PLATINUM SURFACE AT HIGH TEMPERATURES. 
23‘) 
The results of Warburg and von Babo* * have shown that for gases near their 
critical temperature, there is a great increase of viscosity as the critical pressure is 
approached. 
Meyer explains, and at the same time confirms this result, by a calculation based 
on the assumption that the gas is a mixture of double and simple molecules. 
The relatively large value of the exponents of 'p (see page 236) in the case of carbon 
dioxide and nitrous oxide, may be accounted for by the above fact. 
It has been shown that the conductivity of a gas is proportional to its viscosity. 
Taking into account that even the so-called permanent gases diverge consideraldy 
from Boyle’s law at the higher pressures, it seems probable that the constancy of 
this conductivity is far from absolute. 
A glance at figs. 2 to 6 will show that in every case a sharp fall occurs in the 
curve of emissivity when d decreases below a certain limit ;t and from such measure¬ 
ments as have been made at small temperature intervals there can be no doubt that 
at pressures above 10 or 20 atmospheres the conductivity accounts for only a very 
small projjortion of the total heat dissipated, but none the less the conductivity may 
have considerably increased in absolute value. 
The present experiments do not, therefore, definitively settle the question. To 
eliminate convection at the higher pressures, the temperature interval between the 
radiator and the enclosure would have to be a small fraction of a deu'ree. An 
apparatus designed to measure up to ] 000 degrees is not suitable for this kind of 
work. It was therefore thought preferable to leave tlie subject for a future investi¬ 
gation, rather than record values which would only be a rough approximation. 
On the other hand, at ordinary pressures the constancy of the conductivity has 
received ample experimental verification, and there is no difficulty in determining 
the absolute amount of heat lost by each of the three factors : radiation, convection, 
and conduction. 
An Analytical Study of the Total Heat Dissipated. 
Our first object must be to determine what proportion of the total loss is due to 
conduction alone. 
In the present case we have two coaxial cylinders : the one forming the radiator, 
the other the enclosure, and we must determine what is the tliermal conductance of 
the air filling the intervening space. 
carbon dioxide. When it is taken into account that DuLONG and Petit’s formula was derived from 
observations made at pressures below 1 atmosphere and under totally different experimental conditions, 
even this approximate agreement is not devoid of interest. 
* ‘Wicd. Ann.,’ vol. 17, p. 390, 1882. 
t About 300“ for hydrogen and 100“ for all other gases. 
