The Mishnah (5:9) taught that if one poured liquid from a tahor utensil into a tameh utensil, the liquid in the first utensil remains tahor. In other words the concept of nitzuk chibur does not apply; the stream does not connect the liquids in both utensils.
The Gra (Eliyahu Raba, Taharot 8:9) asks an important question. Granted that the stream does not connect the liquid to make them one, nevertheless each drop in the stream should cause the next drop above it to become tameh. Recall the rabbinic decree regarding liquids that become tameh, that even if they came in contact with a sheni le’tumah they would be considered a rishon le’tumah and be able to pass on tumah (see Para 8:7). Consequently each drop should cause the next to be tameh until the liquid in the upper kli would be tameh!
The Gra therefore explains that this case must be where the lower utensil contained trumah liquid that became tameh through a tevul yom. A tevul yom is one that was tameh, immersed in a mikveh and is waiting for nightfall to become tahor. During that time this person is a sheni le’tumah. Importantly for our discussion, trumah that came into contact with a tevul yom is the exception to the above described gezeirah and the trumah cannot transmit tumah. The only way for the liquid in the upper kli to become tameh would be if it were considered connected to the tameh liquid in the bottom one.
The Rambam (Tumat Ochlin 7:1) however explicitly rules that even if liquid was poured directly onto a sheretz – an av ha’tumah – we still do not rule nitzuk chibur and the stream itself is tahor. How can the question of the Gra be answered?
The Sha’arei Yoshar (3:27) answers that in this case the tumah cannot be transferred from drop to drop. The reason is that the case is considered tumat beit setarim. In other words, the tumah is concealed and therefore cannot be a source of tumah. The Mishnah in Keilim (27:10) discussed a small patch that was tameh midras and tore in half making each half less than the minimum size to be susceptible to tumat midras, R’ Yosi ruled that each half would tahor even though they are large enough to contract other forms of tumah. The reason is that the point of contact of the two parts when the patch was complete was not visible. In this case as well, since the point where the drops connect is not recognisable, tumah is not transferred.
The Chazon Ish (5:10) however provides a different answer. He explains that the reason for the gezeirah is that if we made liquid that touched a sheni le’tumah tahor (as it should) then one might make a mistake and consider liquid that touched a sheretz also tahor when it is really tameh m’de’oraita. Consequently the gezeirah was made making liquids that came into contact with tameh food considered rishon le’tumah. In this case however, since there is a constant stream and each drop is never isolated there is no room for error and the gezeirah does not apply.
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