140 Rainbow Phosphorescence. 



seen a fragment of green fiuor spar and two white diamonds 

 emit light for one hour after insolation." 



In preparing these phosphori, it is noticed that the pecu- 

 liarity of their luminous action depends on the primitive 

 condition of the sulphates employed. " Thus the natural 

 crystallized sulphate of baryta affords the orange yellow 

 Bologna phosphorus; the natural sulphate of strontium from 

 Sicily in rod-shaped (bacillary) crystals, yields a bluish green 

 phosphorus, and if by the action of carbon different sulphates- 

 are reduced to the condition of sulphides, their luminous action 

 will vary.'" 



In preparing a phosphorescent sulphide with lime, or car- 

 bonate of lime, it is most convenient to add 85 parts of sulphur 

 to 100 of the lime, or 48 to 100 of its carbonate. The 

 materials are intimately mixed and placed in an earthen 

 crucible in a charcoal furnace. M. Becquerel says it is neces- 

 sary to pay attention to the temperature as well as to its 

 duration. " Operating with fibrous arragonite, and heating 

 the crucible to 500° (0.) for a time sufficient to allow the 

 reaction between the lime and the sulphur to take place, and the 

 excess of the latter to be. eliminated, a feebly-luminous mass 

 affording a bluish tint is obtained. If this mass is raised to a 

 temperature of 800° or 900° (C.) and kept for five-and-twenty 

 or thirty minutes at a point not exceeding the fusion of gold 

 or silver, it yields a brilliant green light. The chemical com- 

 position is the same in both cases ; but it is remarkable that 

 if the process is conducted with carbonate of lime instead of 

 with lime, the refrangibility of the light emitted does not vary 

 with the temperature/'' 



Too high, or too prolonged a temperature destroys the 

 phosphorescence, and charcoal furnaces answer better than 

 coke. 



Among the lime preparations, those made with pure Iceland 

 spar give, after insolation, an orange yellow light, calc spar 

 affords a less vivid tint, Carara marble a very weak yellow 

 light, oyster-shells yellow, chalk a scarcely visible yellow. 

 Arragonite of Vertaison in bacillary crystals, a green of medium 

 intensity, fibrous arragonite a dominant violet tint, with some 

 parts green, and lime obtained from fibrous arragonite a very 

 vivid green. 



If nitric acid is employed to dissolve the lime of these 

 minerals, and it is then precipitated by carbonate of ammonia, 

 the tints of the phosphorescence will vary according to the 

 sources of the lime. 



Phosphori composed of strontium sulphides usually require 

 less heat in their preparation than the lime scries, and an 

 excess of heat destroys their luminosity. The barium phos- 



