126 Mr. James Mackenzie Davidson [April 25, 



duced by the spark, rises a little and forms an easier path for the 

 succeeding spark. The conducting power of the air is so greatly 

 increased that the spark will sometimes prefer to follow the path of 

 this altered air in preference to metal, which is such an excellent 

 conductor of electricity. 



It is only natural that exi)erimentalists should have wondered 

 how this electric discharge would behave if it was allowed to pass 

 through a vessel from which the air had been pumped out. Faraday, 

 I think, was the first to call attention to the peculiar appearance of 

 the electrical discharge in rarefied air. In the 13th Series of his 

 Experimental Eesearches on Electricity, 1838, he observes : — 



" I will now notice a very remarkable instance in the luminous 

 discharge, accompanied by a negative glow, which may perhaps be 

 correctly traced hereafter into discbarges of much higher intensity. 

 Two brass rods 0*3 of an inch in diameter, entering a glass globe on 

 opposite sides, had their ends brought in contact, and the air about 

 them very much rarefied. A discharge of electricity from the 

 machine was then made through them, and whilst that was continued 

 the ends were separated from each other. At the moment of separa- 

 tion a continuous glow came over the end of the negative rod, the 

 positive termination remaining quite dark. As the distance was in- 

 creased, a purple stream or haze appeared on the end of the positive 

 rod, and proceeded directly outwards towards the negative rod, 

 elongating as the interval was enlarged, but never joining the 

 negative glow, there being always a dark space between. This 

 space of about -^-^ or -^^ inch was apparently invariable in its extent 

 and its positive relation to the negative rod, nor did the negative 

 glow vary. Whether the negative end were inductric or inducteous, 

 the same effect was produced. It was strange to see the positive 

 purple haze diminish or lengthen as the ends were separated, and 

 yet this dark space and the negative glow remained unaltered." 



You may look upon this as the first step towards the discovery 

 of X-rays. 



De la Eue, Muller, Gassiot, Spottiswoode and Geissler, all did 

 excellent work in studying electric discharges through vacuum tubes. 

 After the discovery of the mercury pump by Dr. Herman Sprengel 

 the degree of exhaustion of these tubes was rendered far more 

 perfect. Professor Hittorf had done some very fine work in Germany 

 with these highly exhausted tubes, and then Sir William Crookes, in 

 this country, did his remarkable researches between 1874 and 1875. 

 By carrying exhaustion to a higher degree than had ever been pre- 

 viously attained, he came to the conclusion that the particles of air 

 remaining in these highly exhausted tubes behaved so differently 

 from an ordinary gas, that he considered it to have attained a fourth 

 condition of matter, which he called radiant matter, so that we could 

 no longer only speak of solids, liquids and gases, but had to include 

 radiant matter. The gas in the tube assumes this condition when 

 the vacuum is about a millionth of an atmosphere ; in other words, 



