238 
MR. J. E. PETATEL ON THE HEAT DISSIPATED P>Y A 
and therefore 
Heat lost by convection 
in therms 
centim., per degree 
temperature interval, 
per second 
+ hf{T, — T,) 
Civ(l + nT,) 
(hi.), 
where a, a, h, /3, Cj, k, are constants, the pressure, and T,. and the 
absolute temperatures of the radiator and the enclosure. 
The numerical values of each of these three terms of the above equation"^ for air at 
temperatures between 100 and 1000 and pressures between 10 and 160 atmospheres 
will be found in Table VII., and the values of the emissivity for the other gases 
studied are recorded in Tables VIII. to XI. 
If we consider the temperature constant, we obtain for the variation of emissivity 
with pressure :+ 
E = ) 
and we have seen that the exponents of j) are different for every gas studied. It is 
not easy to reconcile this fact with the theory that conductivity, viscosity, and 
specific heat are all three independent of pressure. It must be remembered, 
Iiowever, that the theory is deduced on the assumptions that the molecular paths 
are straight lines, that the radius of the molecular sphere of action is very small 
compared with the mean free path, and that the cohesion of the gas is a negligible 
quantity. 
Neither of these hypotheses seems altogether justifiable at pressures of one or two 
hundred atmospheres. 
* The numerical value of the heat lost by conductivity is merely given to show that, as far as our 
present knowledge goes, it forms a very small proportion of the total loss oliserved at high pressures. 
Though the conductivity of gases at ordinary pressirres is fairly accurately known its temperature 
coefficient is as yet uncertain. No observations are available above 200“ C., and even at lower temperatures 
there is much discrepancy between different observers. For air, for instance, Wixkelmanx gives 0‘0019, 
Eichhorn 0-00199, Schleiermacher 0-0028, and Eckerlein 0-0036. Again, the effective temperature 
of the gas is uncertain, but to make sure of not under-estimating the part played by conductivity we have 
used the maximum value T,.. 
t DuLOXCi and Petit (‘Annales de Chimie et de Physique,’ 1st s., vol. 7, p. 337, 1817) found for the 
loss due to convection and conduction per degree temperature interval 
N>“i ^ 1-233. 
At constant temperature the variation with pressures would be 
where fli = a constant, = pressiu’e, = 0-4.5 for air, 0-38 for hydrogen, and 0-517 for CO 2 . 
In the formula (ii.) above -- is of the order and for small values of S, the emissivity is approxi¬ 
mately equal to rt 2 p“, 
where no = a constant, j) = pressure, and a (see p. 236) is 0-56 for air, 0-35 for hydrogen and 0-82 for 
