438 



HENRY A. ROWLAND 



The reducing factor is -0621, and hence the last results for the jacket 

 at 20 C. become: 



TABLE XXXVI. 



The variation which I find is almost exactly that given by McFar- 

 lane, as is shown by the constancy of the column of ratios. But my 

 coefficients are less than half those of McFarlane. This may possibly 

 be due to the fact that the walls of McFarlane's enclosure were black- 

 ened, and to his surface being of polished copper and mine of polished 

 nickel: his surface may also have been better adapted by its form to 

 the loss of heat by convection. The results of Nichol are also much 

 lower than those of McFarlane. 



The fact that the coefficients of radiation are less with increased 

 temperature of jacket is just contrary to what Dulong and Petit found 

 for radiation. But as I have shown that convection is the principal 

 factor, I am at a loss to check my result with any other observer. 

 Dulong and Petit make the loss from convection dependent only upon 

 the difference of temperature, and approximately upon the square root 

 of the pressure of the gas. Theoretically it would seem that the loss 

 should be less as the mean temperature rises, seeing that the air be- 

 comes less dense and its viscosity increases. Should we substitute 

 density for pressure in Dulong's law, we should have the loss by con- 

 vection inversely as the square root of the mean absolute temperature, 

 or approximately the absolute temperature of the jacket. This would 

 give a decrease of one per cent in the radiation for about 6, which is 

 not far from what I have found. 



To estimate the accuracy with which the radiation has been obtained 

 is a very difficult matter, for the circumstances in the experiment are 

 not the same as when the radiation was obtained. In the first place, 

 although the water is stirred during the radiation, yet it is not stirred 

 so violently as during the experiment. Further, the wheel above the 

 calorimeter is warmer during radiation than during the experiment. 



