through Exhausted Tubes without Electrodes. 451 



passed through the tube when the primary was placed round 

 it at the level of the ring, while a discharge passed as soon 

 as the primary w T as moved above or below the ring. 



Another very convenient tube for showing this effect is 

 the one with the hollow down the middle, fig. 11; when 

 this is pumped so that discharges can pass through the outer 

 tube the spark-length can be adjusted so that the discharge 

 stops immediately when a metal tube, a test-tube containing 

 a strong solution of an electrolyte, or a tube containing air 

 at a pressure at which it is electrically very weak, is placed 

 in the central opening. The discharge is renewed again as 

 soon as the tubes are removed. On one occasion, when the 

 large tube was in a peculiarly sensitive state, I was able to 

 see distinctly the diminution produced by a dielectric in the 

 electromotive intensity parallel to its surface. The discharge 

 stopped as soon as a stick of sulphur or a glass rod sufficiently 

 large almost to fill the opening was inserted, and was re- 

 newed again as soon as these were withdrawn. It requires, 

 however, the large tube to be in an extremely sensitive state 

 for the effect produced by a dielectric to be apparent, and I 

 have only on one occasion succeeded in getting the tube into 

 this condition. The effect on that occasion, however, was so 

 definite and regular that I have no doubt as to the existence 

 of the screening effect due to the dielectric. 



When the tube is of average sensitiveness dielectrics do not 

 produce any appreciable effect, nor can the influence of even 

 comparatively so good a conductor as distilled water be de- 

 tected, and it is not until after the addition of a considerable 

 quantity, 10 to 20 per cent., of sulphuric acid or ammonium 

 chloride, that the insertion or withdrawal of the tube stops or 

 starts the discharge. 



A tube containing air at a low pressure is very efficacious 

 in stopping the discharge, and the result of the comparison 

 of the relative effects of an exhausted tube and a tube of the 

 same size and shape containing a solution of an electrolyte 

 are very remarkable. I found that an exhausted tube which 

 contained air at a very low pressure (less than -j-\f of a milli- 

 metre) produced as much effect on the discharge in the outer 

 tube as a tube containing at least 50,000 times as many 

 molecules of ammonium chloride. This would be expressed 

 in the language of electrolysis by saying that under the 

 electromotive intensity to which it was exposed in this ex- 

 periment the molecular conductivity of the gas is 50,000 

 times that of the electrolyte. The proportion between the 

 number of air molecules and the number of molecules of an 



2 H2 



