548 



NATURE 



\OcL 6, 



merely showi signs of perforation like the card, without any 

 blackening or indication of combustion. Whether these facts 

 point merely to shortness of duration in ihe discharge such as to 

 preclude the c^mmunica'.ijn of heat-vibrations to the bodies 

 traversed, or whether they imply some mode of motion with 

 which heat has nothing to do, are questions which have been 

 thro.vn out by those who have studied the subject. 



In favour of the former view it should be stated that the 

 spectru n of the spark proper, whether w ith or without the jar, 

 shjws bright lines, indicating the presence of metallic vapours. 

 These of course imply a high temperature, although not neces- 

 sarily any great quantity of heat. And if the diu'atiin of the 

 spark itself be ex'.remely small compared with that of the inter- 

 val between twj successive sparks, the period of cooling will be 

 extremely long compared with that of heating, and the observed 

 result is exactly what we might expect. 



There is, however, one feature of the spark discharge proper 

 which ii perhapi especially deserving of remark, namely, the 



similarity, in appearance at least, of its passage through air 

 with that of a spark through glass or other solid and non-con- 

 ducting substances. In the latter case we are familiar with the 

 manner in which it rends its way by a shattering and dislocation 

 of the substance in its immediate path, while it leaves the other 

 parts of the substance untouched, very much as does a bullet 

 when shot through a pane of glass. The path, however, if of 

 any considerable length, is never quite straight, ami it sometimes 

 divides itself into two branches. The anal.jgy above suggested 

 will be complete, and the phenomenon will be brought into 

 harmony with other known facts, if only we regard the spark as 

 being so rapid, so instantaneou-, in its pas-age that the particles 

 of air have not time to exercise their mobility during the period 

 occupied by the spark in its passage throagh them. In this 

 view, air itself in the presence of the electric spark is to be 

 regarded as exhibiting a rigidity and brittleness comparable with 

 that of glass itself. 



If, the Leyden jar having been removed, the terminals of the 



Holtz machine be separated to a distance greater than that over 

 which the spark will leap, a hissing or crackling noise is heard, 

 indicating a rapid intermiltence in the dischirge, and a delivery, 

 S5 to speak, of small quantities of electricity at a tiine. A 

 minute examination of the phenomena occurring with terminals 

 of different forms, and at different di^tan:es, has led to a classi- 

 fication of types of discharge under four main heads : — 



1. The glow discharge: presenting a glow on the positive 

 terminal, and a pencil of light issuing from the negative, and 

 consisting of two portions with a dark space between them. 



2. The brush discharge : con--isling of a brush, viz., a stem 

 and branches at the positive terminal, a pencil of light at the 

 negative, and a dark space as before. 



3. The band discharge : consisting of a band of light pro- 

 ceeding from the positive terminal, sometimes stratified, and 

 separated from the negative glow by a dark space. 



4. The spark discharge : showing in the spectroscope bright 



lines at bnth terminals. Two brushes of metallic vapour, that at 

 the positive terminal being the bnger, that at the negative the 

 shorter and thicker. Two dark spaces are to be noticed in this 

 form of discharge. 



On the other hand, if the terminals be brought nearer together 

 than they were at first, nearer, that is to say, than is suitable for 

 the production of the spark proper, it will be noticed that the 

 .sliarp cracking noise is replaced by a sound similar to that heard 

 when they were beyond striking distance. The intermiltence 

 of the discharge becomes ver)- rapid, and its colour assumes a 

 reddish hue. 



A full explanation of this almost abrupt change in the cha- 

 racter of the discharge would probably involve a more profound 

 acquaintance with the nature of eleccricity than we at present 

 possess. But there is reason to think that tomething like the 

 following takes place :— The path between the terminals once 

 opened offers for a very short time considerable facility for the 



