Phosphorescence and Colour by Electricity. 275 



This muriatic acid solution deposited after some time small 

 fragile crystals of fluoride of calcium ; these, when collected 

 and dried, lost their form ; when heated, they slightly decre- 

 pitated, and were phosphorescent. 



There are certain classes of bodies which exhibit decided 

 differences in this relation to light. All the calcareous mine- 

 rals, as the carbonates of lime and the fluor spars, may be ren- 

 dered phosphorescent, while none of the specimens of quartz, 

 siliceous, and aluminous minerals resorted to, either possessed 

 naturally, or would receive phosphorescence. 



I ought to mention, that in several instances I have observed 

 a slight return of phosphorescence by time after it has disap- 

 peared. One example was in a crystal of fluor spar which had 

 been calcined entire : after it had been deposited in darkness 

 for some months, it was found to have regained feeble phospho- 

 rescence ; and others, which gave no signs of light when heated 

 after the calcination, yet appeared luminous when heated after 

 a long seclusion from light. Other substances, besides these, 

 might be adduced, whose feeble but constant phosphorescence 

 cannot be the result of accidental circumstances alone. Com- 

 mon scollop-shells seem to inherit a remarkable phosphoric 

 structure, as also the substance of calcined oyster-shells and 

 cuttle-fish bones, especially when exposed to light for a short 

 time; instances have occurred, when, after a strong calcination 

 -of these substances, they appeared visible, although they were 

 heated many times and kept in darkness. These degrees of 

 luminosity, although not likely to be confounded with the pre- 

 vious experiments, are yet pointed out, because they were 

 guarded against in the following experiments. After all, the 

 effects of temperature may be far more influential than can 

 be traced at present, being perhaps as capable of disposing 

 structure, as requisite in the ultimate development of the phe- 

 nomena. 



From these assigned reasons I conclude, that phosphores- 

 cence is dependent upon and modified by the structure and 

 mechanical conditions of the substances under investigation. 



The beautiful results thus produced by electricity naturally 

 led me to vary its mode of application ; and as, in the expe- 

 riments described, the electric discharge had been passed 

 directly over the substances, I now inclosed them in glass 



