240 PHENOMENA DEPENDENT ON MOLECULAR PATHS 91 



where s is determined by the ratio of the two limiting values 

 of the coefficient of viscosity, viz. ?/ 1 of the gas when com- 

 pletely dissociated into simple molecules C0 2 and ?; 2 of the 

 gas when composed of only the double molecules C 2 4 , in 

 accordance with the formula 



s = 



D is further the density which the gas would possess under 

 the circumstances of the experiment if it were entirely dis- 

 sociated into simple molecules. 



The three magnitudes T? P rj^ D named above are not 

 capable of direct measurement because we cannot know 

 whether the limiting case in which the gas contains only 

 molecules of one kind, single or double, has been reached in 

 any experiment. 



But from one of the laws of theoretical chemistry l we 

 can theoretically calculate the density D of a gas from its 

 molecular weight. Since now, according to what we have 

 already said, carbonic acid below the critical pressure, or at 

 least below 70 atmospheres, may be considered as an actual 

 gas, we are justified in extending the procedure for the cal- 

 culation of D to the formulae in which we put 



A = ap, B = bpk. 

 We may therefore put these formulas in the shape 



a = A (2 - e) / (1 - e), b = 

 wherein 



represents the density of the completely dissociated gas 

 under the pressure of 1 atmosphere, and therefore the 

 known magnitude which chemists call the theoretical or 

 normal density of the gas. We have now to take into 

 account the circumstance that we must retain for its 

 specification the unit of density assumed by Warburg 

 and Von Babo, so that we must not take for carbonic acid 

 the usual value 1*5198 as referred to air, but its value as 



1 30. 



