KXI'LOSIONS OF COAL-GAS AND All:. 



nearly double the first length, was in effect got by polishing a circular patch of silver 

 of about (! in. diameter opposite the bolometer on the other end cover, and so 

 reflecting the 30 cm. tack upon itself. Were the silver perfectly reflecting a virtual 

 length of 60 cm. would have been obtained by this means, but having regard to its 

 imperfect reflecting power the 30 cm. was increased by about 97 per cent, or 98 per 

 cent.,* this bring! th<^ equivalent length down to 59 cm. When the walls of the 

 vessel were reflecting, the 30 cm. length of gas was obtained by blackening a circular 

 patch of about 6 in. diameter on the end cover opposite the bolometer. This made it 

 impossible for radiation from any point outside the cone of gas to reach the bolometer. 

 When this black patch was rubbed off and the silver polished, the 30 cm. was virtually 

 increased to 59 cm. as explained above. In this case, however, it was possible for 

 radiation reflected from the silvered walls surrounding the bolometer to reach the 

 bolometer after again being reflected from the opposite end cover. In order to 

 prevent this a ring of black paint was placed on the walls round the bolometer. 



In all the following experiments the bolometer was in position A on the end cover. 

 It was placed at a distance of 12 '5 cm. behind the fluorite, so that the solid angle 

 subtended was 0'062. 



Records of the pressure of the gas and rise of temperature of the bolometer during 

 explosion and subsequent cooling of mixtures of coal-gas and air were taken in the 

 usual way. From these records curves of mean gas temperature and of radiation 

 emitted by the cone of gas were obtained with times after ignition as abscissae. In 

 what follows these radiation curves have been differentiated (with respect to time), 

 and the rates at which the cone of gas emits radiation plotted against the mean 

 absolute temperature of the gas. 



Fig. 14 gives the results of these experiments for 15-per-cent. mixtures of coal-gas 

 and air with the fluorite window, an allowance of 5 per cent, having been made for 

 the absorption of the fluorite and 5 per cent, for reflection from the blackened surface 

 of the bolometer. The results have been divided by 0'062 so as to give the emission 

 from unit solid angle. Prof. CALLENDAU calls this the " intrinsic radiance." 



Curve A shows the intrinsic radiance from 59 cm. of the hot gaseous mixture 

 from the maximum temperature reached in the explosion down to 1300 C. 

 (;iks.), when the walls of the vessel were reflecting. 



Curve B. 30 cm., walls reflecting. 



Curve C. 59 cm., walls black. 



Curve D. 30 cm., walls black. t 



Curve S. 15 cm., walls black. 



* Following the figures given by HAGEN and RUBENS for the reflecting power of silver for radiation of 

 wave-length between 2/* and 5/x ('Z. fur Instr. Kunde,' 22 (1902), p. 52). 



t The intrinsic radiance from 15 cm. reflected back upon itself, by means of a small polished silvered 

 (il.ite placed opposite the l>olometer, was also measured ; the results were precisely the same as those 

 shown in Curve D. When this plate was painted black the record giving Curve S was taken. 



3 2 



