PLATINUM SURFACE AT HIGH TEMPERATURES. 
245 
If R/r is made very great E^, is nearly constant with regard to R and is equal 
to K/r, showing that if the enclosure is above a given size, the heat lost by 
conduction is practically independent of its actual dimensions. 
The theoretical treatment of the variations of the emissivity with the tempera¬ 
tures of the gas offers some difficulties. 
On the one hand the conductivity increases in proportion to the viscosity, on the 
other the convection is a function of the density divided by the viscosity. Taking 
the gas at the same density, both at the high and the low temperature, we may safely 
predict that at low pressures, where the effect of conductivity predominates, a rise 
in temperature will correspond to an increased value of the emissivity. At high 
pressures, however, with the data at present available, a theoretical solution is not 
possible. 
The results of two sets of experiments made with carbon dioxide are given 
below— 
Emissivitv in Carbon Dioxide in C.G.S. Units. 
Temperature* 
d in degrees 
Centigrade. 
Pressure 19’9 atmospheres 
at 16° C. 
Pressure 45 - 5 atmosjdieres 
at 16° C. 
Enclosure at 
18° C. 
Enclosure at 
100° C. 
Enclosure at 
18° C. 
Enclosui’e at 
100° C. 
200 
0-0032 
0-0032 
0-0058 
0-0053 
300 
0-0036 
0-0038 
0-0063 
0-0058 
400 
0-0040 
0-0044 
0-0068 
0-0063 
500 
0-0044 
0-0048 
0-0074 
0-0068 
600 
0-0048 
0-0053 
0-0079 
0-0075 
700 
0-0052 
0-0057 
0-0084 
0-0081 
We see that even at 20 atmospheres an increase of 34 per cent, in the absolute 
temperature causes an increase of about 10 per cent, in the value of the emissivity. 
In the case of carbon dioxide, and probably of all easily condensible gases, when 
the pressure is near that which would bring about a change of state, the emissivity 
is diminished by a rise of temperature. 
Finally the question of the average temperature of the gas in the enclosure in its 
relation to that of the radiator offers some interest. It can be determined from 
the variation of pressure, for the apparatus constitutes a rough form of constant 
volume thermometer. On the other hand, when the heat lost by convection is 
small, the average temperature can lie calculated from the equations on page 240. 
If we divide the gas into a series of concentric cylinders, the boundaries of these 
cylinders will be isothermal surfaces, and the fall of temperature from the inner to 
* ^ = temperature of the radiator - temperature of the enclosure in degrees Centigrade. 
