1879.] 



on Molecular Physics in High Vacua. 



157 



has brouglit to sncli perfection, and I think that I shall succeed in 

 even melting that. I first turn on the induction-coil slightly, so as 

 not to bring out its full power. The focus is now playing on the 

 iridio platinum, raising it to a white heat. I bring a small magnet 

 near, and you see I can deflect the focus of heat just as I did the 

 luminous focus in the other tube. By shifting the magnet I can 

 drive the focus up or down, or draw it completely away from the 

 metal, and render it non-luminous. I withdraw the magnet, and let 

 the molecules have full play again ; the metal is now white-hot. I 

 increase the intensity of the spark. The metal glows with almost 

 insupportable brilliancy, and at last melts. 



There is still another property of this molecular discharge, and it 

 is this : — You have seen that the molecules are driven violently from 

 the negative pole. If I place something in front of these molecules, 

 they show the force of impact by the heat which is produced. Can I 

 make this mechanical action evident in a more direct way? Nothing is 

 simpler. I have only to put some easily moving object in the line of dis- 

 charge in order to get a powerful mechanical action. Mr. Gimingham, 



Fig. 19. 



with great skill, has constructed a piece of apparatus which I will 

 presently put in the electric lantern, so that all will be able to see its 

 action. But first I will explain the construction by means of this 

 diagram (Fig. 19). The negative pole (a, h) is in the form of a very 

 shallow cup. In front of the cup is a mica screen (c, d), wide enough 

 to intercept nearly all the molecular rays coming from the negative 

 pole. Behind this screen is a mica wheel (e,/) with a series of vanes, 

 making a sort of paddle-wheel of it. So arranged, the molecular 

 stream from the pole a h will nearly all be cut off from the wheel, and 

 what escapes over and under the screen will hit the vanes equally, and 



