Mr. J. C. Maxwell on the Dynamical Theory of Gases. 217 



in one second transmit through every square foot of horizontal 

 surface a quantity of heat the mechanical energy of which is 

 equal to that of 2344 grains moving at the rate of one foot per 

 second. 



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 tempera- 

 ture, 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 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 /n, 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 /x, 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 conducti- 

 vity of iron diminishes as the temperature increases. 



Also, since 7 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, ni- 

 trogen, 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 that of oxygen, and its viscosity -fL 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. 



