74 



Mr. J. Joly. 



Figures obtained on both, sides of this plate at pressures in the re- 

 ceiver of over 15" of mercury showed much the same minutely divided 

 patterns, in which the + is hardly distinguishable from the — , as 

 occur at ordinary pressures. At about 15" pressure, however, a sudden 

 transition in the nature of the figures occurs. The negative becomes 

 very indistinct, a hazy riug with little veining. The positive becomes 

 a few long straggling lines, radiate to the centre. So abrupt is the 

 transition that a plate was obtained on which about one-third, or 120°, 

 of the first sort of pattern was definitely replaced by a sector of the 

 second sort, a faint halo on the negative and a ring of straggling 

 radiate lines backing it on the positive side. 



As the pressure diminishes the second order of pattern persists and 

 develops. The positive riug spreads outwards, the straggling lines of 

 which it consists becoming shorter. The negative grows so indistinct 

 as to be no longer easily located on the plate ; it is found, how r ever, 

 backing the positive ring on close observation. Ultimately both forms 

 disappear from the plate, the positive persisting at pressures at which 

 the negative is quite indiscoverable. 



It would appear from all these observations that where the discharge 

 is chiefly in the gaseous dielectric it is to some property of this 

 medium that the peculiar characters of the patterns are to be ascribed. 

 Faraday's view that much of the individualities of positive and nega- 

 tive brushes, sparks, &c, w r as to be ascribed to the behaviour of the 

 matter conveying the current may possibly be an approximation to 

 the truth in the present case, although affording no insight into the 

 manner in which such a remarkable distinction in the nature of 

 the figures can be brought about by a difference in the behaviour of 

 the gaseous matter towards positive and negative discharges. This 

 view is, however, I think, rather confirmed by the lessening of the 

 distinction between the patterns as the dielectric plate gets thinner, 

 for it is probable that in the case of a thin screen separating 

 the discharges these occur to a greater extent in the matter of 

 the screen, the parts of which, yielding only slightly to stresses, 

 refuse to show any distinguishing behaviour towards positive or 

 negative. 



That the distinguishing features of the positive and negative pat- 

 terns vary with the nature of the matter which shares with the gas 

 in carrying the discharge is apparent, from Mr. Brown's results, in 

 air discharges over photographic plates. The photographic film in 

 this case probably possessed a considerable degree of conductivity y 

 and so modified the air discharge. In the case of the peroxide and 

 protoxide of tin there was also, as already observed, a modifying, or 

 at least a superadded, effect. It is probable that with the use of 

 jycopodium there is with thick plates little modification of the air 

 discharge. The next experiment is a case in point, although much of 



