1 879.] Molecular Physics in High Vacua. 423 
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 
Fig. 8. 
brought into the darkened theatre you will see the word 
“ — light, a very suitable word for so beautiful a 
phosphorescence — shining brightly in luminous characters. 
The first letter, <p , 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 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 
