458 Dr. F. H. Newman on Active Modifications of 



The substances selected, were those which are fairly 

 volatile in vacuum. The temperature of the experimental 

 bulb A was maintained at —40° G. by immersion in a freezing 

 mixture. Table I. shows that the rate of absorption in- 

 creased as the pressure of the gas increased. The chemical 



Table I. 



Substance. 



Volume of nitrogen (at 760 mm. pressure) absorbed 

 in 30 mins. 



Sodium 



Potassium 



Sodium-Potassium 



alloy 



Sulphur 



Phosphorus 



Iodine 



Arsenic 



Magnesium 



Mercury 



Pressure 

 3 mm. Hg. 



0-04 c.c. 

 005 



0-09 

 011 

 012 

 0-09 

 0-01 

 0-03 

 004 



Pressure 

 96 mm. Hg. 



Pressure 

 304 ram. Hg. 



0'07 c.c. 



0-24 c.c. 



0-07 



0-27 



011 



0-36 



0-14 



0-48 



0-16 



0-51 



o-n 



0-46 



0-08 



021 



0-06 



019 



0-08 



021 



effect due to the a rays, which is not necessarily equivalent 

 to the ionization and probably exceeds the latter, appears to 

 be a function only of the velocity of the a rays and of the 

 number of collisions with the gas molecules. As -the pres- 

 sure decreases, the number of molecules present decreases, 

 and as a result the amount of chemical action decreases. 

 After absorption had occurred no gas was re-liberated when 

 the bulb A was heated to 100° C. The chemical compounds 

 produced must be fairly stable. The disappearance of the 

 gas continued for many hours, but the rate of the absorption 

 decreased with time, and finally ceased altogether. If the 

 substance was re-heated, so that a fresh surface was prepared, 

 the absorption re-commenced. The chemical compound pro- 

 duced at the surface of the substance protects the solid from 

 further action, and accounts for this fatigue effect ; unless 

 the surface under examination was very clean there w r as no 

 absorption at all. When about 3 c c. of the gas had been 

 absorbed by the sodium-potassium alloy, moist air was drawn 

 through the bulb A from F to E, and was then passed through 

 Nessler's solution. The presence of ammonia in the stream 

 of air showed that the nitrides of sodium and potassium had 

 been produced. A null experiment indicated that this 

 ammonia was not present as an impurity in the air. The 

 product obtained with magnesium was boiled with caustic 

 potash, and the formation of a nitride was shown by the 



