ON GASEOUS EXPLOSIONS. 



211 



The transparency of flames is well illustrated by some experiments 

 which Professor Callendar has been making, and which he showed to 

 the Committee. The radiation from a Meker burner (which gives a 

 ' solid ' flame without inner cone) was measured by means of a Fery 

 pyrometer, the reading of which gives a measure of the radiation trans- 

 mitted through a small cone intersecting the flame and having its vertex 

 at this point of observation (see fig. 1). Callendar proposes to give the 

 name ' intrinsic radiance ' to the radiation of a flame measured in this 

 way, divided by the solid angle of the cone. When a second similar 

 flame was placed behind the first in the line of sight, it was found that 

 the reading recorded by the pyrometer was considerably increased, but 

 not doubled; the first flame appeared to be partly, but not completely, 



P = 



Fig. 2. 



Fig. 3. 



transparent to the radiation emitted by the second. A third flame 

 placed behind the first two contributed a further but smaller addition 

 to the radiation, and as the number of flames in the row was increased 

 the radiation received from each fell off according to an exponential 

 law. The total radiation from the whole row (which is that recorded 

 on the pyrometer) tends to a finite limit as the number of flames is 

 increased. The radiation from a depth of 12 cm. is about half, and 

 that from a depth of 100 cm. is within one-half per cent, of that 

 emitted by an infinitely great depth. 



The general result of Callendar 's experiments is to show that flames 

 of a diameter of three centimetres or less burning at atmospheric pres- 

 sure emit radiation approximately in proportion to the volume. If the 

 diameter be increased beyond that figure the radiation will also increase, 



