Reference for Chullin 174:25

מי גשמים נמי כיון דשקיל ורמי אחשבינהו

It refers to the thin blood.<span class="x" onmousemove="('comment',' When blood congeals there settles at the base a clear watery liquid. This liquid will only render susceptible to uncleanness if it has the colour of blood.');">¹⁵</span>

Jastrow

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