docs: def visc_mol_sol(T,rho,X):
      '''
      Molecular plasma viscosity (Spitzer 1962)
      
      Parameters
      ----------
      X : float
         H mass fraction
      T : float
         temperature in K
      rho : float
         density in cgs
  
      Returns
      -------
      nu : float
         molecular diffusivity in [cm**2/s]
  
      Notes
      -----
      According to Eq 22 in Schatzman (1977). Assume log Lambda = 15. 
      (see Table 5.1), a H/He mix (for different mix use Eq. 5.54 in
      Spitzer text book)
  
      Examples
      --------
      see astronomy.visc_rad_kap_sc
  
      '''
      visc_mol = 1.84e-17*(1.+7.*X)*(old_div(T**2.5,rho))
      return visc_mol