QUESTION: The Gemara (according to the explanation of TOSFOS, DH Chatzi Kav) explains that when the Mishnah (49b) states that "Chatzi Kav Atzamos" is Metamei through Ohel, its objective is to teach that less than a Chatzi Kav (i.e. a Rova Kav) is not Metamei through Ohel but is Metamei through Maga and Masa. The Gemara asks why the Mishnah needs to teach this when the same Mishnah already states that an Etzem k'Se'orah is Metamei through Maga and Masa. If an Etzem k'Se'orah is Metamei through Maga and Masa, certainly less than a Chatzi Kav is Metamei. The Gemara answers that the Mishnah refers to less than a Chatzi Kav of bones that are crushed such that there is not a single bone that is the size of an Etzem k'Se'orah. One might have thought that such a pile of crushed bones is not Metamei through Maga and Masa, and therefore the Mishnah needs to teach that even in such a state it is Metamei through Maga and Masa.

The Acharonim (BIRKAS ROSH, CHAZON ISH) point out that the Chidush of the Mishnah cannot be that it is Metamei through Maga, because in order to touch a Chatzi Kav of pulverized bones one must touch individual particles of bone, and it is not possible to touch all of the individual particles of bone at the same time. Moreover, even if one manages to touch all of the particles at the same time, they will not be Metamei. The Gemara in Chulin (124b) teaches that when one touches two different items (that together comprise a Shi'ur) at the same time by coincidence ("Noge'a v'Chozer v'Noge'a"), they are not Metamei him, since each item is a separate entity and is not connected to the other. Rather, one becomes Tamei only when he touches one item which has in itself the entire Shi'ur. Therefore, a Chatzi Kav of pulverized bone cannot be Metamei through Maga. Rather, the Mishnah's Chidush is that it is Metamei through Masa, through carrying the entire collection of pulverized bone.

The SIDREI TAHAROS in Maseches Ohalos (2:1) asks that if the Mishnah is teaching that pulverized bone is Metamei through Masa, why is that a Chidush? We know that an Etzem k'Se'orah is Metamei through Masa, and pulverized bone is comprised of many pieces of Etzem k'Se'orah which were made into tiny pieces. The Gemara earlier (52a) teaches that an Etzem k'Se'orah that was split into two is Metamei through Masa. The Gemara there explains that this is the view of Rebbi Akiva and the Chachamim, and the only one who disagrees is Rebbi Yochanan ben Nuri. Why, then, does the Mishnah here need to teach that when an Etzem k'Se'orah is crushed up it is Metamei through Masa? There is no difference between an Etzem k'Se'orah which was split into two parts and an Etzem k'Se'orah which was split into a hundred or a thousand parts. If one carries all of it, it should be Metamei him.

(a) The simple answer is that the Gemara understands that the Mishnah follows the opinion of Rebbi Yochanan ben Nuri who maintains that an Etzem k'Se'orah which was split into two is not Metamei. Nevertheless, when there is a significant amount, such as a Rova Kav, of crushed bones, the Halachah is that they are Metamei through Masa.

(b) The CHAZON ISH suggests that although an Etzem k'Se'orah which was split into two still retains its status as an Etzem k'Se'orah and therefore is Metamei when one carries all of the different pieces together, nevertheless when it is pulverized into powder the pieces of bone are no longer recognizable as bone and are no longer called "Etzem" at all. Accordingly, one might have thought that they are not Metamei through Masa, and therefore the Mishnah teaches that a Rova Kav of such bones is Metamei through Masa.

(It is not clear whether an amount smaller than a Rova Kav, such as crushed up bones the size of a k'Se'orah, are also Metamei through Masa according to the Chazon Ish. Is the Gemara teaching that crushed up bones do retain the status of "Etzem" to a certain degree (and thus even an amount smaller than a Rova Kav is Metamei through Masa), or is the Gemara teaching that even though crushed up bones do not retain the status of "Etzem" when there is only a small amount, they are Metamei when there is a Rova Kav?)