258 Professor Tyndall [March 16, 



The first ignition of the sodium was accompanied by the development 

 of a large amount of sodium vapour, which spread outwards and sur- 

 rounded, as a cool envelope, the core of intensely heated vapour 

 inside. By the cool vapour the rays from the hot were inter- 

 cepted, but on lengthening the arc the outer vapour in great part was 

 dispersed, and the rays passed to the screen. This relation as to 

 temperature was necessary to the production of the black band ; for 

 were the outside vapour as hot as the inside, it would, by its own 

 radiation, make good the light absorbed. 



An extremely beautiful experiment of this kind was made here last 

 week by Professor Liveing, with rays which, under ordinary cir- 

 cumstances, are entirely invisible. Professor Dewar and Professor 

 Liveing have been long working with conspicuous success at the 

 ultra-violet spectrum, and with Professor Dewar's aid I will now show 

 you this spectrum, as it was shown last week by Professor Liveing. 

 Using prisms and lenses of a certain kiud, and a powerful dynamo 

 machine to volatilize our metals, we cast a spectrum upon the screen. 

 You notice the terminal violet of this spectrum. Far beyond that 

 violet, waves are now impinging upon the screen, which have no 

 sensible eifect upon the organ of vision ; they constitute what we call 

 the ultra-violet spectrum. Professor Stokes has taught us how to 

 render this invisible spectrum visible, and it is by a skilful applica- 

 tion of Stokes' discovery that Liveing and Dewar bring the hidden 

 spectrum out with wondrous strength and beauty. 



You notice here a small second screen, which can be moved into the 

 ultra-violet region. Felt by the hand, the surface of this screen re- 

 sembles sandpaper, being covered with powdered uranium glass, a 

 highly fluorescent body. Pushing the moveable screen towards the 

 visible spectrum, at a distance of three or four feet beyond 

 the violet, light begins to appear. On pushing in the screen, the 

 whole ultra-violet spectrum falls upon it, and is rendered visible 

 from beginning to end. The spectrum is not continuous, but com- 

 posed for the most part of luminous bands derived from the white- 

 hot crucible in which the metals are to be converted into vapour. 

 I beg of you to direct your attention on one of these bands in par- 

 ticular. Here it is, of fair luminous intensity. My object now is to 

 show you the reversal, as it is called, of that band which belongs 

 to the vapour of magnesium, exactly as I showed you a moment 

 ago the reversal of the sodium band. An assistant will throw a bit 

 of magnesium into the crucible, and you are to observe what first 

 takes place. The action is rapid, so that you will have to fix your 

 eyes upon this particular strip of light. On throwing in the mag- 

 nesium, the luminous band belonging to its vapour is cut away, 

 and you have, for a second or so, a dark band in its place. I repeat 

 the experiment three or four times in succession, with the same 

 unfailing result. Here, as in the case of the sodium, the mag- 

 nesium surrounded itself for a moment by a cool envelope of its own 



