420 



NATURE 



{August 28, 1879 



has become so long that the hits in a given time in comparison to 

 the misses may be disregarded, and the average molecule is now 

 allowed to obey its own motions or laws without interference. 

 The mean free path, in fact, is comparable to the dimensions of 

 the vessel, and we have no longer to deal with a continuous 

 portion of matter, as would be the case were the tubes less highly 

 exhausted, but we must here contemplate the molecules indivi- 

 dually. In these highly exhausted vessels the molecules of the 

 gaseous residue are able to dart across the tube with compara- 

 tively few collisions, and radiating from the pole with enormous 

 velocity, they assume properties so novel and so characteristic as 

 to entirely justify the application of the term borrowed from 

 Faraday, that of Radiant Matter. 



Radiant Matter exerts powerful Phosphorogenic Action where it 

 strikes 



I have mentioned that the radiant matter within the dark space 

 excites luminosity where its velocity is arrested by residual gas 

 outside the dark space. But if no residual gas is left, the 

 molecules will have their velocity arrested by the sides of the 

 glass ; and here we come to the first and one of the most note- 

 worthy properties of radiant matter discharged from the negative 

 pole — its power of exciting phosphorescence when it strikes 



against solid matter. The number of bodies which respond 

 luminously to this molecular bombardment is very great, and the 

 resulting colours are of every variety. Glass, for instance, is 

 highly phosphorescent when exposed to a stream of radiant 

 matter. Here (Fig. 2) are three bulbs composed of different 

 glass : one is uranium glass («), which phosphoresces of a dark 

 green colour ; another is English glass (b), which phosphoresces 

 of a blue colour ; and the third {c) is soft German glass— of 

 which most of the apparatus before you is made— which phos- 

 phoresces of a bright apple-green. 



My earlier experiments were almost entirely carried on by the 

 aid of the phosphorescence which glass takes up when it is under 

 the influence of the radiant discharge ; but many other substances 

 possess thii phosphorescent power in a still higher degree than 

 glass. For instance, here is some of the luminous sulphide of 

 calcium prepared according to M. Ed. Becquerel's description. 

 When the sulphide is exposed to light— even candle-light— it 

 phosphoresces for hours with a bluish-white colour. It is, how- 

 ever, much more strongly phosphorescent to the molecular dis- 

 charge in a good vacuum, as you will see when I pass the 

 discharge through this tube. 



Other substances besides English, German, and uranium glass, 

 and Becquerel's luminous sulphides, are also phosphorescent. 



The rare mineral phenakite (aluminate of glucinum) phosphoresces 

 blue ; the mineral spodumene (a silicate of aluminium and 

 lithium) phosphoresces a rich golden yellow ; the emerald gives 

 out a crimson light. But without exception, the diamond is the 

 most sensitive substance I have yet met for ready and brilliant 

 pliosphorescence. Here is a very curious fluorescent diamond. 



Fig. 



green by daylight, colourless by candle light. It is mounted in 

 the centre of an exhausted bulb (Fig. 3), and the molecular 

 discharge will be directed on it from below upwards. On 

 darkening the room you nee the diamond shines with as much 

 light as a candle, phosphorescing of a bright green. 



Next to the diamond the ruby is one of the most remarkable 

 stones for phosphorescing. In this tube (Fig. 4) is a fine collec- 

 tion of ruby pebbles. As soon as the induction-spark is turned 

 on you will see these rubies shining with a brilliant rich red tone, 

 as if they were glowing hot. It scarcely matters what colour the 

 ruby is, to begin with. In this tube of natural rubies there are 

 stones of all colours — the deep red and also the pale pink ruby. 

 There are some so pale as to be almost colourless, and some of 

 the highly-prized tint of pigeon's blood ; but under the impact of 

 radiant matter they all phosphoresce w'ith about the same colour. 



Now the ruby is nothing but crystallised alumina with a little 



Fig. 4. 



colouring-matter. In a paper by Ed. Becquerel,^ published 

 twenty years ago, he describes the ajipearance of alumina as 

 glowing with a rich red colour in the pliosphoroscope. Here is 

 some precipitated alumina prepared in the most careful manner. 

 It has been heated to whiteness, and you see it also glows under 

 the molecular discharge with the same rich red colour. 



The spectrum of the red light emitted by these varieties of 

 alumina is the same as described by Bccquerel twenty years ago. 

 There is one intense red line, a little below the fixed line B in 



^ Annates de Chim;e et de Physique, 3rd series, vol. Ivii., p. 50, 1859.*]; 



