1884.] NATURAL SCIENCES OF PHILADELPHIA, 11 



now described, the phosphorescence of which is nearly as strong 

 as that of fluor spar. 



Phosphorescence is developed when the rock is either struck, 

 scratched or heated. Upon using- metal, glass or any other hard 

 substance to strike or to scratch it, a deep red light is emitted, 

 which continues sometimes for several seconds after the blow. 

 Rubbing with other fragments or grinding in a mortar developed 

 a white light. The most remarkable phosphorescence is devel- 

 oped by heating a fragment of the limestone in a glass tube over 

 a flame. It then glows with a deep red light which lasts for a 

 minute or more after withdrawing the flame. The color of the 

 light emitted resembles that of a red-hot body. Several seconds 

 before dying out, the light becomes white or bluish white. Upon 

 cooling and subsequent heating, phosphorescence is again devel- 

 oped in the same fragment, but much more feebly and for a 

 shorter period, and afcer two or three such heatings, its phos- 

 phorescence is destroyed. 



Experiments made by the speaker upon the temperature at 

 which " Hell-fire rock " became phosphorescent, showed that 

 phosphorescence occurred at a temperature somewhat under 

 500° F. Small fragments phosphoresced much more quickly 

 than large ones. The lurid red light produced by a blow from a 

 hammer varied in duration of AMsibility according to the strength 

 of the blow. The phosphorescence produced by a slight touch 

 lasted only half a second, while a sharp blow produced a light 

 which remained more than twent}^ seconds after the blow was 

 given. Doubtless, a blow with a miner's pick upon the rock 

 would cause still longer phosphorescence. 



It was found that the phosphorescence developed by heating 

 occurred nearly contemporaneously with the decrepitation of the 

 calcite, and this fact may be of value in theoretical considerations. 



A search through the collection of the Academy for limestones 

 having similar properties resulted in finding a limestone from 

 Kaghberry, India, which glowed with a strong yellow phosphor- 

 escent light when heated. No phosphorescence was produced by 

 friction alone, as in the case of the Utah limestone. It was of 

 great interest to find that this Indian limestone, and this one 

 alone of all in the collection, had the precise external characters 

 of that from Utah. It had the same crystalline structure and 

 state of aggregation, crumbling readily in the fingers, and resem- 

 bling a sandstone. It was labeled " Phosphorescent Sandstone,'' 

 although containing no siliceous sand. 



This similarity of external characters between the two phos- 

 phorescent limestones is certainly more than a coincidence. It 

 confirms Becquerel's view that phosphorescence depends upon 

 physical rather than chemical conditions. He has shown that 

 when Ai'agonite is calcined, fused with sulphur and then heated, 

 it phosphoresces with a green light; whereas calcite, similarly 



