8G 
residue which was very hitter to the taste. When treated 
with cold nitric acid this residue assumed a dark blue colour 
like indigo. Upon the application of heat the colour dis- 
appeared. 
“ On the Luminous Sulphides of M. Ed. Becquerel,” by 
William Thomson, F.RS.E. 
My object in bringing this communication before the 
society is to shew what I consider to he some most 
interesting substances, viz., the Sulphides, principally of 
the Alkaline Earths, the luminous properties of which were 
studied many years ago by M. Edmond Becquerel and 
others. These samples which I have I received some time 
ago when in Paris through my friend M. Auguste Guerout, 
Preparateur au Museum d’Histoii'e Naturelle. They were 
prepared by M. Andre, the laboratory assistant of M. Ed. 
Becquerel, who has devoted much attention to the pecu- 
liarities of manipulation required to produce the greatest 
degree of luminosity in the sulphides after holding them 
for a few seconds before the sun’s light or a piece of burn- 
ing magnesium wire or other source of light and then 
placing them in the dark ; doubtless many present will be 
familiar with the results arrived at by Becquerel and others, 
but a short resume of some of them may not be out of 
place here. 
When these sulphides, which must be kept in hermetically 
sealed tubes to prevent oxidation, are exposed to the rays 
at different parts of the spectrum, these rays have very 
different actions in rendering the sulphides luminous, and 
also in some cases of producing slightly different shades of 
colour. The visible part of the spectrum has little or no 
power to render these bodies phosphorescent, the violet 
part, however, has greatest action, and the ultra-violet rays 
produce the maximum effect. M. Becquerel found then, in 
these sulphides, an interesting method of examining the 
