1855] WRITINGS OP JOSEPH HENRY. 357 



pieces of fire-brick mingled with the coal than from the com- 

 bustible itself. The effect however would probably have 

 been greater with bituminous coal. The arrangement for 

 experimenting with coal in a fire-place was very imperfect, 

 and I had recourse to the heat produced by the flame of 

 a spirit lamp, and also of a jet of hydrogen. A flame of 

 this kind was placed before the thermo-pile at such a dis- 

 tance that the needle of the galvanometer stood at 15° ; the 

 end of the platinum wire coiled into a spiral form was then 

 introduced into the flame, and an instant increase of the 

 radiation of heat was observed, the galvanometer advancing 

 to 27°. 



It has long been well known that the introduction of a 

 platinum wire into a pale flame of this character greatly in- 

 creases the radiation of light, and from this experiment it is 

 evident that the radiation of heat is increased in a like 

 degree. After this a number of different substances were 

 employed, such as glass, carbonate of lime, sulphate of lime, 

 stone coal, fire clay, &c. The greatest effect appeared to be 

 produced with pieces of carbonate of lime. The exact order 

 however could not be determined without procuring a series 

 of balls of the same diameter of these different substances. 

 The most striking effect was produced at the very top of the 

 flame, placing the platinum wire in the heated though almost 

 non-luminous air, immediately above the highest point of 

 combustion. 



We cannot suppose in these experiments that the absolute 

 amount of heat produced by the combustion of a given quan- 

 tity of fuel is increased. The most probable conjecture is 

 that the heat of combination is converted into radiant heat, 

 and that the flame itself is cooled in proportion as the radia- 

 tion is increased. In order to bring this idea to the test of 

 experiment, a slip of mica, one-fifth of an inch in breadth, 

 was introduced vertically into the lower part of the flame, 

 while the platinum wire occupied the space just above the 

 top. The slip of mica was placed with its flat side vertically 

 so as not to affect by its radiation the heat of the wire. With 

 this arrangement the radiation of heat from the platinum 



