We present a method to determine the spin temperature T _ { S } and volume density n of HI gas simultaneously along the tangent-point circle of galactic rotation in the Milky Way by using the \chi ^ { 2 } method .
The best-fit T _ { S } is shown to range either in T _ { S } \sim 100 - 120 K or in 1000 - 3000 K , indicating that the gas is in cold HI phase with high density and large optical depth , or in warm HI with low density and small optical depth .
Averaged values at 3 \leq R \leq 8 kpc are obtained to be T _ { S } = 106.7 \pm 16.0 K and n = 1.53 \pm 0.86 H cm ^ { -3 } for cold HI , and 1720 \pm 1060 K and 0.38 \pm 0.10 H cm ^ { -3 } for warm HI , where R = 8 | { sin } l| kpc is the galacto-centric distance along the tangent-point circle .
The cold HI appears in spiral arms and rings , whereas warm HI in the inter-arm regions .
The cold HI is denser by a factor of \sim 4 than warm HI .
The present analysis has revealed the hidden HI mass in cold and optically thick phase in the galactic disk .
The total HI mass inside the solar circle is shown to be greater by a factor of 2 - 2.5 than the current estimation by optically thin assumption .