260 Dr. Collie and Prof. Ramsay. Behaviour of [Feb. 13, 



to repeat the experiments on the helium ribbon, and the tube was 

 filled with the first sample of helium that came to hand ; it had been 

 prepared from broggerite, and had received some contamination from 

 air and nitrogen. To our surprise, we failed to obtain a ribbon dis- 

 charge at ordinary atmospheric pressure ; only a rain of sparks could 

 be seen, and it was not until the pressure had been lowered by several 

 hundred millimetres that the discharge became ribbon-like. This 

 observation led to the experiments, of which we now give an account. 



The distance between the electrodes was arranged at 170 mm. ; the 

 hammer of the coil was fixed in a definite position, and a freshly 

 charged accumulator of 8 volts was employed to drive the coil. The 

 first gas experimented with was dry pure air. 



1. Air. — Spark began to pass feebly at a pressure of . . 117 mm. 



The band w T as continuous, but " fluffy " at . . 70 ,, 



The continuous band began to disappear at. . 83 ,, 

 On removing air, the continuous band was 



visible at 74 ,, 



The pressure appears to be 73 or 74 mm., at which the change first 

 occurs. On lowering pressure still further, the band grows less well- 

 defined, and finally fills the tube as a luminous discharge, such as is 

 seen in an ordinary vacuum-tube. 



2. Hydrogen. — Sparking at 57 mm. 



Band visible at 42 „ 



Becoming indistinct at edges at 22 ,, 



Very "fluffy" at 14 „ 



Again giving sparks at 43 „ 



The pressure in this case appears to be 42 or 43 mm. 



3. Oxygen. — The band was clear, but quite continuous at 33 mm. 



On raising pressure, intermittent sparks ap- 

 peared at 86 ., 



Band began to be free from sparks at 81 „ 



Band disappeared again, and sparking began 



at 86 „ 



The pressure of the oxygen appears to be somewhat above 81 mm. 

 to produce the looked-for change in character. The amount of 

 luminosity was very feeble. 



4. Garhon Dioxide. — Continuous band at 92 mm. 



Distinct sparking at 102 ,, 



On reducing pressure, sparking 



ceased at 94 



Here the critical pressure appears to be 92 — 94 mm. 



