Thanks deecee for your help. I had written all info off the lamp and taken it to Home Depot. They didn't carry GE (which is what it is), and they said you can't change the bulb in a mercury lamp to a sodium bulb (of course, they've given me other advice that I found wasn't correct). Anyway, I threw my note away as to what it said on the fixture. I will look again this weekend and get back to you. I do remember it being 175 watt and the number H37 rings a bell.
You are saying that it is possible to use a sodium bulb in a mercury lamp?