BY 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’s 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 shght friction. Speaking of blend, 
Dana says :— Merely the rapid motion of a fea- 
ther across some specimens of sulphuret of zinc, 
will often elicit hght 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.+ 
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 ‘ Mineralogy.’ 
+ 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. 
