Well to get back to the original question and hopefully away from the animosity since there has been waaaaay too much of that lately).
@Eliza, if this lady has been a long standing friend as you said then presumably matters between you are such that you can raise the issue in a fairly straight forward way, that was what happened with my lady and I, and even if she does not reciprocate your feelings it should clear the air between you if nothing else.
While it is true that on average people are more often heterosexual, at the same time, there is more than enough evidence these days that things are not quite as binary or straight forward as automatically being one thing or the other.
For instance, a good friend of mine has been married to a lady for the past 8 years, is generally attracted to women, but has on two or three occasions fallen very much in love with close male friends. He describes himself as %80 hetero.
I'll also say this sort of thing seems culturally easier for women to explore, since it is much more common and acceptable in western culture for women friends to share physical affection with each other, and I certainly know ladies who, though they are primarily attracted to men have developed feelings towards their female friends on occasion as well.
I wouldn't suggest going in all guns blazing , start with exploring the idea gently and let your friend get used to it over time, and of course give her the space to say "no" and remain friends if that is her desire as well. Indeed, that was what originally I thought my lady and I would be doing.
I'll also say if she does not! share your attraction that does not mean you can't be friends either. My lady's best friend is gay and she's said she's always been attracted to my lady, but that doesn't get in the way of them being friends since she's aware my lady doesn't share her attraction, or indeed of me being friends with her either.
Hope some of this is vaguely helpful.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)