40 Mr. T. Wedgwood's Experiments and Observations 
defined speck of red light, and are not luminous below the part 
struck. The greatest apparent quantity of light is produced 
by hard, uncoloured, transparent, and semitransparent bodies, 
whose surfaces soon acquire an asperity by rubbing together, 
as quartz, agate, &c. From an examination of the table, it 
appears that white lights are emitted from colourless transpa- 
rent bodies ; faint red, or flame-coloured, from white semi- 
transparent bodies ; deeper red from more opake and coloured 
bodies, and the deepest red from opake and from deep-coloured 
bodies. Extremely faint lights, such as those given by fluor, 
marble, &c. are of a bluish white ; quartz, very lightly rubbed, 
gives a very faint light of a bluish hue ; when rubbed a little 
harder, it emits a flame-coloured light ; when rubbed with 
violence, its light approaches to whiteness. Opake red feld- 
spat gives a deep red light by attrition ; exposed to a strong 
heat in the furnace, it becomes white, and somewhat transpa- 
rent, and when cool, gives out, on attrition, as white a light as 
quartz ; clear, blackish flint, made opake by heat, gives a 
redder light than before ; deep-coloured glass gives out a red 
defined light without any flash, whilst clear uncoloured glasses 
emit a white flashing light of some brightness. 
Bodies are not luminous by simple pressure ; but when they 
are at all broken by the pressure, the fragments rubbing on 
each other produce some light. Mr. Boyle, indeed, found a 
particular diamond to emit light when pressed by a steel bod- 
kin ; but the diamond is phosphorescent in so many ways, and is 
so curious and singular a body, both in properties and consti- 
tution, that it can scarcely be expected to exhibit the same 
appearances as the common class of earthy bodies. 
Alum, indurated by having been kept long in a state of 
