VOL. LXXXIl.] PHILOSOPHICAL TRANSACTIONS. 133 



quantity of light is produced by hard, uncoloured, transparent, and semi-trans- 

 parent 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 transparent bodies ; faint red, or flame- 

 coloured, from white semi-transparent bodies ; deeper red from more opaque 

 and coloured bodies, and the deepest red from opaque 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. Opaque red feldspat gives a 

 deep red light by attrition ; exposed to a strong heat in tlie furnace, it becomes 

 white, and somewhat transparent, and when cool, gives out, on attrition, as 

 white a light as quartz; clear, blackish flint, made opaque by heat, gives a 

 redder light than before ; deep-coloured glass gives out a red defined light without 

 any flash, while 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 

 bodkin ; but the diamond is phosphorescent in so many ways, and is so curious 

 and singular a body, both in properties and constitution, that it can scarcely be 

 expected to exhibit the same a[)pearances as the common class of earthy bodies. 



Alum, indurated by having been kept long in a state of fusion, and being 

 then much harder than loaf sugar or borax, both which are luminous from mo- 

 derate attrition, gives no light, though rubbed with much violence. If two 

 pieces of glass or quartz be strongly rubbed against each other, and then applied 

 to the fine down of a feather, the down is not sensibly affected ; if the same 

 glass be rubbed on woollen cloth, and placed near the feather, the down is imme- 

 diately attracted. Rock crystal, quartz, feldspat, white unglazed earthen-ware, 

 Derbyshire black marble, and probably all phosphorescent bodies, insoluble in 

 water, give out their light on rubbing them under water, as copiously as in air. 

 Hard white sugar, from the outside of the loaf, gives out its light when rubbed 

 in oil. Bodies seem equally luminous in atmospheric, pure, fixed, and inflam- 

 mable air. 



All hard earthy bodies emit a peculiar smell on attrition. The most remark- 

 able for this property are chert, quartz, feldspat, biscuit earthen- ware, and rock 

 crystal : this smell does not differ much in kind, though it does considerably in 

 intensity. Many of the softer bodies yield the same smell, but in a less degree, 

 and probably none are entirely without it. It appears to be strongest where the 

 friction is greatest: it has no dependence on the light produced by attrition, as it 



