PLATINUM SURFACE AT HIGH TEMPERATURES. 
241 
E . 
E—R. 
Air. 
Oxygen. 
Hydrogen. 
Carbon dioxide. 
0-000.50 
0-00047 
0-00052 
0-00049 
0-00285 
0-00282 
0-00044 
0-00041 
0-1617 (E—R) . . . 
Kioo. 
Heat lost by convec-l 
tion X 0-1617 . J 
0-000076 
0-000067 
0-000009 
0-000079 
0-000066 
0-000013 
0-000456 
0-000455 
0-000001 
0-000066 
0-000046 
0-000020 
The fact thus brought out, that the convection is a maximum in carbon dioxide 
and a minimum in hydrogen,* may cause some surprise, for the coefficient of vis¬ 
cosity of the former is only half that of the latter. It must, however, be remembered 
that the force causing convection is the buoyancy of the heated gas which is propor¬ 
tional to its density. The heat conveyed by the stream of gas is proportional to 
the volume of the gas in motion, its mean rise of temperature, its mean velocity, 
its S]3ecific heat, and its density ; thus we have for a given temperaturet 
Convection = A - ^ . 
7 
Where A = a constant, p = the density of the gas, C = the specific heat, y = the 
viscosity. 
According to the above formula, the relative convection In carbon dioxide, oxygen, 
air, and hydrogen is represented by the numbers 20, 8’6, 8'5, and 1’2. If we take 
into consideration that the experimental numbers 0‘000020, O'OOOOIS, O'OOOOOO, and 
0'00001 are obtained from the difference of two nearly equal quantities, the agree¬ 
ment may be considered satisfactory. 
We are now in a jjosition to express in absolute units the heat lost respectively 
by convection, conduction, and radiation. 
* This fact is confirmed by the experiments of Kuxdt and Warburg (see ‘ Pogg. Ann.,’ vol. 156, p. 179, 
1875). They found that, in a certain apparatus, the heat dissipated was independent of pressure up to 
•30 millims. in the case of air, and up to 154 millims in the case of hydrogen. 
t This relation between the heat lost by convection and the density, viscosity, and specific heat of a gas 
only holds good when the speed of the convection currents is very low ; at higher temperature and 
pressures the phenomenon is more complex. 
VOL. CXCVII.-A. 2 I 
