June ;, 1883] 



NA TURE 



139 



cussion which I contemplate for the years in question would 

 probably occupy one computer for a year and a half, involving 

 an outlay of about 70/." 



I 



ON THE DARK PLANE WHICH IS FORMED 



OVER A HEATED WIRE IN DUSTY AIR 1 

 N the course of his examination of atmospheric dust as 

 rendered evident by a convergent beam from the electric 

 arc, Frof. Tyndall noticed the formation of streams of dust-free 

 air rising from the summits of moderately heated solid bodies 

 (Proc. Roy. Inst., vol. vi. p. 3, 1870). "To study this eftect a 

 platinum wire was stretched across the beam, the two ends of 

 the wire beiig connected with the two poles of a galvanic 

 battery. To regulate the strength of the current a rheostat was 

 placed in the circuit. Beginning with a feeble current, the 

 temperature of the wire was gradu illy augmented ; but before it 

 reached the heat of ignition, a flat stream of air rose from it, 

 which, when 1 .oked at edgeways, appeared darker and sharper 

 than one of the blackest lines of Fraunhofer in the solar spec- 

 trum. Right and left of this dark vertical band the floating 

 matter rose upwards, bounding definitely the non-luminous 

 stream of air." . . . 



"When the fire is white hot it sends up a band of intense 

 darkness. This, I say, is due to the destruction of the floating 

 matter. But even when its temperature does not exceed 

 that of boiling water, the wire produces a dark ascending 

 current. This, I say, is due to the distribution of the floating 

 matter. Imagine the wire clasped by the mote-filled air. My 

 idea is that it heats the air and lightens it, without in the same 

 degree lightening the floating matter. The tendency, therefore, 

 is to s'art a current of clean air through the mote-filled air. 

 Figure the motion of the air all round the wire. Looking at its 

 transverse section, we should see the air at the bottom of the 

 wire bending round it right and left in two branch currents, 

 ascending its sides, and turning to fill the partial vacuum created 

 above the wire. Now as each new supply of air, filled with its 

 motes, comes in contact with the hot wire, the clean air, as just 

 stated, is first started through the inert motes. They are dragged 

 after it, but there is a fringe of cleansed air in advance of the 

 motes. The two purified fringes of the two branch currents 

 unite above the wire, and, keeping the motes that once belonged 

 to them right and left, they form by their union the dark band 

 observed in the experiment. This process is ince-sant. Always, 

 the moment the mote filled air touches the wire, the distribution 

 is effected, a permanent dark band being thus produced. Could 

 the air and the particles under the wire pass through its ma-s, we 

 should have a vertical current of particles, but no dark band. 

 For here, though the u otes would be left behind at starting, they 

 would hotly follow the ascending current, and thus aboli-h the 

 darkness." 



Prof. Frankland (Proc. Roy. So-., vol. xxv. p. 54 2 )> on tne 

 other hand, considers that what is proved by the above described 

 observations is that " a very large proportion of the suspended 

 particles in the London atmosphere consists of water and other 

 volatile liquid or solid matter." 



Last summer ( 1 88 1 ) 1 repeated and extended Tynclall's beautiful 

 experiment, not feeling satisfied with the explanation of the dark 

 plane given by the discoverer. Too much stress, it appeared to 

 me, is placed upon the relative lightening of the air by heat. 

 The original den-ity is probably not more than about i/ioooth 

 part of that of the particles, and it is difficult to see how a slight 

 further lightening could produce so much effect. In other 

 respects, too, the explana'ion was not clear to me. At the same 

 time I was not prepared to accept Prof. Frankland's view that 

 (he foreign matter is volatilised. 



The atmosphere of smoke was confined within a box (of 

 about the size of a cigar-box), three of the vertical sides of 

 which were composed of plates of gla^s. A beam of sunlight 

 reflected into the darkened room from a heliostat was rendered 

 convergent by a large lens of somewhat long focus, and made to 

 pass in its concentrated condition through the box. The third 

 glass side allowed the observer to see what was going on inside. 

 It could be removed when desired so as to facilitate the intro- 

 duction of smoke. The advantages of the box are twofold. 

 With its aid much thicker smoke may be used than w< inld be 

 convenient in an open room, and it is more easy to avoid 



1 Paper read at the Royal Society, December 21, 1882. by Lord 

 Rayleigh, F.R.S , Professor of Experimental Physics in the University 

 of Cambridge. 



draughts which interfere greatly with the regularity of the 

 phenomena to be observed. Smouldering brown paper was 

 generally used to produce the smoke, but other suh-tances, such 

 as sulphur and phosphorus, have been tried. The experiment 

 was not commenced until the smoke was completely formed and 

 had come nearly to rest. In some respects the most striking 

 results were obtained from a copper blade about J-iuch broad, 

 formed by hammering flat one end of a stout copper rod. The 

 plane of the blade was horizontal, and its length was in the line 

 of sight. The unhammered end of the rod projected from the 

 box, and could be warmed with a spirit-lamp. The dark plane 

 was well developed. At a moderate distance above the blade it 

 is narrow, sometimes so narrow as almost to render necessary a 

 magnifying glass ; but below, where it attaches itself to the 

 blade, it widens out to the full width, as shown in the figure. 



1 



Whether the heated body be a thin blade or a cylindrical rod, 

 the fluid 1 asse- round the obstacle according to the electrical 

 law of flow , the stream-lines in the rear of the obstacle being of 

 the same form as in front of it. This peculiarity of behaviour 

 is due to the origin of the motion being at the obstacle itself, 

 especially at iis hinder surface. If a stream be formed by other 

 means, and impinge up in the same obstacle without a difference 

 of temperature, the motion is of a different character altogether, 

 and eddies are formed in the shadow. 



The difference of temperature necessary to initiate these 

 motions with this dark plan': accompaniment is in ignificant. On 

 |uly 20, 1881, a glass rod, about i-inch in diameter, was em- 

 ployed. It was heated in a spirit-lamp, and then inserted in the 

 smoke box. The dark plane gradually became thinner as the 

 rod cooled, but could be followed with a magnifier for a long 

 time. While it was still quite distinct the experiment was 

 stopped, and on opening the box the glass rod was found to be 

 scarcely warmer than the fingers. It was almost impossible to 

 believe that the smoky matter had ben evaporated. 



In order to test the matter more closely, smoke was slowly 

 forced through a glass tube heated near the end pretty strongly 

 by a spirit-lamp, and then allowed to emerge into the concen- 

 trated sunshine. No distinct attenuation of the smoke could be 

 detected even under this treatment. 



It is not necessary to dwell further upon these considerations, 

 as the question may be regarded as settled by a decisive experi- 

 ment tried a few- days later. '1 he glass rod before u ed was 

 cooled in a mixture of salt and ice, and after w.ping was placed 

 in the box. In a short time a dark plane extending downwards 

 from the rod, clearly developed itself and persisted for a long 

 whde. Thi- re.-ult not merely shows that the dark plane is not 

 due to evapiration, but alsj excludes any explanation depending 

 upon an augmentation in the difference of densities of fluid and 

 foreign matter. 



The experiment was varied by using a (J -lube through which 

 cooled water could be made to flow. When the water was not 

 very cold the appearances were much the same as with the solid 

 rod ; but when by means of salt and ice the tube was cooled 

 still further, a curious complication presented itself. Along the 

 borders of the dark plane the smoke appeared considerably 

 brighter than elsewhere. Sometimes when the fbw was not 

 very regular it looked at first as if the dark plane had been 

 replaced by a bright one, but on closer examination the dark 

 plane could be detected inside. There seems no doubt but that 

 the effect is caused by condensation of moisture upon the smoke 

 due to the chilling which the damp air undergoes in passing 

 close to the cold obstacle. Where the fog forms more light is 

 scattered, hence the increased brightness. That the fog should 

 not form within the smoke-free plane itself is what we might 

 expect from the interesting observations of Aitken. 



With respect to the cause of the formation of the dark plane, 

 the most natural view would seem to be that the relatively dense 

 particles are thrown outwards by centrifugal force as the mixture 

 flows in curved lines round the obstacle. Even when the fluid 

 is at rest a gradual subsidence must take place under the action 

 of gravity ; but this effect could at first only manifest itself at 

 the top where the upper boundary of the gas prevents the 



