502 



Mr. J. E. Petavel. An Experimental 



made, it was decided to try the effect of using coal gas instead of 

 hydrogen. After the blow-pipe had been supplied for twenty minutes 

 with coal gas, the curve shown in fig. 16, curve I, was obtained ; 

 fifteen minutes later a similar set of readings were recorded. The 



Fig. 36. — Curve I shows the effect of using coal gas to heat the platinum, the light is 36 per cent, 

 above the normal. Curve II was taken an hour and a half later, the coal gas having been 

 replaced by pure hydrogen. The error is already reduced to 2 per cent. 



10 SO 60 40 50 60 70 80 90 100 HO 120 130 140 150 



Time in Seconds. 



entire surface of the metal was covered with a white film. The inten- 

 sity of the light was 36 per cent, above the normal. The coal gas 

 was then stopped, and the hydrogen turned on, the surface gradually 

 clearing as the carbon oxidised out. In an hour and a half the curve 

 fig. 16, curve II, was obtained, the error having already been reduced 

 to less than 2 per cent. 



From the above results I believe I am justified in stating that the 

 probable variation in the light emitted by molten platinum under the standard 

 conditions is not above 1 per cent. 



"With more perfect apparatus, and with the experimental conditions 

 altered in the direction that has been suggested, the accuracy of this 

 standard would certainly be increased. 



Physiological considerations fix a limit to the accuracy of photo- 

 metric observations. It is not impossible that the accuracy of the 

 platinum standard may attain to or even surpass this limit. 



The present work has occupied nearly three years, and it was 

 thought advisable before devoting more time to the subject to publish 



