270 SCIENCE IN SHORT CHAPTERS. 



Cheapside and Piccadilly, and the excitement caused by 

 the brilliancy of the new gas lamps ; but now we are dissat- 

 isfied with these, and demand electric lights for common 

 thoroughfares, or some extravagant combination of con- 

 centric or multiplex gas-jets to rival it. 



The latest novelty is a device to render darkness visible 

 by capturing the sunbeams during the day, holding them 

 as prisoners until after sunset, and then setting them free 

 in the night. The principle is not a new discovery ; the 

 novelty lies in the application and some improvements of 

 detail. In the "Boy's Own Book," or "Endless Amuse- 

 ment," of thirty or forty years ago, are descriptions of 

 "Canton's phosphorus," or "solar phosphori," and recipes 

 for making them. Burnt oyster-shells or oyster-shells 

 burnt with sulphur, was one of these. 



Various other methods of effecting combination between 

 lime or baryta with sulphur are described in old books, the 

 result being the formation of more or less of what modern 

 chemists call calcium sulphide and barium sulphide (or 

 otherwise sulphide of calcium or .sulphide of barium). 

 These compounds, when exposed to the sun, are mysteri- 

 ously acted upon by the solar rays, and put into such a 

 condition that their atoms or molecules, or whatever else 

 constitutes their substance, are set in motion in that sort 

 of motion which communicates to the surrounding medium 

 the Avavy tremor which agitates our optic nerve and pro- 

 duces the sensation of light. 



Until lately, this property has served no other purpose 

 than puzzling philosophers, and amusing that class of boys 

 who burn their fingers, spoil their clothes, and make holes 

 in their mothers' table-covers, with sulphuric acid, nitric 

 acid, and other noxious chemicals. The first idea of turn- 

 ing it to practical account was that of making a sort of 

 enamel of one or the other of these sulphides, and using it 

 as a coating for clock-faces. A surface thus coated and 

 exposed to the light during the day becomes faintly lumi- 

 nous at night. 



Anybody desirous of seeing the sort of light which it 

 emits, may do so very easily by purchasing an unwashed 

 smelt from the fishmonger, and allowing it to dry with its 



