80 VISCOSITY OF GASES 203 



denominator of the formula for 77, we obtain for the co- 

 efficient of viscosity of the mixed gases the expression 



N 



In this formula, however, the numbers N which are not 

 accessible to direct observation may be replaced by the 

 values of the pressure, on the assumption that the mixed 

 gases obey Boyle's and Avogadro's laws. Denoting by 

 p 1 the pressure which the first gas, consisting of the mole- 

 cules m lt would exert by itself, and by p 2 that which the 

 second gas alone would produce, and lastly by p the whole 

 pressure exerted by the mixture, which in accordance with 

 Dalton's law is given by 



the formulae 



N l :N=p l :p and N 2 : N = p 2 : p 



hold good, and by these the expression ^for the coefficient of 

 viscosity takes the form 



p p 



p 



Puluj has made measurements of the internal friction 

 of mixtures of carbonic acid and hydrogen, has compared 

 his results with the formula, and has found a really good 

 agreement. So, too, has Breitenbach. 1 By these observa- 

 tions a striking fact was confirmed, which was first noticed 

 by Graham 2 as he allowed mixtures of hydrogen with 

 other gases to flow through capillary tubes. Although the 

 viscosity of hydrogen is less than that of carbonic acid, a 

 slight admixture of hydrogen has the effect of increasing 

 and not of decreasing the viscosity of carbonic acid ; nor 

 does a diminution begin until the mixture contains a largish 

 amount of hydrogen. 



1 Wied. Ann. 1899, Ixvii. p. 820. 2 Phil. Trans. 1846, p. 622. 



