T CLEAVAGE, ETC. 27 



When chloride of calcium that has been melted 

 in a crucible, is rubbed upon the sleeve in a dark 

 room, it glows with a greenish light. This was 

 first observed by Homberg, hence the name of 

 Homberg 1 8 phosphorus, by which this substance 

 was formerly known. It is very phosphorescent 

 by percussion. 



Certain varieties of blend (sulphuret of zinc) 

 become phosphorescent by percussion and some- 

 times after very slight friction. Speaking of blend, 

 Dana says : " Merely the rapid motion of a fea- 

 ther across some specimens of sulphuret of zinc, 

 will often elicit light more or less intense from 

 this mineral."* Other substances require a 

 stronger rubbing, for instance, quartz, flint, etc. 

 In the case of quartz, an odour of ozone is per- 

 ceived, a fact to which I called attention in the 

 ( Comptes Rendus' of the Paris Academy, in 1860. f 

 Borax and sugar become luminous also in the 

 dark, when rubbed. Otto de Guericke observed 

 that the globe of sulphur with which he con- 

 structed the first electric machine, became lumi- 

 nous when he rubbed it in the dark. 



Hawksbee and Picard both discovered that the 



* Dana's c Mineralogy.' 



f I find that it requires upwards of two hundred flashes to 

 produce a quantity of ozone equivalent in its effects to one drop 

 of nitric acid. Also, that with white quartz the light is white, 

 but with red quartz or calcined yellow quartz, it acquires a 

 crimson tint, owing to the oxide of iron in the stones. 



