402 



MR. W. T. DAVID ON THE RADIATION IN 



TABLE XVI. 15-per-cent. Mixtures of Coal-gas and Air. Fluorite Window. 



of wave-length 3'6/i (the mean between 2'8/x and 4'4jt, which are the principal 

 maxima of emission and absorption in the Bunsen flame spectrum) emitted by a full 

 radiator at the temperatures given in the first column according to PLANCK'S formula 



E^dV 6 (**- 1)' 1 



when c is taken as 14,700. 



There is in all probability a considerable error in the values given in the second 

 column of this table and, consequently, in the ratios in the fourth column. It is 

 unlikely that these ratios should ever be less than unity, for it will be remembered 

 that the observed values of the intrinsic radiance from 30 cm. and 59 cm. of the 

 gaseous mixture enclosed in the vessel with reflecting walls were about 30 per cent, 

 greater than those from the same thicknesses of an identical mixture enclosed in the 

 vessel with black walls. 



Th'e similarity between the figures in the third and last columns shows that the 

 variation of the radiation from the gas with temperature is very much the same as 

 that given by PLANCK'S formula for a single wave-length of 3'6/x, which at high 

 temperatures (1800 C. abs. to 2400 C. abs.) varies approximately as the square of 

 the absolute temperature. The very rapid variation of the total radiation from the 

 gaseous mixture with temperature shown in figs. 9 and 15 (which varies approxi- 

 mately as the fourth power of the absolute temperature*) is in part due to the 

 decreasing transparency of the gaseous mixture as it cools. PASCHEN found that the 

 emission of radiation of wave-length 4'4/x from a thickness of CO 2 greater than 7 cm. 

 was, between the temperatures 150 C. and 500 C., only a little below that of a 



* It should be noticed that this is accidental. Had the vessel been much larger (or smaller) the 

 radiation curve would lie far above (or below) the 0* curve in the initial stages of cooling (see dotted 

 curves, fig. 15) on account of the high transparency of the gaseous mixture; but after the temperature of 

 the mixture has fallen to about 1500" C. (abs.) it would coincide with the fl 4 curve, the gaseous mixture 

 having by this time become fairly opaque. 



