XXX. Of the Light produced by Inflammation . 
By George Fordyce, M. D. F. R. S. 
R * 9 ’ \X 7 HEN a body is heated to a certain de- 
* ▼ gree, it becomes luminous, and is faid to 
be ignited. One of the means of producing heat is in- 
flammation; and this, as is well known, is fufficient for 
ignition. But, I apprehend, that, befides the light pro- 
duced by ignition, there is alfo light produced by the 
inflammation itfelf. For the inveftigation of this prin- 
ciple, it will be neceflary to conlider ignition in the firft 
place. 
Subftances, heated to between 6 and 700° of Fahren- 
heit’s thermometer, begin to be luminous in the dark. 
If they be colourlefs, the light which is firft obferved is 
red ; as the heat is increafed, there is a mixture of yellow 
rays; and, laftly, a due proportion of all the coloured 
rays to form a pure white, which has been commonly 
called, by chemilfs, a melting heat* The intenfenefs of 
this light depends much upon the denfity of the heated 
body ; for, while metals, heated to this degree, throw out 
a ftrong light, the vapour at the end of the flame of a 
blow-pipe, properly applied to a lamp, is not vifibly lu- 
minous, though the heat be fo great as immediately to 
give a white heat to glafs. The colour of this light is 
affected 
