392 



MR. W. T. DAVID ON THE RADIATION IN 



The total amount of radiation emitted by the hydrogen mixture amounts to about 

 16 per cent, of its heat of combustion. A 15-per-cent. mixture of coal-gas and air 

 having the same maximum temperature emitted 26 per cent, of its heat of com- 

 bustion ; in the latter case, however, the rate of cooling of the gaseous mixture after 

 explosion is much slower than that of the hydrogen mixture. 



The Curve H in fig. 9 is the differential of the radiation Curve H in fig. 8. It 

 gives the average rate at which the blackened walls of the explosion vessel receives 

 radiation in calories per square centimetre of surface per second from the hydrogen-air 

 mixture during cooling. 



Table IX. compares the emission of the 25'4-per-cent. hydrogen mixture and that 

 of a 15-per-cent. mixtiire of coal-gas and air in the same vessel and at the same mean 

 gas temperatures. The hydrogen mixture after explosion contains 30 per cent, of 

 water vapour and the coal-gas mixture contains 8 '5 per cent, of CO 2 and 20 per cent, 

 of water vapour. The densities of the two mixtures are very nearly the same. 



TABLE IX. 



The hydrogen mixture emits just as strongly at high temperatures as the coal-gas 

 mixture does ; at lower temperatures the hydrogen mixture emits rather more 

 powerfully. This, at first sight, seems rather extraordinary, in view of the results 

 given on p. 390, for in the hydrogen mixture there is no C0 2 and the quantity of 

 water vapour is only 50 per cent, greater than that in the coal-gas mixture. We 

 shall see presently (p. 397) that the water vapour is more transparent to the radiation 

 which it emits than is the mixture CO 2 . H 2 O to its radiation. This may to some 

 extent account for the equality of the emission in the two mixtures. Probably also 

 there are larger temperature differences in the hydrogen mixture. The much quicker 

 rate of cooling suggests that the temperature gradient in this mixture is greater 

 than that in the coal-gas mixture, so that at the same mean gas temperature the 

 hottest portions of the hydrogen mixture may be at higher temperatures than the 

 same portions of the coal-gas mixture. 



