THE DYNAMICAL THEORY OP GASES. 77 



Substituting for /J its value in terms of y by equation (115), and for &, 

 its value in terms of p. by equation (125), and calling p a , />, and the simul- 

 taneous pressure, density, and temperature of the standard gas, and s the spe- 

 cific gravity of the gas in question, we find 



-lTFD iSt 5 <-> , 



For air we have y= 1*409, and at the temperature of melting ice, or 

 274'6C. above absolute zero, /*- = 918*6 feet per second, and at 16 0> 6 C., 



Hi = 0*0936 in foot-grain-second measure. Hence for air at 16*6C. the conduc- 

 tivity for heat is 



(7=1172 (150). 



That is to say, a horizontal stratum of air one foot thick, of which the upper 

 surface is kept at 17"C., and the lower at 16 C., would 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 1C. different in temperature, would, when the mean temperature 

 is 25C., transmit in one minute through every square foot of surface as much 

 heat as would raise one cubic foot of water 0*0127 C. 



Now the dynamical equivalent in foot-grauvsecond measure of the heat 

 required to raise a cubic foot of water 1C. is 1*9157 x 10'. 



It appears from this that iron at 25C. 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 fi, found that lead should couduct 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. 



* " Experimental Inquiry into the Laws of the Conduction of Heat in Bars," Edinburgh Transactions, 

 186162. 



