EECENT PROGRESS IN PHYSICS. 377 



w.ith a blunt conical point, a phosphorescent continuous glow is ob- 

 tained the more readily as the point is finer. 



2d. Increase of power in the machine. — Rounded ends, which give 

 only brushes when the machine is in weak action, give the glow readily 

 when the machine is in good order. 



3d. Puirefaction of the air. — A brass ball 2| inches in diameter 

 being made positively inductive in an air-pump receiver, became 

 covered with glow in part, " when the pressure was reduced to 4.4 

 inches. By a little adjustment the ball could be covered all over with 

 this light. Using a brass ball 1.25 inch in diameter, and making 

 it inducteously positive by an inducti/e negative point, the phenomena 

 were exceedingly beautiful. The glow came over the positive ball, 

 and gradually increased in brightness until it was at least very lu- 

 minous ; and it also stood up, like a low iiame, half an inch or more 

 in height."— (1529.) 



The negative gloio is difficult to obtain in air at common pressures ; 

 " and it is as yet questionable whetber, even on fine points, what is 

 called the negative star is not a very reduced, but still intermitting 

 brush, or a glow." — (15i^0.) 



In rarefied air the negative glow can easily be obtained. If the 

 rounded ends of two metal rods about 0.2 of an inch in diameter 

 are about four inches apart in rarefied air, the glow can be easily ob- 

 tained on both rods, covering not only the ends but an inch or two of 

 the part behind. Balls are also covered with the negative glow in 

 rarefied air, whether their surface is inductive or inducteous. — (1531.) 



The glow occurs in all the gases examined for it by Faraday. He 

 thought he obtained it also in oil of turpentine, though it was very 

 dull and small.— (1534.) 



"The glowis always accompanied by a wind,proceedingeither directly 

 out from the glowing part or directly towards it ; the former being the 

 most general case." If the arrangements are made so that the ready 

 and regular access of air to a part exhibiting the glow be interfered 

 with or prevented the glow then disappears. — (1535.) 



Frequently it is possible to change the brush given by the end of 

 a rod into a glow, by simply aiding the formation of a current of air 

 at its extremity. — (1535.) 



§ 87. Dark discharge. — If to the rounded end of a metallic rod pro- 

 jecting from the prime conductor of a machine a similar rod be held 

 at a little distance, it is easy to obtain the appearance of light at the 

 ends of both rods, while the intervening space between the positive 

 and negative light remains dark ; besides this familiar phenomenon, 

 Faraday notices a very remarkable case of dark discharge. 



'' Two brass rods, 0.3 of an inch in diameter, entering a glass globe 

 on opposite sides, had their ends brought into contact, and the air 

 about them very much rarefied. A discharge of electricity from the 

 machine was then made through them, and while that continued the 

 ends were separated from each other. At the moment of separation 

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

 termination remaining quite dark. As the distance was increased a 

 purple stream or haze appeared on the end of the positive rod, and 

 proceeded directly onward towards the negative rod, elongating as the 



