The Window is Free
Now, somewhat miraculously, the bottom window is now free. You need to hang onto this window with a free hand, it will likely pop out quite easily and you might be surprised on how heavy it is. This is a great time to clean both sides of the window, paint the meeting rails of the top and bottom window. Regardless of what else you do, put this window away for the time being into an area where neither you, nor any pedestrians, will accidentally kick it and break it.
Incidentally, its at this point, your cats will attempt to either escape through the open window (freedom!) or knock over the window you carefully set aside out of the way of humans. The more productive cats will somehow do both simultaneously. Some advanced children may follow the cats outside.
You’ll also discover if your house is negatively or positively pressurized because any dust you have accumulated on the window sill will either blow in or out of the house. Interesting.