A few articles (such as "Misconceptions about Medieval Medicine: Humors, Leeches, Charms, and Prayers") state that Hippocrates observed that blood, when removed from the body, would separate into four layers, and that this stratification partially inspired his humoral theory. What passages from antiquity describe this observation? Is it mentioned anywhere in the Hippocratic corpus? If so, where?
This thesis appeared in the "The Humans nature" book. Ancients thought it was the work of Hippocrates, but sooner it was the work of his son-in-law Polibius.
It is got from STORIA DELLA FILOSOFIA (REALE - ANTISERI), the first volume.
Oh, wiki says this, too. http://en.wikipedia.org/wiki/On_the_Nature_of_Man (On the Nature of Man is a work in the Hippocratic Corpus.)
What about observations, it is not strange to me, that very few articles mention observations conducted by Hyppocrates. In the Antic world the knowledge had to be pure, not tainted by the low reality. It had to go from inside. Remember Aristoteles, that thought that flies have 4 legs and never tried to recount. So, even if Hippocrates did measured something (he could, for medicine couldn't be without practice), he would never write about it somewhere. IMHO, the articles mentioned fall into the vice of anachronism. They give post-Bacon philosophy to the scientists of the pre-Bacon times.
Even if there will be found some later work that would mention such observations of Hippocrates, it would be mostly probable, that some later scientist simply covered so his own observations by the authority of Hippocrates - very usual practice in the Middle Ages.