Mischna zu Bamidbar 21:30

Some materials raise the mikveh up [to the required quantity] and do not make it invalid. Some make it invalid and do not raise it up [to the required quantity]; And some neither raise it up [to the required quantity] nor make it invalid. These raise it up to the required quantity and do not make the mikveh invalid. Snow, hail, frosted dew, ice, salt, and thin mud. Rabbi Akiva said: Rabbi Ishmael once argued against me saying; snow does not raise up the mikveh [to its required quantity]. But the men of Madeba testified in his name that he had once told them: go and bring snow and with it prepare a mikveh from the outset. Rabbi Yohanan ben Nuri says: hailstones are like drawn water. How do they raise it up [to the required quantity] and not render it invalid? If the mikveh contained forty seahs less one, and a se'ah of them fell in and made up [the required quantity], they thus make up [the required quantity] but do not render it invalid.