Quote 7
No; she was drinking in a very elixir of life through that open window. (18)
In contrast to her sister's fear that she is making herself sick, Mrs. Mallard seems to think that she's never felt more alive. In opening herself up to this idea of freedom and letting go of the shackles of marriage, Mrs. Mallard seems to be reviving herself with a "very elixir of life." Unlike other elixirs, though, this one can't promise immortality. It's as if it's so powerful that it burns right through her system, leaving her with nothing to defend herself with against the shock of seeing her husband again.
Quote 8
She breathed a quick prayer that life might be long. It was only yesterday she had thought with a shudder that life might be long. (19)
Up to this point, Mrs. Mallard has been dreading the rest of her life. It's only once she thinks her husband has died and she's free that she's excited about living. In another example of Chopin's ironic style, though, this "prayer" goes unanswered.