268 MR JOHN AITKEN ON THE 



a space of dry air, into which the fog particles evaporate as they approach, and 

 so rapid is the diffusion towards the wood that the air is kept dry enough to 

 evaporate the particles as quickly as they approach. 



Attraction Due to Cold. 



To explain the attraction of the dust particles by cold surfaces, we have only 

 to reverse the explanation given of the repulsion due to heat. At the cold sur- 

 face the outward moving molecules of air have less kinetic energy than the 

 warmer inward moving ones, and the dust is thus driven towards the cold 

 surface by the greater energy of the hot molecules. 



This explanation of the action of hot and cold surfaces may not at first 

 sight seem satisfactorily to account for the peculiar movements of the dust 

 particles as they approached the hot wire, in the experiment shown in fig. 13. 

 We might here ask ourselves, for instance, Why were some of the particles 

 carried close to the wire, and then driven away from it % The inertia of 

 the particles is clearly not sufficient to cause them to advance against the force 

 which produced their rapid repulsion. Then why did they approach so close 

 to the wire, and then appear driven away with such violence ? It looks as if 

 the particles had become heated to a temperature sufficient to drive off their 

 occluded gases and condensed vapours, and that the repulsion in this case was 

 due to the rapid escape of these gases and vapours. No doubt, something 

 will be due to their escape, but I do not think it is the principal cause of the 

 repulsion, because the particles are so small, the gases and vapours will escape 

 from their surfaces all round them, and their effects will therefore nearly balance. 

 Further, the escape of these gases will not explain why the particles were 

 always driven towards cold surfaces. The following seems to be the principal 

 reason why the particles are always driven sideways and not downwards. The 

 rate at which a particle of dust will be repelled from the surface of a body is 

 not necessarily the same in all directions round the body, but will depend on 

 the closeness of the isothermal lines at the different places; and as in the experi- 

 ment the temperature varies very much more rapidly towards the cold glass 

 at the sides than it does downwards, the result is, a more powerful impulse is 

 given sideways than downwards; and further, the cold glass surfaces diffe- 

 rentiate the molecular movements near them, and cause an attraction. 



Let me illustrate this point further, and show by a parallel case in the action 

 of gravitation, why it is that the particles of dust when repelled move towards 

 the side, and not downwards. Suppose there is a very long, but narrow and 

 regularly shaped mountain, with its highest point near the middle, and the sides 

 sloping regularly and quickly to the summit, while the ridge descends slowly. 

 In ascending such a mountain, we can either go up the long easy slope of the 



