260 MR JOHN AITKEN ON THE 



were made for studying the action of surfaces placed on both sides of the wires. 

 Fixing the plates parallel to each other, and at a distance of 2 or 3 mm. apart, 

 with the platinum wire between them, I carefully watched the motions of the 

 particles carried up in the air current. As the particles approached the wires 

 they gradually changed the direction of their motion, and instead of coming 

 straight up they curved towards the sides, some of the particles striking and 

 adhering to the side plates at a point below the wire. Some rose higher and 

 stuck opposite to it, others went higher still, while others passed on to the top 

 and escaped. 



I had for some time been trying to arrange an experiment in which I should 

 be able to watch the movements of the individual particles of dust, so as to see 

 them moving away from the hot surface. My intention was to examine the 

 movements of the particles with a microscope of low power, or with a powerful 

 magnifying glass. My great difficulty, however, was to get the movements 

 due to the convection currents sufficiently slow to enable me to follow the 

 moving particles when much magnified. After making the experiment last 

 described, I saw it was possible to arrange for this much- desired observa- 

 tion. The use of the large particles of magnesia enabled me to dispense with 

 the microscope, and use only a magnifying glass of moderate power ; and by 

 bringing the plates on each side of the wire close together, the velocity of the 

 upward convection current could be greatly reduced by the friction of these 

 surfaces, and by their cooling effect on the gases. The two side plates of glass 

 were accordingly brought closer together, to a distance of about 1 millimetre. 

 Fig. 13 represents the arrangement magnified five times, the U-shaped wire 

 being shown in section between the plates. The ascending current was now 

 very slow, and no difficulty was experienced in following the movements of the 

 individual particles, so I had at last the satisfaction of seeing the particles 

 being repelled by the hot wire. 



When the wire, heated to a red heat in air filled with magnesia dust, was 

 examined by means of a magnifying glass, the spectacle which presented itself 

 was most curious and interesting. At a distance below the wires, the particles 

 could be seen coming straight up between the glass plates, but as they approached 

 the wires they seemed to get uneasy, and as if wishing to avoid the heat, 

 some of them attached themselves quickly to the glass, others went further up, 

 but soon curved towards the sides and adhered to them; while others boldly 

 advanced straight up, almost to the wires, when their motion was suddenly 

 arrested and they were driven downwards and sideways, and attached themselves 

 to the glass. If the wires were hot enough, not a single particle got past them, 

 and the glass plates had each a patch of magnesian powder adhering to 

 its surface below the level of the wires. The direction of movement of the par- 

 ticles is roughly indicated by the lines in fig. 13. 



