Talmud for Parah 12:5
אוֹחֵז הוּא הַטָּהוֹר בַּקַּרְדֹּם הַטָּמֵא בִּכְנָפוֹ, וּמַזֶּה עָלָיו. אַף עַל פִּי שֶׁיֵּשׁ עָלָיו כְּדֵי הַזָּיָה, טָהוֹר. כַּמָּה יְהֵא בַמַּיִם וְיִהְיֶה בָהֶם כְּדֵי הַזָּיָה. כְּדֵי שֶׁיַּטְבִּיל רָאשֵׁי גִבְעוֹלִין וְיַזֶּה. רַבִּי יְהוּדָה אוֹמֵר, רוֹאִים אוֹתָם כְּאִלּוּ הֵן עַל אֵזוֹב שֶׁל נְחשֶׁת:
One who is pure can grasp an impure axe in a fold of his clothing and have one sprinkle onto it. Even if [after the sprinkling] it has on it a [sufficient] amount of water for sprinkling, he is pure. [Generally carrying <i>chatat</i> waters renders one impure, but in this case the waters have already done their mitzva, as noted in the previous Mishna.] And how much must the waters be in order to be an amount for sprinkling? [Enough] that one may dip the tips of the stalks and sprinkle. Rabbi Yehuda says: We view them as if they were on a hyssop of copper [i.e. we also take into account water which has been absorbed and is no longer visible, since copper does not absorb].