A PLATINUM SURFACE AT HIGH TEMPERATURES. 513 



Considering now Equation II., we see that here also the second term decreases as 

 the temperature rises ; for it has been shown that whereas a surface coated with 

 platinum-black or lamp-black almost totally absorbs all radiation of short wave-length, 

 it reflects a comparatively large proportion of the infra-red rays. The curve obtained 

 by any form of radiometer will therefore be steeper than that representing the actual 

 relation between total radiation and temperature. 



The foregoing conclusions may be summed up as follows : 



The rate of change of the total radiation with temperature will be too small when 

 measured by the heat lost by the radiating body, whereas it will be too large when 

 measured by the heat gained by an irradiated surface coated with lamp-black or 

 platinum-black. 



Finally we may note that, as both m and n decrease at higher temperatures, the 

 accuracy of both methods increases as the temperature rises. 



Let us now see to what extent the actual experimental work verifies the conclusions 

 we have just arrived at. 



In fig. 10 (Plate 21) the values obtained by the first method are represented by 

 the results of Dr. J. T. BOTTOMLEY and of SCHLEIERMACHER, whereas the second 

 method was used by F. PASCHEN and by myself. 



To facilitate the comparison of the slopes of the curves, all the results are reduced 

 to the same arbitrary value at a temperature of 800 C. 



We see that Curve No. II., obtained by BOTTOMLEY, lies between the Curves No. I. 

 and III. obtained respectively by SCHLEIERMACEER and by PASCHEN, or precisely in 

 what is for theoretical reasons the most probable position of the true curve of 

 radiation. 



I have recently made a series of determinations by aid of the bolometer described 

 above ; the results within the limits of the probable experimental errors fall on this 

 same curve. 



It seems very probable that Curve II. represents the actual law of thermal 

 radiation between 400 and 800 C., but even were this not so, we might safely admit 

 that the true curve must fall somewhere between Curves I. and III., fig. 10. All 

 expressions therefore giving values outside these limits may be discarded without 

 further study. The formulae of STEFAN, of DULONG and PETIT, and of ROSETTI, fail 

 when tested by this criterion. WEBER'S formula, on the other hand, agrees closely 

 with Curve II. from 400 to 800 C. At all higher temperatures the rate of change 

 of radiation with temperature calculated by this formula, is certainly too great. 



Taking the value of WEBER'S constant at 800 C. as unity and comparing the 

 calculated results with Curve II., we obtain I'Ol, '96 and '94 for the values of the 

 constant for 700, 600 and 500 degrees centigrade, showing that the expression 

 remains nearly correct over this range of temperature. 



Table XYI. and fig. 11 give the results of a series of observations on the radiation 

 from platinum at temperatures ranging from 500 C. to the point of fusion of the 

 metal. 



VOL. cxci. A. 3 u 



