Your question is asked by the Rash in Taharos 3:2 (at the end). He concludes that there is a difference between Kabalas Tum'ah And Hashakah to a Mikvah. As far as Kabalas Tum'ah is concerned, we know that touching is not considered connected. However, as far as the Mikvah is concerned, since one may be immerse in the snow itself, through touching the Mikvah it is as if the whole snow has become a Mikvah.

D. Zupnik

You will find the Rash printed in any standard Gemara Nidah of the Vilna Shas.

Touching is not considered a Chibur in Ochlim. With regard to Mashkim, in general a Chibur is made by being in one "Ashboren," which means that it is not on a slope or otherwise resting in a manner that it will flow apart. Otherwise, the water in the Mikvah itself would also not be considered to be connected. In true Mashkim, Ashboren is a Chibur even for Tum'ah. Snow is different, for we treat it like Ochlim. The question then arises, if it is Ochlin how can it be a Chibur? The Rash answers that since we allow it to attach to a Mikvah, we treat it as Mashkin as far as the Hashakah is concerned.

D. Zupnik

When Tum'ah touches something, that thing alone becomes Tamei. (Although in some circumstances that thing can then proceed to be m'Tamei something else, that is a new Tum'ah and not a result of the original touching. Here we are dealing in a case where the snow that became Tamei cannot proceed to be m'Tamei something else (i.e. the snow that is next to it). This may be because we are talking here where the original cause for the Tum'ah was a Tevul Yom (Gra, Tosefta Taharos 2:3) or for other reasons.)

We need to determine whether all the pile of snow is considered one item or whether each of its constituent parts is an individual item. The Gemara brings a Beraisa that rules that only the part touched by the Tum'ah is Nitma which indicates to us that the Tana holds that each particle of snow is considered to be a separate item.

The Beraisa continues by saying that Hashakah on part of the pile of snow is Metaher the whole pile. This appears to be a contradiction because it seems to indicate that we consider the whole pile of snow as one item. However, this is not the case. Each particle is considered separate, and nonetheless Hashakah works for the whole pile without the whole pile needing to be immersed in the Mikvah. This is because Hashakah is Metaher by making the water or snow that touches the Mikvah Batel to the Mikvah and hence a part of the Mikvah. Once the snow that touches the Mikvah has become part of the Mikvah, it in turn can be Metaher the particles of snow that touch it by making those particles themselves a part of the Mikvah. This process will repeat itself as necessary until all the snow is considered to be part of the Mikvah and Tahor.

As for how one may Tovel in the snow, in fact the Beis Yosef (YD 201) brings a Machlokes Rishonim on the issue. A minority of Rishonim are of the opinion that Tevilah in snow is only valid after the snow has melted. One of the reasons given is that for Tevilah in snow to be valid, it would have to be in a way that the body of the Toveil touches forty Se'ah of snow at one instant, something that is considered impossible. However, the majority of Rishonim, as well as the Beis Yosef in Shulchan Aruch and the Rama (YD 201:30) permit Tevilah in frozen snow.

You ask how this is possible if the snow is not considered attached. We may answer that since it is all in one place, even if it is not considered one item, since his body is immersed in the snow it is considered a Tevilah. We can prove this from the fact that falling water is not considered one item (Gittin 16a) but one may still be Toveil in a wave that falls on oneself (Chulin 31a).

You can find the Rosh next to the Mishnayos at the back of a Nidah Gemara.

Dov Freedman