436 



NATURE 



[Sept. 4, 1879 



{Phrynosoma cornuliim) from Texas, presented by Mr. Ernest 

 E. Sabel ; a Sulphur-breasted Toucan i(RaniJ>haslos carinatus) 

 three Black-necked Stilt Plovers (ffimantopus nigricollis), two 

 Cayenne Lapwings ( Vanellus cayeimensis) from South America, 

 a Slow Loris {NycHcebus tardi^radus) from Malacca, a Radiated 

 Tortoise (Tesludo radiata) from Madagascar, two Electric 

 Silurus {Malapterurus beninensis) from West Africa, purchased ; 

 a Squirrel-like Phalan^er (Belidetis sciurea), bom in the Gardens. 



ON RADIANT MATTER 1 



II. 



Radiant Matter exerts strong Mechanical Action where it Strikes 



■^XTE have seen, from the sharpness of the molecular shadows, that 

 ' radiant matter is arrested by solid matter placed in its path. 



Fic. 10. 



If this solid body is easily moved the impact of the molecules 

 •will reveal itself in strong mechanical action. Mr. Gimingham 

 has constructed for me an ingenious piece of apparatus which 

 when placed in the electric lantern will render this mechanical 

 action visible to all present. It consists of a highly-exhausted 

 glass tube (Fig. 11), having a little glass railway running along 

 it from one end to the other. The axle of a small wheel revolves 

 on the rails, the spokes of the wheel carrying wide mica paddles. 

 At each end of the tube, and rather above the centre, is an 

 aluminium pole, so that whichever pole is made negative the 

 stream of radiant matter darts from it along the tube, and striking 

 the upper vanes of the little paddle-wheel, causes it to turn round 

 and travel along the railway. By reversing the poles I can arrest 

 the wheel and send it the reverse way, and if I gently incline the 

 tube the force of impact is observed to be sufficient even to drive 

 the wheel up-hill. 



This experiment therefore shows that the molecular stream 



Fig. it. 



from the negative pole is able to move any light object in front 

 of it. 



The molecules being driven violently from the pole there 

 should be a recoil of the pole from the molecules, and by 

 arranging an apparatus so as to have the negative pole movable 

 and the body receiving the impact of the radiant matter fixed, 

 this recoil can be rendered sensible. In appearance the apparatus 

 (Fig. 12) is not unlike an ordinary radiometer with aluminium 

 disks for vanes, each disk coated on one side with a film of mica. 

 The fly is supported by a hard steel instead of glass cup, and the 

 needlepoint on which it works is connected Ijy means of a wire 

 with a platinum terminal sealed into the glass. At the top of the 

 radiometer bulb a second terminal is sealed in. The radiometer 

 therefore can be connected with an induction-coil, the movable 

 lly being made the negative pole. 



For these mechanical effects the exhaustion need not be so 

 high as when phosphorescence is produced. "Ihe best pressure 



" A lecture delivered to the British Association for the .Advancement of 

 Science, at Sheffield, Friday, August 2z, 1879, by William Crookes, F.R.S. 

 Continued from p. 423, 



for this electrical radiometer is a little beyond that at which the 

 dark space round the negative pole extends to the sides of the 

 glass btilb. When the pressure is only a few millims. of mercury, 

 on passing the induction current a halo of velvety violet light 

 forms on the metallic side of the vanes, the mica side remaining 

 dark. As the pressure diminishes, a dark space Ls seen to sepa- 

 rate the violet halo from the metal. At a pressure of half a 

 millim. this dark space extends to the glass and rotation com- 

 mences. On continuing the exhaustion the dark space further 

 widens out and appears to flatten itself against the glass, when 

 the rotation becomes very rapid. 



Here is another piece of apparatus (Fig. 13) which illustrates 



Fig. 12. 



Fig. 13. 



the mechanical force of the radiant matter from the negative 

 pole. A stem [a] cai-ries a needle-point in which revolves a light 

 mica fly {b b). The fly consists of four square vanes of thin 

 clear mica, supported on light aluminium arms, and in the centre 

 is a small glass cap which rests on the needle-point. The 

 vanes are inclined at an angle of 45° to the horizontal plane. 

 Below the fly is a ring of fine platinum wire {cc), the ends of 

 which pass through the glass at dd. An aluminium terminal 

 (e) is sealed in at the top of the tube, and the whole is exhausted 

 to a very high point. 



By means of the electric lantern I project an image of the 

 vanes on the screen. Wires from the induction-coil are attached, 



:.«;: 



:..•] ^ 



Fig. 14. 



so that the platinum ring is made the negative pole, the aluminium 

 wire (e) being positive. Instantly, owing to the projection of 

 radiant matter from the platinum ring, the vanes rotate with 

 extreme velocity. Thus far the apparatus has shown nothing 

 more than the previous experiments have prepared us to expect ; 

 but observe what now happens. I disconnect the induction-coil 

 altogetlier, and connect the two ends of the platinum wire with 

 a small galvanic battery ; this makes the ring c c red-hot, and 

 under this influence you see that the vanes spin as fast as they 

 did when the induction-coil was at work. 



Here, then, is another most important fact. Radiant matter 

 in these high vacua is not only excited by the negative pole of an 

 induction-coil, but a hot wire will set it in motion with force 

 sufficient to drive round the sloping vanes. 



