180 Professor Sir James Dewar [Jan. 19, 



free space was by smearing the interior surface of a glass box with 

 glycerine, closing and allowing it to stand for a few days, when the dnst 

 settled, adhering to the glycerine. On applying the light beam test the 

 box was found to be what Tyndall termed " optically empty." Aitken 

 has devised an apparatus for counting the particles in the air, sug- 

 gested by experiments on cloud condensation due to the cold produced 

 by sudden expansion of air saturated with water vapour. Very briefly 

 his method is to make these solid particles the nuclei of small rain- 

 drops, which can then be counted. Ordinary London air contains 

 about 100,000 suspended particles, organic and inorganic, per cubic 

 centimetre. The particles get coated with all kinds of adherent 

 organic matter which, coming in contact with the bubble, affects its 

 surface tension locally, thereby inducing instability. 



When bubbles blown with pure air were formed in vessels closed 

 to prevent communication with the outside air, it was soon recognised 

 that, when the interior of the vessel had become free from suspended 

 matter by long standing and deposition on the walls the surface of 

 which was coated with glycerine soap solution, the bubbles lasted 

 much longer. 



The methods of purification employed for the purpose of obtain- 

 ii;g long-lived soap bubbles were (1) simple displacement by purified 

 air ; (2) deposition of the solid material by electrical discharge in the 

 space to be utilised. To demonstrate the two methods, domed 

 glass shades about 2 ft. high were used, each with a black wood 

 base, in which was a circular glass window. A concentrated 

 beam of light from an arc lamp was reflected up through the window, 

 thus strongly illuminating the solid impurities in the air-space enclosed 

 by the shade. Highly compressed air from an ordinary steel bottle, 

 purified by passing it through three small glass towers containing, 

 respectively, soda, lime, and cotton-wool damped with glycerine, 

 was introduced in the bottom of the shade through holes in a 

 circle of lead pipe secured on its wooden base. In about a minute 

 the Ijeam disappeared as the impure air was swept out through a 

 small hole in the top of the shade. To show the electrical method 

 of 2:»urification a similarly fitted shade had an insulated st^el point 

 hung within it from the centre of the dome. This was connected to 

 one pole of a Wimsharst machine, while the other was earthed 

 along with tlie lead pipe descril)ed above. Within two minutes of 

 starting the point-discharge the illuminated cloud had practically 

 disappeared. When this treatment was ap])lied to a vessel containing 

 soap solution, a heavy mist of large floating water particles first 

 appeared, of some persistence. 



Bubbles from the Evaporation of Liquid Ain. 



The evaporation of liquid air readily affords a supply of pure 

 mixed oxygen and nitrogen. Apparatus for this purpose is shown in 



