centres being in a line. Sometimes the little cylinders 

 are nearly separated, merely touching at a corner. This 

 action Is very extraordinary, and has never been explained. 

 In addition to this, when a lamp breaks down on an 

 alternating circuit, the wirn sometimes goes at one poin' 

 and sometimes it breaks in several places, and tangles 

 itself up in an extraordinary way ; at other times it breaks 

 up into numerous little pieces, which will be found lying 

 on the Inside of the globe. Some of the other lamps show 

 a change under the action of the current, but it is not so 

 marked as in the case of tantalum. 



One of the most interesting of the new lamps Is the 

 zircon. It is said to be made of zirconium and tungsten, 

 and lamps of this material have been made for 200 volts, 

 a matter of the greatest importance from a distribution 

 point of view. It is possible that the conductor is really 

 a zirconide of tungsten, and this opens up a new series 

 of compounds. A zircon lamp for 100 volts has really 

 six separate loops of wire mounted in series inside a 

 hiilb. ,\ recent improvement is to pro^'ide an extremelx 



