Reference for Mikvaot 7:1
יֵשׁ מַעֲלִין אֶת הַמִּקְוֶה וְלֹא פוֹסְלִין, פּוֹסְלִין וְלֹא מַעֲלִין, לֹא מַעֲלִין וְלֹא פוֹסְלִין. אֵלּוּ מַעֲלִין וְלֹא פוֹסְלִין, הַשֶּׁלֶג, וְהַבָּרָד, וְהַכְּפוֹר, וְהַגְּלִיד, וְהַמֶּלַח, וְהַטִּיט הַנָּרוֹק. אָמַר רַבִּי עֲקִיבָא, הָיָה רַבִּי יִשְׁמָעֵאל דָּן כְּנֶגְדִּי לוֹמַר, הַשֶּׁלֶג אֵינוֹ מַעֲלֶה אֶת הַמִּקְוֶה. וְהֵעִידוּ אַנְשֵׁי מֵידְבָא מִשְּׁמוֹ שֶׁאָמַר לָהֶם, צְאוּ וְהָבִיאוּ שֶׁלֶג וַעֲשׂוּ מִקְוֶה בַּתְּחִלָּה. רַבִּי יוֹחָנָן בֶּן נוּרִי אוֹמֵר, אֶבֶן הַבָּרָד, כַּמָּיִם. כֵּיצַד מַעֲלִין וְלֹא פוֹסְלִין. מִקְוֶה שֶׁיֶּשׁ בּוֹ אַרְבָּעִים סְאָה חָסֵר אַחַת, נָפַל מֵהֶם סְאָה לְתוֹכוֹ וְהֶעֱלָהוּ, נִמְצְאוּ מַעֲלִין וְלֹא פוֹסְלִין:
There are those that raise [i.e. that count towards the measure of it waters to reach forty <i>se'ah</i>, to complete] a <i>mikveh</i> [a gathering of water collected by natural means and immersed in for purification] and do not invalidate it, [those that] invalidate but do not raise, [and those that] do not invalidate and do not raise. These raise and do not invalidate: snow, and hail, and frost, and ice, and salt, and mud that can be poured. Rabbi Akiva says: Rabbi Yishmael would rule against me, saying, "Snow does not raise a <i>mikveh</i>." But the people of Midva testified in his name that he said to them, "Go out and bring snow to make a <i>mikveh</i> from the outset." Rabbi Yochanan ben Nuri says: A hailstone is like water. How do these raise and not invalidate? If there is a <i>mikveh</i> containing forty <i>se'ah</i> [a specific unit of volume] less one, and a <i>se'ah</i> [of one] of these fell into it, and raised it [to be a complete <i>mikveh</i> containing forty <i>se'ah</i>], we find thus that these raise and do not invalidate.