1879.] on Molecular Phi/slcs in Higli Vacua. 147 



follow altogether the negative polo. If the negative pole points in 

 the direction of the positive all very well, but if the negative pole is 

 entirely in the opposite direction it does not matter : the line of rays 

 is projected all the same in a straight line from the negative. 



I have hitherto spoken of and illustrated these phenomena in 

 connection with green phosjihorescence. It does not follow, however, 

 that the phosphorescence is always of that colour. This colouration 

 is a property of the particular kind of glass in use in my laboratory. 

 I have here (Fig. 7) three bulbs composed of dilfereut glass ; one is 

 uranium glass (a), which phosphoresces of a dark green colour ; 

 another is English glass {h), which phosphoresces of a blue colour ; 

 and the third (c) is soft German glass — of which most of the ajipa- 

 ratus before you is made — which phosphoresces of a bright apple-green 



Fig. 7. 



colour. It is therefore plain that this particular green phospho- 

 rescence is solely due to the glass which I am using. Were I to use 

 English 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 phosphorescence which glass takes uji when it is under the 

 influence of the electric discharge in vacuo; but many other sub- 

 stances possess this phosphorescent 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 

 description. When it is exposed to light — even candlelight — it 

 phosphoresces for hours with a rich blue colour. I have prepared a 

 diagram with large letters written in this luminous sulphide ; before 

 it is exposed to the light the letters are invisible, but Mr. Gimingham 

 has just exposed it in another room to burning magnesium, and now 

 it is brouglit into the darkened theatre you will see the word " <^o)9," — 

 liglit, 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 prepared from oyster-shells. 

 The other letters, shining with a blue light, are sulphide of calcium 

 prepared from precipitated carbonate of lime. Once the phospho- 



