of Gases and Molecular Force. 515 



proved to hold over the great range of temperature up to 

 1216° C. Barus found that his results for air could be well 

 represented by the empirical form y/v = (T/273) 1 , and, further, 

 that the results for hydrogen could be equally well represented 

 by the same form : hence for hydrogen the value of C in our 

 theoretical equation would from his experiments have the 

 same value as' for air, namely 113 ; and the theoretical law 

 applies to hydrogen as well as to air up to high temperatures. 

 Holman's experiments on carbonic dioxide furnish a still 

 better test of the theory. Here are his values of 7]/rj at the 

 given temperatures with the values of C calculated therefrom: — 



Temp. 18° 41° 59° 79-5° 100-2° 1194° 



fyij exper 1-068 1-146 1213 1285 1-351 1-415 



C 315 265 286 292 277 274 



n/rioGslcxA 1066 1-148 1*211 1-280 1-351 1-414 



Temp. 142° 158° 181° 224° 



7,/^exper 1-484 1-537 1*619 1*747 



C 266 270 284 279 



v / Vo calcul 1490 1-541 1614 1*746 



Giving each value of C a weight proportional to the tempera- 

 ture-interval from which it is found, we get the mean value 

 C = 277, with which the calculated values of rj/rj Q in the last 

 table were obtained. The agreement between the experi- 

 mental and calculated numbers is again within the limits of 

 experimental error. 



The law of the connexion of viscosity and temperature 

 being thus established, we can now examine some important 

 consequences of the theoretical formula (2), 



•064c* m* T* 

 ^= 7 7JT 



w(i + «J 



The value of C, that is of m 2 f(l/2a)/c, is proportional to 

 the potential energy m 2 f(l/2a) of two molecules in contact, 

 and it is therefore desirable to obtain values of C for as many 

 substances as possible ; and as C is a function of 2a it 

 will be advantageous to use the values of G to calculate 

 relative values of 2a for different substances by means of the 

 above formula, so that we may be in possession of relative 

 values of the diameters of molecules and of the mutual 

 potential energy of two molecules whose centres are at the 

 distance apart of a diameter. The equation of the kinetic 

 theory of forceless molecules, 77 Qcm*T*/(2a) 2 , was applied by 



2M2 



