I April lo, 1879] 



NATURE 



533 



EXPERIMENTAL RESEARCHES ON THE RE- 

 PULSION RESULTING FROM RADIATION^ 



II. 



HAVING completed the experimental investigation 

 of the amount of repulsion produced by radiation 

 on disks of various kinds, and coated with different sub- 

 stances, I turned my attention to the amount of repulsion 

 produced when polarised light is allowed to fall on a plate 

 of tourmaline suspended in vacuo in a torsion balance. 

 It was originally thought that a slice of tourmaline, being 

 black to a ray of light polarised in one plane, and white to 

 a ray polarised in the other plane, would be repelled when 

 the incident light was quenched by it, and not affected 

 when the incident light passed through it. Experiments, 

 however, prove that this action does not exist in any 



' Fig. 4. 



appreciable degree, the repulsion resulting from radiation 

 being almost entirely a surface action, whilst the action 

 of a tourmaUne on a ray of polarised light is one in which 

 thickness is necessary. 



I next examined the effect of shape in influencing 

 the amount and direction of repulsion. These experi- 

 ments were for the most part tried with the apparatus 

 shown in Fig. 3 (p. 513, part I.). Through the open top 

 access can readily be obtained, and disks, plates, &c., 

 can be quickly tested by being fixed to the extremities of 

 a pair of aluminium arms, with a glass cap in the centre, 

 rotating on the needle-point. Plates, 12 millims. square, 

 cut from thin aluminium foil, were mounted diamond- 

 wise on arms, and supported on the needle-point inside 

 the bulb. The plates were lampblacked on sides facing 



Ulevatlon 

 Scale 



i 



Fig. s- 



opposite ways, and the apparatus was well exhausted. 

 The vanes behaved like an ordinary metal radiometer in 

 respect to light and radiant heat. Fig. 4 shows the elevation 

 and plan of the fly, the dotted side representing the one 

 which was lampblacked. The arrows show the direction 

 of positive rotation when exposed to the light of a stan- 

 dard candle 3*5 inches off. The outer corners of the 

 aluminium plates were now turned up at an angle of 45", 

 4 millims. of the two sides being turned up, leaving 8 

 millims. flat, as shown in Fig. 5. They were lampblacked 

 on the inside, as shown in the figure by dots. A lighted 

 candle 3*5 inches off caused very slow and feeble positive 

 rotation. On shading the light from the black side, the 

 bright side was repelled, causing positive rotation ; and 

 on shading the light from the bright side the black was 



' Continued from p. 514^ 



repelled, causing negative rotation.^ The positive repul- 

 sion was, however, rather stronger than the negative 

 repulsion, so that, when both -sides were illuminated, the 

 force was only that due to the difference of these re- 

 pulsions. 



A hot glass shade is a convenient means of heating 

 the bulb, by immersing it in a hot-air bath, without 

 the liability of introducing action of rays other than 

 those emitted by hot glass. On inverting a hot glass 

 shade over the bulb in the above experiment, negative 

 rotation was produced which changed to positive on cool- 

 ing. Both these rotations were stronger than that given 

 by the candle. The experiment was varied (i) by 6 

 millims. of the sides being turned up instead of 4 ; (2) by 

 folding the plates across the vertical diagonal and then 

 across their horizontal diagonal; (3) by attaching flat 



Fig. 6. 



plates to the arms at an angle of 45°, blacking them on 

 the insides away from the bulb, and repeating the experi- 

 ment with plates blacked on the outsides. The results 

 obtained show that when flat plates are taken blacked on 

 alternate sides, the rotation is normal or positive, i.e., the 

 black side is repelled. When the outer comers of each 

 plate is turned up so as to keep the blacked surface on 

 the concave side, the positive rotation is either diminished, 

 stopped, or converted into negative rotation, according to 

 the amount of surface of the plate which has been turned 

 up. The favourable presentation of the surface of the 

 vanes to the inside of the bulb has more influence on the 

 movement than has the colour of the surface. Radio- 

 meters constructed with silver flake vanes set at an angle 



Fig. 7. 



of 45° and blacked on the outside prove the most sensitive 

 for light hitherto constructed. 



I now endeavoured to clear up many anomalous results 

 which had attended the application of heat either by hot 

 shades or by hot water to radiometers. There was an 

 antagonistic action between the effect of shape and that 

 of colour of surface, the two actions sometimes acting 

 together and sometimes in opposition. 



Five radiometers were made exactly alike in size of 

 bulb, shape of vanes, and degree of exhaustion, only 

 differing in the material of which the vanes were com- 

 posed. No. I was made of mica, 0*003 iiich in thickness ; 

 No. 2, of mica, o'ooo5 inch in thickness; No. 3, of pith, 



' I call the rotation positive when the black or driving side is repelled, 

 and negative when the side which under ordinajy circumstances would be 

 the driving side, moves towards the light. 



