The intensity of line emission of neutral hydrogen (HI) is proportional to concentration of gas and is inverse proportional its temperature.Therefore the reduction of intensity emission of HI in dust clouds could not be interpeted simple. It has been observed the decreasing of HI line absorption at radiation frequencies and coordinates of the dense dust cloud which covered the most part of NGC 6618 nebula (figures 1 and 2). It permits to state that just gas density is reduced because of it's conversion into molecular form. If the temperature of the dust cloud is 10 K, then concentration of atomic gas decreases 10 times as much ,that corresponds to velocity of conversion of HI into molecular form (3 - 5) .

Figure 1. The structures of HI absorption line in four points of NGC 6618 nebula.

Figure 2. The decreasing of optical thickness of HI line in dust cloud and locations of H2CO line.