438 Br. F. Gr. Donnan on the Relative Rates of 



A sample of helium was now introduced into the apparatus. 

 After the effusion had proceeded for a short time, a complete 

 block occurred. The helium was pumped out, and. the mercury 

 having been sucked out of the seal, the apparatus was cut above 

 the tap and the effusion-plug removed for examination. There 

 was a whitish deposit on the platinum. The hole was cleared 

 by cautiously heating the platinum to redness and drawing air 

 through it. On sealing the apparatus together again the hole 

 was found to be clear, but on again admitting the helium 

 another block occurred. It was evident that the helium con- 

 tained some impurity which attacked the platinum or condensed 

 in some way in the aperture. The helium was accordingly 

 removed and the hole again cleared out as described. On 

 sealing together again and testing with oxygen and hydrogen, 

 a greatly increased viscosity-effect was observable. When the 

 effusion-tube w<as cut out and the platinum foil microscopically 

 examined, the cause of the viscosity could be seen in the form 

 of two holes connected by a fissure. This was probably due 

 to the corroding action of the impurity * combined with the 

 frequent heating. 



A new disk of foil was now pierced and soldered on to the 

 platinum tube ; but it was also found to show a very high 

 viscosity-effect. The origin of this was finally traced to the 

 cracking of the enamel at the junction of the platinum and 

 glass tubes. 



Owing to the difficulty of making this joint, a new effusion- 

 plug w T as constructed as follows : — A thick circular disk of 

 platinum was made by welding several pieces of foil together. 

 A circular hole was punched in the centre of this, and a thinf 

 disk of foil,which had been punctured in the manner previously 

 described, was welded on so as to cover tbe hole in the thick 

 disk. The latter was then fastened by means of Ohatterton 

 cement to a glass tube whose end had been turned over and 

 ground to a flat flange ; and this tube was sealed to the rest 

 of the apparatus. After some trials an effusion-plug was 

 obtained which showed only a comparatively small viscosity- 

 effect : - m. s. m. s. 

 Oxygen .... 4 3*1 

 Hydrogen ... 1 2*2 1 3'3 

 Oxygen .... 4 3 



* A piece of glass which was immersed in the helium was found to 

 fume in the air. As oxygen had been previously removed from the 

 helium by bruning phosphorus, the impurity was probably some volatile 

 lower oxide of phosphorus. 



t This was the thinnest foil procurable from Johnson and Matthey 

 that was free from microscopic holes. Tt was about - x -^ in. thick. 



