394 Prof. De Volson Wood on 



find its value, we have 



Hydrogen. Air. Oxygen. 



Specific heat* .... 3-4093 0'2375 0-2175 

 Velocity of sound feet per| ^3 1Q9() 104Q 



second at t=49o *2 . J 



and ^ = 32-2, y= 1*4, J = 772. These, substituted in the second 

 member of (6), give 



xy for hydrogen .... 6'599 



„ air 6*706 



?• oxygen 6*596 



3) 19*901 

 Mean .... 6*63 



This value, which is nearly constant for the more perfect 

 gases, we propose to call the modulus of the gas, and represent 

 it by /x; and for the purposes of this paper we will use 



/i = 6*6. 



This relation of the product xy being a constant, has, so far 

 as we are informed, been overlooked by physicists, and is 

 worthy of special notice, since it determines the value of one 

 of the factors when the other has been found. Kronig, 

 Clausiusf? and Maxwell give for x the constant number 3, 

 but variable values for y $. 



We are confident that the value of x is not strictly constant; 

 or if it is, it exceeds 3, since the effect of the viscosity of a 

 gas would necessitate a larger velocity to produce a given 

 tension than if it were perfectly free from internal friction. 

 For our purpose it will be unnecessary to find the separate 

 values of x and y; but if we have occasion to use the former 

 in making general illustrations, w r e will call it 3, as others 

 have done heretofore. If the correct value of x exceeds 3, it 

 will follow that the velocity of the molecules exceeds the values 

 heretofore computed§. According to Thomson, Stokes showed 

 that in the case of circularly polarized light the energy was 



* Stewart on 'Heat/ p. 229. 



t Phil. Mag. 1857 [4] xiv. p. 123. 



% l Theory of Heat/ pp. 314 and 317. Maxwell states that the value 

 for y is probably equal to 1*634 for air and several of the perfect gases. 

 This would make £ = 4 nearly. 



§ Maxwell gives for the mean square of the velocities (or, in other 

 words, the velocity whose square is the mean of the squares of the actual 

 velocities) of the molecules, in feet per second at 493 0, 2 F. above absolute 

 zero, hvdrogen 6232, oxygen 1572, carbonic oxide 1672, carbonic acid 

 1570. "Phil. Mag. 1873, p. QS. Our equation (4) gives for air 1593, 



