136 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XIII.) 



effect may be detected by shifting the eye at the time of observation, or avoided, by 

 using blackened rods. 



1533. It is curious to observe the relation of glow, brush, and spark to each other, 

 as produced by positive or negative surfaces ; thus, beginning with spark discharge, 

 it passes into brush much sooner when the surface at which the discharge commences 

 (1484.) is negative, than it does when positive; but proceeding onwards in the 

 order of change, we find that the positive brush passes into glow long before the ne- 

 gative brush does. So that, though each presents the three conditions in the same 

 general order, the series are not precisely the same. It is probable, that, when these 

 points are minutely examined, as they must be shortly, we shall find that each dif- 

 ferent gas or dielectric presents its own peculiar results, dependent upon the mode in 

 which its particles assume polar electric condition. 



1534. The glow occurs in all gases in which I have looked for it. These are air, 

 nitrogen, oxygen, hydrogen, coal gas, carbonic acid, muriatic acid, sulphurous acid 

 and ammonia. I thought also that I obtained it in oil of turpentine, but if so it was 

 very dull and small. 



1535. The glow is always accompanied by a wind proceeding either directly out 

 from the glowing part, or directly towards it ; the former being the most general 

 case. This takes place even when the glow occurs upon a ball of considerable size : 

 and if matters be so arranged that the ready and regular access of air to a part ex- 

 hibiting the glow be interfered with or prevented, the glow then disappears. 



1536. I have never been able to analyse or separate the glow into visible element- 

 ary intermitting discharges (1427. 1433.), nor to obtain the other evidence of inter- 

 mitting action, namely an audible sound (1431.). The want of success, as respects 

 trials made by ocular means, may depend upon the large size of the glow preventing 

 the separation of the visible images : and, indeed, if it does intermit, it is not likely 

 that all parts intermit at once with a simultaneous regularity. 



1537- All the effects tend to show, that glow is due to a continuous charge or 

 discharge of air ; in the former case being accompanied by a current from, and in 

 the latter by one to, the place of the glow. As the surrounding air comes up to the 

 charged conductor, on attaining that spot at which the tension of the particles is 

 raised to the suflScient degree (1370. 1410.), it becomes charged, and then moves off, 

 by the joint action of the forces to which it is subject ; and, at the same time that it 

 makes way for other particles to come and be charged in turn, actually helps to form 

 that current by which they are brought into the necessary position. Thus, through 

 the regularity of the forces, a constant and quiet result is produced ; and that result 

 is, the charging of successive portions of air, the production of a current, and of a con- 

 tinuous glow. 



1538. I have frequently been able to make the termination of a rod, which, when 

 left to itself, would produce a brush, produce in preference a glow, simply by aiding the 

 formation of a current of air at its extremity ; and, on the other hand, it is not at all 



