As was shown by I. S. Shklovsky, a radio spectral line from interstellar deuterium on λD
= 91·6 cm. (fD
= 327·38424 Mc./s.) may be expected . According to Shklovsky the difference between the effective temperature T
eff in this line and outside it must be of the order of 1/500 T
eff in the direction of the galactic centre, if the concentration of deuterium equals 10−3 that of hydrogen. In this case, contrary to the emission by hydrogen, an absorption line is expected. As T
eff equals about 300° the depth should be of the order of 0·6°.