86 VISCOSITY OF GASES 223 



that we must not see in this any ground for doubt ; for 

 Graham's measures were made at quite a different tem- 

 perature from those of the observers at Tubingen. On the 

 contrary I should sooner think that Graham's observa- 

 tions are of small value because they were made with a 

 short tube, the section of which was neither circular nor 

 regular. 1 If, however, Lothar Meyer's observations also 

 were to be affected by a constant error, that would be 

 without effect on the mutual agreement of the numbers ; 

 and the conclusion remains that the entrance of chlorine or 

 bromine or iodine into a chemical combination substantially 

 increases the viscosity of the vapour. 



Within each group, however, the value of the viscosity 

 is the same for all substances. In this lies a confirmation 

 of the proposition stated in 79, that the viscosity-coefficients 

 of gases whose molecules are made up of a considerable 

 number of atoms are of nearly the same magnitude. They 

 are certainly not so different from each other as the co- 

 efficients of viscosity of bodies in which fewer atoms are 

 bound together to form the molecule ; this is seen also from 

 a comparison of the numbers just given, both with each 

 other and with other numbers tabulated earlier. 



If, as this shows, vapours obey the laws of viscosity 

 in many respects like gases, there still remain essential 

 differences to take into account, and these we have now to 

 consider more in detail. 



87. Dependence on the Temperature 



The first of these differences concerns the mode in which 

 the viscosity of vapours depends on the temperature. 

 Vapours exhibit a much more marked variation with tem- 

 perature than gases. Hence Sutherland's formula, which 

 is in excellent agreement with the behaviour of gases, is 

 only imperfectly satisfied by many vapours. 



We can at once see that the validity of the formula may 

 be limited ; for it is not possible by a determination of the 

 value of C to represent every possible ratio in which the 

 viscosity may increase with the temperature. 



1 Capillary tube K, Phil. Trans. 1849, pp. 353, 357. 



