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Mr. J. Aitken. On some Phenomena [Apr. 28 



the differentiation which takes place in the particles forming the 

 cloudy condensation. As has been already explained, the small drops 

 rapidly diminish in size while the large ones increase, and as in these 

 experiments the drops are very close to each other, these changes take 

 place the more rapidly. These changes are also taking place in the 

 steam jet, but, owing to the constant supply of new drops, the older 

 ones are swept away before the change is observed. The following 

 experiment will, however, show that these changes are taking place 

 in the steam jet also. If, while the jet is condensing dense and the 

 transmitted light is yellow, we imprison some of the jet by closing 

 both ends of the tube, we shall find in an extremely short time that 

 the colour will change to blue, after which it will fade as the drops 

 increase still further in size, and fall. In this experiment we have a 

 proof of the statement that when the jet is electrified the drops are 

 smaller than when not electrified, and not larger, as has been supposed ; 

 as this experiment shows, if we begin with drops transmitting yellow 

 light, that as the drops diminish in numbers and increase in size, the 

 transmitted light changes to blue. 



The conditions of the experiments for producing colour by cloudy 

 condensation, produced by expansion, have been varied in a number of 

 ways. After the air has been cooled by the expansion, the layer of 

 cloudy air in contact with the walls of the tube rapidly acquires heat 

 from the metal, and the rise in temperature quickly evaporates the 

 cloudy particles and causes a clear space all round next the walls, so 

 limiting the colour to the centre of the tube. The receiver was 

 therefore increased in diameter to get rid of the disturbing effects 

 of the heating of the air on the walls of the tube, so as to have a 

 larger mass of air beyond this influence ; but no decided advantage 

 has been obtained. It was afterwards found that the difficulty of 

 studying these colour effects in small tubes can be easily overcome 

 by wetting the inside of the tube. With this precaution the air 

 next the walls is kept saturated and the temperature of the walls is 

 lowered by the heat given off to evaporate the water, with the result 

 that the colour is the same all over the field and close up to the 

 walls. 



Large tubes might be used for showing these colour phenomena to 

 an audience, a parallel beam of light being sent through them, which 

 would become coloured when the dusty air in them was expanded. 

 One large tube tried has a diameter of 7 cm., and is 50 cm. long. With 

 a receiver of that capacity it would be hopeless to attempt to produce 

 any colour effects with an ordinary air-pump alone ; a vacuum receiver 

 has, therefore, been added to the apparatus. This receiver is made of 

 metal ; it is 15 cm. diameter and 60 cm. long, with round ends. There 

 are two tubes attached to it, one for connecting it with the air-pump, 

 and the other is provided with a stopcock, to which a tube is attached, 



