def visc_mol_sol(T,rho,X):
visc_mol = <NUM_LIT>*(<NUM_LIT:1.>+<NUM_LIT>*X)*(old_div(T**<NUM_LIT>,rho))<EOL>return visc_mol<EOL>
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