When you put a piece of glass in an old window sash you hold it in place with glazers pins, a flat metal pierce that you press into the would around the perimeter of the glass. Theis holds the glass in place but doesn't seal the window. If it is an old house like mine, the glazing will chip off in time and thus the seal is lost. The good news is that it is easy to fix. Go to your local Home Depot etc. and get the following.
1 qt can (not the tube) of glazing putty 1 1/2 in plastic (not metal) putty knife glazers pins (in the are where they sell glass) Mineral Spirits Rags
If you can remove the window its easier but not required.
1.Try to clean of any old glazing if possible it usually chips away with a carpet knife, but be careful not to chip or carck the glass (old glass is not as strong and chips easily) 2. Put on some Latex gloves if possible and scoop a 3 fingerful of glazing putty from the can 3. Roll the putty into a 3/8" roll and place it on the edge of the glass 4. With you Plastic putty knife, force the putty into the corner where the glass and the wood meet. Work your way around the glass 5. Next apply pressure with the corner of the blade and drag the knife around the edge. This will leave some excess that will need to be scraped off. Use the mineral spirits to clean the glass.
6. At this point, you are done. When it becomes rubbery after a day or so, you can gently run your finger over it to smothe it.