According to Wikipedia, Hennig Brand discovered phosphorous. Later, Robert Boyle desired to replicate Brand's discovery, but didn't know that urine was used - only that it involved something that "belonged to the body of man".
That quote is apparently taken from the 1939 edition of Mellor's Modern Inorganic Chemistry; however, I'm unable to access a copy of that. Looking at the 1933 edition, the quote doesn't seem to appear (though interestingly, does imply an important role for Boyle's assistant, A. G. Hanckwitz), so I can't verify the context of the quote.
My question is: what substances from the human body did Boyle (or Hanckwitz) try before settling on urine? I'm not particularly knowledgeable about chemistry beyond school, never mind historical chemistry/alchemy, so I don't know if urine would've been known as the most plausible candidate, or just one of dozens of possibilities given the vagueness of the hint available. Is there any record, by Boyle or Hanckwitz, of how much trial and error this process involved?