250 



NATURE 



[Jfuly lo, 1879 



MOLECULAR PHYSICS IN HIGH VACUA ' 

 II. 

 T HAVE hitherto spolcen of and illustrated these phenomena in 

 connection with green phosphorescence. It does not follow, 

 however, that the phosphorescence is always of that colour. 

 This coloration is a property of the particular kind of glass in 

 use in my laboratory. I have here (Fig. 7) three bulbs com- 

 posed of different glass : one is uranium glass (a), which phos- 



F.G. 7. 



phoresces 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 phosphoresces of a bright apple-green colour. It 

 is therefore plain that this particular green phosphorescence is 

 solely due to the glass which I am using. Were I to use Eng- 

 lish glass I should have to speak of blue phosphorescence, but I 

 know of no glass which is equal to the German in brilliancy. 



My earlier experiments were almost 

 entirely carried on by the aid of the 

 ] hosphorescence which glass takes up 

 when it is under the influence of the 

 electric discharge in vacuo ; but many 

 other substances possess this phos- 

 phorescent power, and some have it 

 in a much higher degree than glass. 

 For instance, here is some of the 

 luminous sulphide of calcium prepared 

 according to M. Ed. Becquerel's de- 

 scription. ^Vhen it is expo.-ed to light 

 — even candlelight — it phosphoresces 

 for hours with a rich blue colour. I 

 have prepared a di.igram with large 

 letters written in this luminous siU- 

 phide ; before it is exposed to the light 

 the letters are invisible, but Mr. 

 Gimingham has just exposed it in 

 another room to burning mignesium, 

 and now it is brought into the dark- 

 ened theatre you will see the word 

 " rpiSs," — ^'g'Af, a very suitable word 

 for so beautiful a phosphorescence — 

 shining brightly in luminous characters. 

 The first letter, <)>, shines with an orange 

 light ; it is a sulphide of calcium pre- 

 jjared from oyster-shells. The other 

 letters, shining with a blue light, are 

 sulphide of calcium prepared from pre- 

 cipitated carbonate of lime. Once the 

 phosphorescence is excited the letters 

 shine for several hours. I will put the 

 diagram at the back, and we shall see 

 how it lasts during the remainder of 

 the lecture. This substance, then, is 

 phosphorescent to light, but it is also 

 much more strongly phosphorescent to 

 the molecular discharge in a good 

 vacuum, as you will see when 1 pass 

 the discharge throngh this tube (Fig. 8). The white plate 

 {a, i) in the centre of the tube is a sheet of mica painted 

 over with the luminous sulphide of which the letter (p was 



* A short-hand report of a lecture delivered at the Royal Institution on 

 Friday. Apr.! 4. 1879. By William Crookes, F.R.S. Contributed by the 

 author. Continued from p. 131. 



FlC. 8. 



composed in the diagram you have just seen. On connecting 

 the poles with the coil, the mica screen glows with a strong 

 yellowish green light, bright enough to illuminate all the appa- 

 ratus near it. But there is another phenomenon to which I now 

 desire to draw attention : on the luminous screen is a kind of 

 distorted star-shaped figure. A little in front of the negative 

 pole I have fixed a star (c) cut out in aluminium, and it is the 

 image of this star which you see on the screen. It is evident 

 that the rays coming from the negative pole project an image of 

 anything that happens to be in front of it. The 

 discharge, therefore, must come from the pole in 

 straight lines, and does not merely permeate all 

 parts of the tube and fill it with light as it would 

 were -the exhaustion less good. Where there is 

 nothing in the way the rays strike the screen and 

 produce phosphorescence, and where there is an 

 obstacle they are obstructed by it, and a shadow is 

 thrown on the screen. I shall have more to say 

 about this shadow presently ; I merely now wish 

 to establish the fact that these rays driven from the 

 negative pole produce a shadow. 



I must draw your attention to an important ex- 

 periment connected with these molecular rays, but 

 unfortunately it is a very delicate one, and very 

 difficult to show to many at once ; but I hope, if 

 you know beforehand what to look for, you will all be 

 able to see what I w ish to show. In this pear-shaped 

 bulb (Fig. 9A) the negative pole {a) is at the pointed end. In the 

 middle is a cross (d) cut out of sheet aluminium, so that the rays 

 from the negative pole projected along the tube will be partly 

 intercepted by the aluminium cross, and will project an image of 

 it on the hemispherical end of the tube which is phosphorescent. 

 I think you will all now see the shadow of the cross on the end 

 of the bulb (c, d), and notice that the cross is black on a luminous 



Fig. 9 A. 



ground. Now, the rays from the negative pole have been pass- 

 ing by the side of the aluminium cross to produce the shadow; 

 they have been hammering and bombarding the glass till it is 

 appreciably warm, and at the same time they have been pro- 

 ducing another effect on that glass — they have deadened its sensi- 

 bility. The glass has got tired, if I may use the expression, by 

 thejenforced phosphorescence. Some change has been produced 



Fig. 9 B. 



by this bombardment which will prevent the glass from respond- 

 ing easily to additional excitement ; but the part that the shadow 

 has fallen on is not tired— it has not been phosphorescing at all, 

 and is perfectly fresh ; therefore if I throw this star down— I 

 can easily do so by giving the apparatus a slight jerk, for it 

 has been most ingeniously constructed with a hinge by Mr. 

 Gimingham— and so allow the rays from the negative pole to fall 



