•56 



NA TURE 



{Dec. 13 1883 



tions strongly confirm. Hence relatively warmer and 

 moister upper currents will'llow backward over the colder 

 and drier air immediately in the rear of the centres of 

 cyclones ; and upper currents also presenting contrasts 

 of temperature and vapour will overlap the outskirts of 

 anticyclones. These considerations suggest how very di- 

 verse interpretations of the movements of the cirrus cloud 

 in their relation to cyclones and anticylones have origi- 

 nated, and may also indicate lines of research into some of 

 the more striking optical scenic displays of the atmosphere. 



ELECTRIC SHADOWS 

 'T^HE brilliant researches of Crookes upon the electric 

 -'■ discharges in highly attenuated vacua, which some 

 four years ago culminated in the discovery of the pheno- 

 mena of "radiant matter," revealed, amongst other 

 singular and curious effects, the existence of electric 

 shadows. In the tube; employed by Crookes, wherein 

 the rarefaction had been carried to miUionths of the nor- 

 mal air pressure, objects cut out in sheets of metal or 

 other good conductors of electricity were found to cast 

 shadows against the glimmering surfaces of the ghss 



the notice of these researches for Nature in 1881, I 

 made the following remark : — " These dust-figures have 

 an obvious relation with those obtained by Wiedemann 

 from the discharge of Leyden jars through a pointed con- 

 ductor against the surfaces of various bodies. It would 

 be interesting to ascert.iin whether by this process also 

 shadow-figures can be produced." The suggestion then 

 thrown out has not been lost, for during the current year 

 a memoir has appeared on the subject of electric shadows 

 from the pen of Prof. Augusto Righi, of Padua, giving 



when interposed in the path of the discharge. Tlie 

 deflection of these shadows by the magnet was also 

 observed by Crookes. About eighteen months afterwards 

 some analogous phenomena were observed and described 

 by Prof. W. Holtz of Berlin ; the main difference between 

 the phenomena observed by Crookes and by Holtz being 

 that in the experiments of the latter the shadows were 

 obtained at the ordinary pressure of the air by means of 

 the discharge from a Holtz's influence machine. Of these 

 researches some account was given at the time in Naiuke 

 (vol. xxiv. p. 130) by the writer of this article. It will 

 be sufficient here to recall the more salient points. In 

 the place of the usual discharging knobs of the Holtz 

 machine were fixed a wooden disk covered with silk on 

 the one side, and a metallic point on the other. The 

 discharge from the latter causes the surface of the former 

 to assume a faint, phosphorescent glow, visible only in 

 complete darkness ; and on this faintly illuminated sur- 

 face shadows were cast when conducting bodies — such, 

 for example, as crosses or rings cut from thin brass or 

 foil, strips of damp cardboard, wires, and other similar 

 objects. It was also noticed by Holtz that these shadow- 

 figures could be temporarily fixed by dusting upon them 

 some fine powder, such as lycopodium. In preparing 



the results of an investigation of shadows produced by 

 this very method. 1 propose to give here a resume of 

 the phenomena observed by Righi. 



Righi discusses in an introductory way the suggestion 

 of Crookes as to the relation between the length of the 

 mean free path of the molecules and the distance to 

 which the "radiant" discharge can be traced from the 

 electrode. He observes that even in cases where the 

 mean free path (as determined by the temperature of the 



gas) be very short, as in air at ordinary pressure, the 

 motion of the gaseous molecules as a whole may yet be 

 in nearly straight lines of considerable length, owing to 

 the fact that the electric force in the space where dis- 

 charge is taking place will necessarily t^nd to urge an 

 electrified molecule along the lines of electric force, and 

 will act in the same direction whether the charge on any 

 single molecule remain upon it or whether it be shared 

 with other molecules .igainst which it may impinge in its 



