47 
on the Production of Light from Bodies. 
the fire, should continue to shine from its own light, with un- 
diminished lustre, as long as the fire is kept up ; for it has been 
shewn, that if a phosphorescent body remain long upon the 
heater, of any temperature between 400° of Fahrenheit and a 
red heat, its light diminishes more and more, till at last it is 
scarcely perceptible ; and then an increase of heat is necessary 
to render it more luminous. 
APPENDIX. 
After a considerable part of the above paper was printed, I repeated the experi- 
ment with boiling oil, related in p. 35, with every possible precaution. I poured 
the powder into the flask through a funnel which reached to the bottom, so that none 
of it might be lodged on the sides ; for, not having attended to this circumstance before, 
I was apprehensive that the experiment was delusive, and that the agitation of the hot 
oil might have washed down some fresh particles which had not been before heated; — 
then slowly pouring in a little oil, I boiled it for a few minutes, and removed it into 
a dark place. When the powder had become but faintly luminous, upon agitation, 
the experiment succeeded exactly. I then boiled and agitated the oil for six or seven 
times successively, with the same result, except that the light of the powder grew 
something fainter each time. If any one has the curiosity to repeat this singular 
experiment, he may attend to the following directions. — Upon each removal from the 
fire, stop the neck of the flask with a cork, having a small hole pierced through it—* 
wrap the neck round with tow — agitate the liquor by quickly raising and lowering the 
flask. 
The sparks which leave the surfaces of hard bodies during attrition, may be exactly 
imitated by burning a bit of dusty paper; — the particles of dust are carried by the cur- 
rent of air through the blaze, and are made red hot in their passage. 
