Question

a gas which turns Nessler's and reagemy Brown

Answer 1

Hi mate, 

             Ammonia is the gas which turns Nessler's Reagent 'Brown' in colour.
Nessler's reagent is the alkaline solution of Potassium Mercuric Iodide, which gives a pale brown solution with Ammonia.
  
      Hope this helps you.. 

Answer 2

Answer:

Ammonia

Explanation:

Ammonia a gas colourless and pungent odour in nature turns nessler's reagent from clear colourless to pale brown or gives a reddish brown precipitate on passage of ammonia in excess.