and Spark-length for Small Values of the latter. 629 



tables show, but the slopes obtained by the two series of 

 observations are nevertheless in good agreement. 



Further evidence that the carriers of the discharge within 

 the limits specified come from the metal and not from the 

 gas, is found in the fact that the slope of the curve below the 

 elbow is completely independent of the pressure of the gas, 

 as is shown in fig. 4, and also independent of the nature of 

 the gas, as shown in fig. 9. 



Furthermore, the character of the discharge as observed in 

 the behaviour of the voltmeter is completely different below 

 the elbow from its character on that portion of the curve to 

 the right of the elbow. When the discharge took place at 

 or above the minimum spark-potential, the pointer of the 

 voltmeter experienced a sudden deflexion of a number of 

 divisions towards zero, and quickly returned to the old value 

 as the potential was built up by the cells. On the other hand, 

 when the discharge took place at points on the curve below 

 the elbow, the pointer always fell completely to zero and 

 remained there. Moreover, a test of the circuit with the 

 galvanometer showed that coherence had taken place. That 

 the coherence was not due to any give in the supports because 

 of electrostatic attraction between the electrodes is definitely 

 proved by the facts presented in figures 9 and 13, showing 

 that the elbow varies with the nature of the electrode and 

 with the nature of the gas. (This last result is discussed in 

 the following section.) 



(3) Observations in Hydrogen and Carbon Dioxide at 

 Atmospheric Pressure with Platinum Electrodes. — In order to 

 check still further the participation of the metal ions in the 

 discharge at small distances, some observations were taken 

 with platinum electrodes in an atmosphere of hydrogen and 

 of carbon dioxide. The results are shown in Table X. and 

 graphically in fig. 9 (PI. XIII.). 



The minimum spark-potentials for these gases are, according 

 to Carr, respectively 280 and 420, while according to these 

 results they are 285 and 420. Now if the metal ions are the 

 carriers of the discharge for all potentials below the minimum 

 value for the gas, then the curve in any gas should follow 

 the slope found for air until the potential reaches the minimum 

 value for the gas, when the curve should bend to the hori- 

 zontal, i. e. the discharge should at this point begin to take 

 place in the gas. This must be true, for if the curve should 

 hold to the straight line beyond this point, the discharge 

 would be produced by the metal ions at a higher potential 

 than was necessary for discharge at the same distance in the 

 gas itself. Taking Can's values for the critical pressure for 



