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 peculiar 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 

 1776). 



I say that we must not confound these two 

 manifestations of light, though they are both 

 molecular vibrations of the body submitted to 

 experiment ; yet they differ : for instance, fluor- 

 spar heated gently over a fire becomes phospho- 

 rescent; heated to a still higher degree, the 



