ELECTRIC DISCHARGE IN GASES 115 



enclosed gas. It was also found by Pliicker that the luminous 

 discharge was capable of deflecting a suspended magnetised 

 needle, and is itself acted upon by a magnet. 



It will be worth while to begin by a brief account of the 

 principal facts about the electric discharge. If the two terminals 

 of any source of high potential electricity are separated by a gas 

 such as air at common atmospheric pressure, and the voltage is 

 gradually increased, at a certain difference of electric pressure 

 the air is ultimately unable to bear the strain and a current 

 passes momentarily producing a spark. If now the gas con- 

 tained in the experimental tube is expanded by the use of an 

 air pump, the difference of potential in the two terminals re- 

 quired to cause a discharge is less, and as the pressure on the gas 

 is diminished the character and appearance of the discharge 

 changes. Straight, well-defined sparks are no longer produced, 



C Jl 



FIG. 47. ELECTRIC DISCHARGE UNDER REDUCED PRESSURE. 

 C= Cathode. A = Anode. 



but a line of light, extending the whole length of the tube, is 

 gradually developed, while the negative pole becomes covered 

 with a violet-coloured glow. If the pressure of the gas is reduced 

 to about half a millimetre of mercury or less, the discharge 

 changes again in appearance and stratification appears, the glow 

 separating into distinct portions with dark spaces between. 



Next the cathode or negative electrode there is a non-luminous 

 space, especially noticeable, which is commonly referred to as 

 Crookes' space, as these phenomena have been studied by him 

 for many years. In order to explain some of the phenomena 

 observed in highly exhausted vessels, Crookes attributed them 

 to new properties developed in the gas in consequence of the 

 reduction in the number of molecules present. " The modern 

 idea of the gaseous state is based on the supposition that a given 

 space contains millions of millions of molecules in rapid move- 

 ment in all directions, each having millions of encounters in a 

 second. In such a case the length of the mean free path of the 

 molecules is exceedingly small as compared with the dimensions 



