Zevachim 25B

ורידין לתוך הכלי. איתמר נמי, אמר רב אסי אמר רבי יוחנן: ורידין צריך שיראו אויר כלי.

the veins of the offering into the vessel, so that all the blood will empty into it. It was also stated: Rav Asi says that R' Yoḥanan says: The veins must see the airspace of the vessel.

בעא מיניה רבי אסי מרבי יוחנן: היה מקבל, ונפחתו שולי מזרק עד שלא הגיע דם לאויר ⁠– מהו? אויר שאין סופו לנוח ⁠– כמונח דמי, או לא כמונח דמי?

§ R' Asi raised a dilemma before R' Yoḥanan: If the priest was collecting the blood, and the bottom of the bowl broke before the blood reached the bowl’s airspace, what is the halakha? Is blood that enters the airspace of a bowl that will not ultimately allow it to settle inside nevertheless considered as though it settled inside the vessel, so that when it spills through the break it is considered to have already been collected in the bowl? If so, it remains fit for sprinkling. Or is it not considered as though it settled, but is instead considered to have spilled directly from the animal’s neck onto the floor, and is therefore disqualified?

אמר ליה, תניתוה: חבית שמונחת תחת הזינוק ⁠– מים שבתוכה ושבחוצה לה, פסולין. צירף פיה לזינוק ⁠– מים שבתוכה פסולין, ושבחוצה לה כשירין.

R' Yoḥanan said to him: You learned the answer in a baraita: With regard to a jug that is placed under a pipe to catch water running from it, both the water that has already flowed inside the jug and that which is still outside it but about to fall in are unfit to be used as water of purification, which purify one who contracts impurity from a corpse. The water of purification must be flowing water, and all of this water is considered to have been collected in the jug. But if one joined the mouth of the jug to the pipe, the water that is inside the jug is unfit, but water that is still outside it is fit, even though it will ultimately fall in, since it is still in the pipe and is not considered to be above the airspace of the jug. One can infer from the baraita that all liquid that enters the airspace of a vessel is considered to have been collected in it.