250 MR JOHN AITKEN ON THE 



very high, the dark plane rising over each leg was very thin (see fig. 10), but as 

 the temperature rose, the planes extended on each side till the two planes met and 

 formed one large one (see fig. 11). An examination by means of a magnifying 

 glass showed that this broad dark plane was clue to the evaporation, or to the 

 disintegration of the particles, as they could be seen streaming upwards and 

 disappearing into the dark space under the wires. They there arrived at a 

 space the temperature of which was sufficient to convert them into gases or 

 vapours. The dark plane in this case was thus due to a change of the particles 

 from the solid to the gaseous state. Hence the great differences in the size of the 

 dark planes of different dusts, each kind of dust having a different temperature 

 at which it evaporates or becomes disintegrated. The sulphate dust, for instance, 

 gives a smaller dark plane than the chloride, because the sulphate requires a 

 higher temperature to drive it into the gaseous state than the chloride. 



This result is quite different from that got with temperatures which were 

 not sufficient to vaporise the particles and make them invisible. It was there- 

 fore now desirable to make experiments with some substance which a high 

 temperature could not destroy. For this purpose I selected calcined magnesia 

 and calcined lime, also soda and magnesia dusts, produced by burning the metals. 

 With these dusts a different result was obtained. A high temperature had no 

 other effect than forming a thin dark plane over each wire (see fig. 10). But 

 even these stable forms of dust were subjected to a repulsion, the particles 

 passing near the wire being driven to a small distance from it on each side. It 

 may be possible that some of the particles of these dusts are vaporised, but if 

 so, the amount must be very small, and can have but little influence on the 

 formation of the dark plane. 



Another effect noticed in these, and in the experiments at lower tempera- 

 tures, was that whenever there was much water vapour present, there was a 

 faintly indicated dark plane formed by the evaporation of the water from the 

 particles. If nothing but the dust of the air was present in a fog formed with 

 steam, then the wires were surrounded by a very thick dark plane, due to the 

 evaporation of the fog particles ; and if any artificial dust was present, then the 

 thick dark plane was still visible, but not black, as the particles were only 

 reduced in size by the evaporation of the water from them. All these different 

 effects of the hot wire can be illustrated at one time, if we put into the dust- 

 box some indestructible dust, also some sal-ammoniac and sulphate dusts, in 

 proper proportion, and then add some water vapour. When the wire is heated 

 in such a mixture, we get a result like that shown in fig. 12. In the centre we 

 have the true dark plane, in the wider space there is only the indestructible 

 powder present. The next boundary shows the vaporising zone of the sulphate, 

 1 he next the vaporising zone of the sal-ammoniac dust, and the last that of water. 

 In fig. 12, a is the true dark plane, in which there is nothing but gases and 





