Old SF Movie I hope someone can identify..?

Here's a real challenge I think. I can just remember a few scenes of an SF movie I saw as a kid during the 60's.Some kind of Alien either lands on earth and is captured or is skulking around. He gets trapped in a large structure like an observatory and the last few scenes he is climbing on the girders of the interior trying to escape. I think I remember he is running out of some special air and he is also invisible but fading in and out of visibility. In the final scene he slips and falls to the ground and I guess dies....any ideas...?? That's all I can remember... Much thanks