2.36 MR JOHN AITKEN ON THE 



a glass tube passed through the stopper, and terminated in a point inside the 

 flask. Means were taken to insure the insulation of this conductor inside the 

 flask. This was done by surrounding the insulating tube with another tube, and 

 causing the entering dry air to pass into the flask through the space between 

 the tubes. The insulation was thereby kept good, and the glow of the dis- 

 charge at the point was quite visible in the midst of the moist air. 



On experimenting with this apparatus, it was found that electrification for a 

 short time by means of an ordinary cylindrical electrical machine was sufficient 

 to deposit almost all the dust, only the very slightest signs of condensation being 

 visible after electrification. What formed the nuclei of the very few cloud par- 

 ticles which appeared it is difficult to say. Whether they were undeposited dust 

 particles, or particles thrown off the conductor, or some product of the electric 

 discharge, this experiment does not determine. That they may be some pro- 

 duct formed from the air by the electric discharge is suggested by the following 

 experiment. First purify the air in the flask, either by passing it through 

 a cotton-wool filter, or by electrification, then reduce the pressure to super- 

 saturate it, and now electrify. At once a cloud forms all round the conductor, 

 and extends to near the sides of the vessel. This cloud is evidently not 

 formed by anything thrown off the conductor, forming nuclei, as it appears at 

 the same moment all round the point. It is more probable that the nuclei of 

 these cloud particles are formed by the discharge of the electricity producing 

 in the air nitric acid, or ozone, on which the supersaturated vapour condenses. 

 That the nuclei so formed are not solid particles there seems to be but little 

 doubt, because if we allow filtered air to enter so as to increase the pressure 

 and evaporate the particles, cloudiness does not reappear on again reducing 

 the pressure, which it certainly would do if the nuclei had been solid particles. 

 The number of nuclei that remain after electrification is very small, if the air 

 is not supersaturated with vapour ; and practically we may say that electrifica 

 tion deposits all the very fine dust, and I may remark here that it does it in a 

 very rapid manner. The air in the flask can be purified much quicker by means 

 of electricity than by the air-pump and cotton-wool filter. It may be noted 

 here that the dust of the atmosphere has but little effect on the brilliancy 

 of the glow of the point discharge. With a large amount of dust, with the 

 ordinary dust, with no dust, and with the electrification used, no difference 

 of importance in the brightness of the glow was detected. 



The Lungs and Dust. 



When we see a beam of sunlight shining into a darkened room through a 

 small opening, and revealing, by illuminating, the suspended dust, making the 

 beam look like a solid body, we have great difficulty in realising that our atmo- 



