158 



THE POPULAR SCIENCE MONTHLY. 



ing 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 center, 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 uj) hill. 



This experiment, therefore, shows that the molecular stream 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 appa- 

 ratus 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 ordi- 



nary 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 needle-point on which it w^orks is con- 

 nected by 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 fly being made the negative pole. 



