Electric Discharge in Gases. 89 



pressure. In fact, in accordance with what has been explained 

 above as to the resistance of the positive light, there exists 

 round the electrode a space, varying from within outwards, 

 which may very well produce such deflections in rays which 

 traverse it. 



(7) The mechanical action of the kathode-rays, such as 

 movements of surfaces upon which they impinge, so far as 

 they do not arise from the thermal phenomena, is seen from 

 Maxwell's results, according to which a ray of light exerts on 

 the wave-front a pressure which is numerically equal to the 

 energy in the unit volume. 



I have previously calculated concerning ttie heating which 

 takes place at the kathode how high a value this energy 

 may possess, of which an idea may be formed from the fact, 

 amongst others, that in the focus of a hemispherical kathode, 

 as is well known, platinum-foil is heated to intense glow, and 

 glass is melted. For this last phenomenon it is important that 

 in the highly exhausted space the heat yielded by the kathode- 

 rays to the foil shall be given up to the surrounding air by 

 convection only with extreme slowness. 



(8) The mutual repulsion of two kathode-rays as observed 

 by Crookes, and which, as I have been able to observe, actually 

 occurs under the conditions which he states, i. e. with strong- 

 discharges and when the rays form a small angle with each 

 other, whereas the rays cross when they meet under a larger 

 angle, may be explained by the pressure exerted on the wave- 

 front of the ray of light. 



(9) Hertz has already (loc. cit.) pointed out the possible 

 connexion of the deflection of the kathode-rays with the 

 rotation of the plane of polarization ; without, however, ex- 

 pressing a definite opinion. I believe that with the above 

 assumption as to the nature of the kathode-rays, the two phe- 

 nomena will be found to be related in a definite manner. 



Let SN (fig. 9) be a magnet along whose axis a ray of 

 light is sent, vibrating, we will suppose, in the plane of the 

 paper ; then, if we place a piece of glass before the north 

 pole, the vibrations are disturbed so that the upper part of the 

 ray is rotated forwards. 



If, now, we imagine the ray moving in a direction at right 

 angles to the plane of the paper, instead of the direction S N, 

 a similar rotation must take place, for the position of the 

 gether-vibrations relative to the magnet remains unaltered. 

 But the ray of light must always be at right angles to the 

 direction of vibration. If, therefore, this be rotated, the ray 

 of light must also be rotated, or it suffers a deflection, the ray 

 being elevated from N onwards. 



