1882.] ivhich is formed over a Heated Wire in Dusty Air. 415 



in the same degree lightening the floating matter. The tendency, 

 therefore, is to start a current of clean air through the mote-filled air. 

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

 verse 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 incessant. 

 Always the moment the mote-filled air touches the wire, the distribu- 

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

 the air and the particles under the wire pass through its mass, we 

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

 here, though the motes would be left behind at starting, they would 

 hotly follow the ascending current, and thus abolish the darkness." 



Professor Frankland,* on the other hand, considers that what is 

 proved by the above described observations is that " a very large pro- 

 portion of the suspended particles in the London atmosphere consists 

 of water and other volatile liquid or solid matter." 



Last summer (1881) I repeated and extended Tyndall'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 density 

 is probably not more than about y"oVo 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 explanation was not clear to 

 me. At the same time I was not prepared to accept Professor 

 Frankland's view that the 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 com- 

 posed of plates of glass. 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 introduction of smoke. The advantages of the box 

 are twofold. With its aid much thicker smoke may be used than 

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

 draughts which interfere greatly with the regularity of the phenomena 

 to be observed. Smouldering brown paper was generally used to 

 # u p roc -R j g 0Cij » VG i_ 25, p. 542. 



