\begin{table}%t6
\caption{Mass and $\rm H_2$ column- and volume-density estimated from 
continuum observations.}
\label{tmass2}
%\centering
\par
\begin{tabular}{cccccc}
\hline \hline
 Tracer      &   $\theta_{\rm s}$ & $D$ & $N_{\rm H_2}$ & $M_{\rm cont}$ & $n_{\rm H_{2}}$  \\
      &   (\asec\ ) & (pc) & (\cmq\ ) & ($M_\odot$) & (\cmc\ ) \\
\hline
 3~mm (PdBI)  &   1.5   & 0.036 & 1.5 $\times$ $ 10^{25}$ & 150 & 14.0 $\times$ $ 10^7$ \\
3.4~mm (OVRO$^{\dagger}$)    &   4 & 0.09 & 0.4 $\times$ $ 10^{25}$ & 324 & 1.6 $\times$ $10^7$ \\
SED ($^*$) & 8 & 0.20 & 1.5 $\times$ $10^{24}$ & 400 &  2.5 $\times$ $10^{6}$ \\
850 $\mu$m (SCUBA)   & 18 & 0.43 & 3.6 $\times$ $10^{23}$ & 520 &  2.8 $\times$ $10^5$ \\
\hline
\noalign{\smallskip}
\multicolumn{6}{l}{\hspace*{-4mm}$^{\dagger}$ From Molinari et~al. (\cite{mol98b}).} \\
\multicolumn{6}{l}{\hspace*{-4mm}$^*$ From the SED of Fig.~\ref{sed}.}\\
\end{tabular}
\end{table}