What happens to the fumes of ammonia and hydrochloric acid in the tube

They react to produce solid ammonium chloride - seen as a white smoke.
NH3(g) + HCl(g) ----> NH4Cl(s)

They will mix together and react to form a white cloud of Ammonium chloride.
NH3(g) + HCl(g) ====> NH4Cl(s).

HClis used in industrial areas where Ammonia is used, to detect ammonia leaks.