240 ME JOHN AITKEN ON THE 



was then found that, though the dark current was much enfeebled, it was still 

 produced. To study this effect, Professor Tyndall stretched a platinum wire 

 transversely under the beam, the two ends of the wire being connected with 

 the poles of a voltaic battery, and the necessary appliances for regulating the 

 strength of the current. " Beginning with a feeble current, the temperature of 

 the wire was gradually augmented ; but long before it reached the heat of 

 ignition a flat stream of air rose from it, which, when looked at edgeways, 

 appeared darker and sharper than one of the blackest lines of Fraunhofer in 

 a purified spectrum " (see fig. 5). He goes on to say — " Right and left of this 

 dark vertical band the floating matter rose upwards, bounding definitely the 

 non-luminous stream of air. What is the explanation ? Simply this : The hot 

 wire rarefied the air in contact with it, but it did not equally lighten the float- 

 ing matter. The convection current of pure air therefore passed upwards 

 among the inert particles, dragging them after it right and left, but forming 

 between them an impassable black partition." * 



This explanation of Professor Tyndall's has been received by most of us 

 without question ; yet I think that if we try to form a mental picture of the 

 process which is here supposed to go on, we shall have some difficulty in doing 

 so. Professor Tyndall supposes the distribution of the floating matter is due 

 to the heat, which lightens the air, but does not in the same degree lighten the 

 floating dust ; the tendency, therefore, he says, is to start a current of clear air 

 through the mote-filled air. No doubt the lightening of the air will slightly 

 increase the tendency of the motes to fall, but the increased freedom to fall 

 from this cause will be extremely slight and inappreciable, and will be entirely 

 negatived and overruled by the upward movement of the hotter air, and the 

 result will be simply to cause the particles to lag a little behind the air in 

 their movements. 



Our confidence in Professor Tyndall's explanation was not, however, shaken 

 till Lord Rayleigh, in going over Professor Tyndall's experiments and extend- 

 ing them, discovered that the explanation given of the formation of the dark 

 plane was not correct, and showed that it could not be due to heat lightening 

 the air, and so enabling it to shake itself free from the dust motes, because he 

 discovered that cooling the air produced a precisely similar result (see fig. 2). 

 Lord Rayleigh introduced a cold glass rod into smoky air, and then found 

 that " a dark plane extending doivnwards from the rod, clearly developed itself, 

 and persisted for a long while." t He says — " This result not merely shows that 

 the dark plane is not due to evaporation, but also excludes any explanation 

 depending upon an augmentation in the difference of densities of fluid and 



* Proe, Roy. Inst., vol. vi. p. 3, 1870; also Essays on the Floating Matter in the Air, p. ■">. 

 I o mans, Green, & Co., 1881. 



' Papei read before Royal Society, December 21, 1882; also Nature, vol. xxviii. p. 139. 



