2Si 
MU. J. E. PETAVEE ON THE HEAT DISSIPATED BY A 
ture indicated by the thermometer ; this again would cause the emissivity to he 
under-estimated. Taking the maximum values of the temperature and pressure, we 
find that in air at I GO atmospheres and 1200° C. the emissivity is 0'01259, and the 
heat dissij^ated = EdTr.d = 5'249 C.G.S. units per second, per unit length. The inside 
diameter of the enclosure is 2’OG centims., its superficial area G'472 sq. centims. 
I^er unit length. Taking the conductivity of steel as O'il, this would correspond to 
a fall of temperature of 7°’3 C. 
Thus the temperature interval we estimated at 1200, really was 1193. This cause 
of error would he very serious were it not for the fact that it decreases not only with 
the square of the temperature hut also with the pressure. At IGO atmospheres and 
500° C. the flow of heat is l'G12 C.Ct.S. units, making the error 2°'3 C. ; at 100 atmo¬ 
spheres and 500° C. the error is 1°'G C., and for 30 atmospheres and 500° C. = 1°'0 C. 
Over the greater part of the range of ohservatlon the error due to this cause is 
therefore below one-third per cent., hut it rises to about two-thirds per cent, at the 
highest temperatures and pressures 
Results obtained. 
In all cases the heat dissipated Ijy a hot body surrounded by gas is the sum of 
three distinct quantities, all three being functions of the temperature, and one 
at least being also a function of the pressure. The formula for the total radiation 
exj^ressed in words will therefore read— 
Total heat dissipated = Convection -f- Conduction fi- Radiation. 
Or Emissivity = t) + Fo (^) -E Eg (^).(i.). 
Of these four quantities, two, Conductivity and Ptadiation, have been determined 
by previous experimenters, and the determination of a third will enable us to solve 
the equation for any value of t or p. The heat carried oil* by convection not being 
directly measurable, our only resource is to determine it by dift'erence. 
The experimental results will he found plotted in figs. 2 to G. It will he seen 
that in all cases above about 10 atmospheres the emissivity is practically a linear 
function of the temperature, from which fact we conclude that the loss by radiation 
must he relatively very small. We may therefore write for any given pressure 
E = m -{- ud. 
Both the values ni and n increase as the })ressure rises : closer observation shows 
that for any one gas they may he expressed as ex})onential functions ol the pressure. 
We thus obtain for the total emissivity an expression of the form 
E = ap°- + hp^B^ 
