37 
on the Production of Light from Bodies. 
agitated in the dark, illuminates the whole of the flask. The 
light of boiling oils proceeds, probably, from some kind of in- 
flammation, as it is scarcely discernible unless the vessel be 
agitated ; and, if a little oil be thinly spread on the heater, a 
subtle lambent flame, of a bluish hue, instantly arises. The 
same thing takes place if horn, bone, hair, saliva, or any animal 
matter be laid upon the heater. 
II. 
The experiments on the light produced from different bodies 
by attrition, were chiefly made by rubbing in the dark two 
pieces of the same kind against each other : all that I tried, 
with a very few exceptions, were luminous by this treatment. 
The following is a list of them, arranged in the order of the 
apparent intensity of their light, and as the lights are either 
white, or some shade of red, I have affixed figures to denote 
these differences; (o) denoting a pure w'hite light ; (1), the 
faintest tinge of red, or flame colour ; (2), a deeper shade of 
red; (3) and (4), still deeper shades. 
1. Colourless, transparent, oriental rock crystal ; and siliceous 
crystals (o). 
2. Diamond (o). 
3. White quartz ; white semitransparent agate (1). 
4. White agate, more opake (2). 
Semitransparent feldspat, from Scotland (2). 
Brown opake feldspat, from Saxony (4). 
Chert of a dusky white, from North Wales (3). 
5. Oriental ruby (4). 
6. Topaz ; oriental sapphire (o). 
7. Agate, deep coloured, brown and opake (4). 
