Chemistry is feeling that you are in sync, with another persons thoughts, feelings, and viewpoints. I think it runs on an unconcious level, and you just feel it, when it happens. Only time will tell if this feeling lasts, as it's more important to me that two people have the same point of view on important issues like communication, expressing themselves on a meaningful level, and most important, are they good in their heart and intentions to the world at large.
I have several close friends that I know in my heart, would never do anything to hurt me, and only want to help me in times when help is needed, and vica versa. I would drop everything and run for them, for anything, that is needed, at a moments notice. This is what close relationships should be about.
They are family to me.
When starting a relationship, one would do well to become friends with that special person instead of rushing to become intimate. If one becomes intimate instantly, if the relationship falls apart, then you don't have a friendship.
Chemistry is somthing that is there, or not there. You instantly will know when it's happening.
BTW - Rodney Dangerfield used to comment that sex, should be like two dogs, One sniff, and "she's for me.
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"It doesn't cost "nuttin", to be nice". 
MOM
I just want to do the right thing! Uncle Joe, (my hero).