Rainbow Phosphorescence. 141 



phori on the contrary are prepared with greater and more 

 sustained heat. Chlorides of calcium, and strontium tend to 

 give blue and violet tints, those of barium yield green tints, 

 while carbonates obtained from nitrates and acetates of baryta 

 afford yellow orange phosphori, and analogous combinations of 

 calcium and strontium give very luminous green ones. 



A luminous orange phosphorus from barium is made by 

 intimately mixing powdered crystalline sulphates with 12 to 

 15 per cent, of lamp black, moistened with a little alcohol. 

 When the mass is dry it is calcined in a crucible for 45 to 60 

 minutes, at a temperature not exceeding cherry red, or the 

 melting point of silver. The resulting mass is powdered and 

 calcined a second time. 



We have mentioned that, as a rule, these phosphori give 

 the same tints whatever may be the colour of the light they 

 are exposed to in order to excite them, but M. Becquerel cites 

 three exceptional cases. 



1. Sulphide of barium obtained by reducing the sulphate 

 with lamp black, gives an orange yellow phosphorescence when 

 illuminated by the action of the rays in the spectrum situated 

 towards the end and beyond the violet (from lines H to P), 

 while the effect of the rays from the blue to the violet (F to H) 

 is to induce a redder phosphorescence. 



2. The sulphide of calcium obtained from oyster- shells, 

 which gives a red light when excited by rays from the blue 

 (F) to the ultra-violet as far as 0, has a green tint imparted 

 to it when excited by the rays beyond and P, which are non- 

 luminous to human eyes. 



3. A phosphorus obtained by the action of sulphide of 

 potassium on oyster- shells, is excited to an indigo-violet lumi- 

 nosity by exposure to rays of that tint, while rays beyond the 

 violet excite it to emit a blue colour. 



It is interesting to observe from M. BecquerePs ex- 

 planations and from a beautifully-coloured plate attached to his 

 work, that while the most luminous parts of the spectrum, the 

 yellow, actually exert a destructive effect on the light of these 

 phosphori, they are all capable of excitation by non-luminous 

 rays beyond its violet extremity. 



We are afraid that experimenters will only succeed in 

 making the more easy of their phosphorescent compounds, 

 unless they possess a good deal of patience and considerable 

 knowledge of chemical manipulation. When well prepared 

 the varieties of colour are very distinct, and the luminous 

 effects brilliant and pleasing. They not only afford an agree- 

 able recreation, but they suggest curious speculations on the 

 molecular condition of the several compounds. Light appears 

 to excite a peculiar vibration of their particles without affecting 



