184: PHOSPHORESCENCE. 
in straight lines; their motion is more probably 
circular. Indeed, my ingenious friend M. Porro 
has endeavoured to show the great resemblance 
which seems to exist between these molecular 
movements and those of celestial bodies; and it 
has been supposed by some philosophers that the 
molecules of matter are as distant from each other, 
in proportion to their size, as the planets them- 
selves. 
But, in the present state of knowledge, all these 
considerations are premature. 
We must not confound the luminosity produced 
in substances that are heated to a certain degree 
which is supposed to be identical, or nearly so, 
for all, with that pecuhar phosphoric radiation 
which a great number of substances emit at 
different degrees of heat, and some, at the ordi- 
nary temperature of summer. As early as 1776, 
Dr. Fordyce observed, that heated bodies began 
to be luminous in the dark at from 600° to 700° 
of Fahrenheit’s thermometer (see his interest- 
ing paper in the ‘ Philosophical Transactions’ for 
LAG): 
I say that we must not confound these two 
manifestations of hght, though they are both 
molecular vibrations of the body submitted to 
experiment; yet they differ: for mstance, fluor- 
spar heated gently over a fire becomes phospho- 
rescent; heated to a still higher degree, the 
