[ 34- 2 ] 
caufe it to part with, it has never after been found, if 
kept in darknefs, to give any more light by that 
degree of heat. But if it be expofed again to the 
common light of the day, the experiments may be 
repeated with the fame fuccefs as before. This has 
frequently been done, with fome dry phofphorus in 
glafs balls which have been hermetically fealed about 
four years, without the leafl injury to the phofphorus y 
as it appears to be as good now as it was at firft. 
Experiment VII. 
Let one end of a bar of iron of about an inch fquare, 
or a poker, be made red hot, and laid horizontally in 
a darkened room, till by cooling it ceafes to fhine, 
or is but barely vilible. Then bring a little dry 
phofphorus, which has been expofed to light in a 
glafs ball hermetically fealed, as near the hot iron as 
pofiible, by holding the ball in contadt with it j and 
the phofphorus, though invifible before, will in a few 
feconds begin to (bine ; and will difcharge its light 
fo very fall as to be entirely exhaufled of it in lefs 
than a minute ; and then will fhine no more by the 
fame treatment, till after it has been expofed to light 
again. By this heat, light received from a candle, or 
even from, the Moon, may be feen feveral days after. 
And phofphorus that will afford no more light by the 
heat of boiling water, will fhine again by the heat of 
the iron. By this heat alfo, phofphorus which had 
been kept in darknefs more than fix months, was 
found to give a confiderable degree of light. 
It was the opinion of the great Sir Ifaac Newton, 
that the rays of light are very fmall bodies emitted 
from 
