88 
ME. CLEEK MAXWELL OX THE DYXAMICAL THEOEY OF GASES. 
Principal Forbes* has deduced from his experiments on the conduction of heat in 
bars, that a plate of wrought iron one foot thick, with its opposite surfaces kept 1° C. 
different in temperature, would, when the mean temperature is 25° C., transmit in one 
minute through every square foot of surface as much heat as would raise one cubic foot 
of water 0 o, 0127 C. 
Now the dynamical equivalent in foot-grain-second measure of the heat required to 
raise a cubic foot of water 1° C. is 1-9157 xlO 10 . 
It appears from this that iron at 25° C. conducts heat 3525 times better than air at 
16°-6 C. 
M. Clausius, from a different form of the theory, and from a different value of 
found that lead should conduct heat 1400 times better than air. Now iron is twice as 
good a conductor of heat as lead, so that this estimate is not far different from that of 
M. Clausius in actual value. 
In reducing the value of the conductivity from one kind of measure to another, we 
must remember that its dimensions are MLT -3 , when expressed in absolute dynamical 
measure. 
Since all the quantities which enter into the expression for C are constant except 
the conductivity is subject to the same laws as the viscosity, that is, it is independent 
of the pressure, and varies directly as the absolute temperature. The conductivity of 
iron diminishes as the temperature increases. 
Also, since y is nearly the same for air, oxygen, hydrogen, and carbonic oxide, the 
conductivity of these gases will vary as the ratio of the viscosity to the specific gravity. 
Oxygen, nitrogen, carbonic oxide, and air will have equal conductivity, while that of 
hydrogen will be about seven times as great. 
The value of y for carbonic acid is 1-27, its specific gravity is ^ of oxygen, and its 
viscosity of that of oxygen. The conductivity of carbonic acid for heat is therefore 
about -§■ of that of oxygen or of air. 
* “ Experimental Inquiry into the Laws of the Conduction of Heat in Bars,” Edinburgh Transactions, 1861-62. 
