\begin{table}%T2
%\centering
\par
\caption{Electron density indicators in \ngc.}\label{tab-2}
\begin{tabular}{lcccc}
\hline
\hline
Ion &Ioniz. & Lines& Observed &$N_{{\rm e}}$ \\
&pot. (eV) & used  & ratio & (cm$^{-3}$)\\
\hline
\ion{[S}{ii]} & 10.4 & 6731/6716 & 2.0 & 10~000\\
\ion{[O}{ii]} & 13.6 & 3626/3729 & 2.0 & 5200\\
\ion{[Cl}{iii]} & 23.8 & 5538/5518 & 2.1 & 16~000\\
\ion{[Ar}{iv]}  & 40.7 & 4711/4740 & 0.77 & 6000 \\
\par
\hline
\end{tabular}
\end{table}