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