Gemara

97b: Some thighs were salted with the Gid ha'Nasheh inside. Ravina forbade them, and Rav Acha permitted them.

Question (Rav Acha, to Ravina): Do you forbid because Shmuel taught that salted food is like Rose'ach (boiling hot), and pickling is like cooking? Shmuel forbids the thigh only if it was cooked with the Gid. If it was roasted, it suffices to peel off the layer around the Gid!

Do not say that Shmuel said that salted food is like Rose'ach, i.e. like cooking. Since he said that pickling is like cooking, we infer that salting is not. Rather, it is like roasting!

This is left difficult.

134b (Beraisa): In a place where people are Moleg calves (pour boiling water over them, and eat them with the skin) one may not flay the Zero'a. (Rather, he must give it to the Kohen with the skin.)

Rishonim

Tur (YD 105, citing the Rashba): Some say that only heat of a Kli Rishon, which cooks, emits. A Kli Sheni, and Iruy (pouring from a Kli Rishon), which is like a Kli Sheni, do not emit. This is wrong. The heat of Beis ha'Shechitah (the neck at the time of Shechitah) and of Chelev of the flanks, emits, and this is less than a Kli Sheni. Rather, every heat through fire emits, even if it is not Yad Soledes Bo.

Rejection (Tur): These are different. The heat of Beis ha'Shechitah and of Chelev of the flanks are Mavli'a (cause what they touch to absorb) only Kedei Klipah. To make all the Heter absorb, the heat of Yad Soledes Bo is required. Perhaps the Rashba means that also when hot (food) falls into hot, it forbids only Kedei Klipah. I saw some Mechaberim say so. This is wrong. Rather, it totally forbids if it is Yad Soledes Bo.

Beis Yosef (DH Kosav ha'Rashba): The Rashba cannot mean that when hot falls into hot, it forbids only Kedei Klipah. Later in this Siman, the Tur infers from the Rashba that the heat of salting (which is less) forbids everything! Also, in Pesachim (76a) we say that when hot falls into hot, it forbids. When hot falls into cold, one must remove Kedei Klipah. We must say that hot into hot forbids more than Kedei Klipah!

Ran (31b DH v'Chasvu): Absorptions do not depend on cooking. The heat of Beis ha'Shechitah is less than that of a Kli Sheni, and even so it is Mavli'a (Chulin 8b).

Beis Yosef (DH veha'Rashba): The Rashba says that even if we consider Iruy like a Kli Sheni, Meligah it is forbidden. This implies that even after pouring into the Kli Sheni, one may not put in (meat), for even though a Kli Sheni does not cook, it emits and is Mavli'a. Heat of a Kli Sheni is no worse than Beis ha'Shechitah or the flanks shortly after Shechitah. Chachamim forbade putting them on meat, lest the Chelev flow and be absorbed in the meat. In some places, they permit Meligah of kids (Chulin 134b). This is after salting and removing the Chelev. Alternatively, it is in a Kli Sheni. However, even though a Kli Sheni does not cook, one should be concerned lest it causes Chelev to be absorbed. Absorptions do not depend on cooking. The heat of Beis ha'Shechitah is less than that of a Kli Sheni, and even so it is Mavli'a (Chulin 8b).

Poskim

Shulchan Aruch (YD 68:13): Some say not to do Meligah even in a Kli Sheni. If he did, it is permitted.

Shach (29,30): It is forbidden l'Chatchilah only if it is Yad Soledes Bo. The Heter for guests or for needs of Shabbos is even if it is Yad Soledes Bo.

Shulchan Aruch (105:2): The heat of a Kli Rishon that is Yad Soledes Bo cooks and totally forbids. The heat of a Kli Sheni does not cook. Some say that also, it does not emit (what is absorbed in the Kli), and it does not Mavli'a. Some say that it emits and is Mavli'a, and forbids Kedei Klipah (the thickness of a peel). It is proper to be concerned for this l'Chatchilah, but b'Di'eved it is permitted without a peel. It suffices to rinse.

Beis Yosef (DH v'Im): The Tur says that if Isur mixed with Heter and it is Yad Nichbis Bo (hot enough to scald the hand), it forbids like cooking. If he discusses a Kli Rishon, this is cooking itself! If he discusses a Kli Sheni, even Yad Soledes Bo does not cook, like Tosfos (Shabbos 40b DH u'Shma) says! It seems that the Tur himself says so in OC 318. It is difficult to distinguish between cooking regarding Shabbos and regarding Isurim. I say that the Tur said so because Stam cooking is in a Kli Rishon on the fire. The Tur teaches that there is one law for a Kli Rishon on the fire, after it was removed, and a Kli Sheni. It forbids like cooking only as long as it is Yad Soledes Bo. However, this is astounding. Tosfos wrote that since a Kli Rishon was on the fire, its walls are hot and it retains its heat a long time. Therefore, as long as Yad Nichbis Bo, it forbids. A Kli Sheni, even if it is Yad Soledes Bo, is permitted, since its walls are not hot, rather, they cool off. The Tur himself said so in OC 318! Rather, he discusses only a Kli Rishon. People think that this is only while it is on the fire, or is still boiling like when it was on the fire, but not after it was removed and stopped boiling. The Tur teaches unlike this. Even if it stopped boiling and it is off the fire, as long as it is Yad Soledes Bo, it cooks, like Tosfos said. I say that even if a Kli Sheni is boiling like a Kli Rishon, it does not cook. The Gemara said Stam that a Kli Sheni does not cook. This is a great general rule. It never cooks. However, below the Tur brings the Rashba, and says that a Kli Sheni forbids if it is Yad Soledes Bo. Therefore, it seems that the Tur discusses a Kli Rishon and Kli Sheni. As long as it is Yad Soledes Bo, even though it does not cook, it is Mavli'a and emits.

Taz (4): The Yam Shel Shlomo (Chulin 8:71) says that even though a Kli Sheni does not cook even if it is Yad Soledes Bo, and it does not emit and absorb at once, e.g. if Isur and Heter are inside, they do not forbid the other. However, if Isur fell into a Kli Sheni of Heter, or Heter fell into Isur, it absorbs and totally forbids, since it is Yad Soledes Bo, like a Kli Rishon, and all the more so Iruy.

Mishbetzos Zahav: Perhaps this means that if two cold solids fell into a Kli Sheni, it cannot heat them enough to absorb from each other. However, if one poured Isur from a Kli Rishon into Heter, they absorb. One could explain that even in this case he says that a Kli Sheni does not cause absorptions, unless one is clear (a liquid). My first Perush is primary.

Taz (4): I explain the Tur this way, for I was not satified with the Perushim I saw. Why did the Tur mention hot falling into hot? The Rashba did not discuss this! Rather, the Tur learned from the words of the Rashba 'every heat through fire emits, even if it is not Yad Soledes Bo.' He gave one stringent rule to equate all levels of heat in a Kli Sheni. All are like the most stringent, i.e. hot into hot, even the most lenient, i.e. not Yad Soledes Bo. All forbid everything. He said that there is no proof from the heat of Beis ha'Shechitah (which is not Yad Soledes Bo, yet it forbids), for it forbids only Kedei Klipah. Rather, he must hold that a Kli Sheni always forbids only Kedei Klipah, i.e. even when it is not Yad Soledes Bo. This answers the Beis Yosef's astonishment at the Tur. The Tur said that this is not true for a Kli Sheni. Even when cold falls into hot, it forbids all only if it is Yad Soledes Bo, and all the more so hot into hot. To emit its absorptions into something else, it is like a Kli Rishon. Even though it is unlike a Kli Rishon regarding cooking, regarding emissions and absorptions there is no difference. If so, the Rashba should not have made one general rule of the law of a Kli Sheni. It turns out that even in a Kli Sheni that is Yad Soledes Bo, everything is forbidden, like I said in the name of Maharshal. The Beis Yosef explained the conclusion of the Tur this way, that Yad Nichbis Bo forbids like cooking, i.e. even in a Kli Sheni. If so, all the stringencies of a Ben Yomo mentioned in Siman 94 apply also to a Kli Sheni, unlike the Rema. In opractice, one should be stringent like the Maharshal, especially since the Tur holds like him. In a case of a big loss, one may rely on the Beis Yosef in Siman 68 and the Rema here in Sa'if 3 to be lenient b'Di'eved to say that a Kli Sheni does not emit and Mavli'a at all, even if it is Yad Soledes Bo. If it is not Yad Soledes Bo, rinsing suffices.

Shach (5): One should be careful even for something that is normally rinsed afterwards. Even though one may put it on cold Isur, l'Chatchilah one should be careful about a Kli Sheni. R. Tam holds that Iruy cooks Kedei Klipah, like a Kli Rishon. The Rashbam agrees that Iruy (of an Isur, or milk over meat) is Mavli'a Kedei Klipah, just it does not emit and Mavli'a at the same time. Therefore, one may do Iruy over a chicken (if it causes blood to be emitted, it will not be reabsorbed). He holds that a Kli Sheni does not Mavli'a at all. R. Tam holds that Iruy is not totally Mavli'a. This is unlike Maharshal (Teshuvah 62), who says that it is totally Mavli'a. The Prishah (68:8) says that R. Tam holds that it cooks totally. This is astounding. R. Tam and all who hold like him, including Semag, hold that a Kli Sheni is Mavli'a Kedei Klipah. This is unlike the Maharshal said in Isur v'Heter (33). Semag says that even a Kli Cheres absorbs in a Kli Sheni, and all the more so food absorbs. Many Rishonim and Acharonim hold that it does not absorb at all; the Rema says so in Toras Chatas (33:1). However, in OC 451:1 the Rema connotes that a Kli Cheres used with Chametz in a Kli Sheni is forbidden! Perhaps this is a stringency of Chametz. In any case, one should forbid a Kli Cheres if it is not a big loss. The Ramban and Rashba hold that even a copper Kli (Sheni) absorbs.

Shach (5): Some Poskim say that even though a Kli Sheni does not cook, it emits and absorbs. Similarly, one should peel a food if it is not a big loss. In any case we do not totally forbid in a Kli Sheni. In his Teshuvah, the Maharshal said that it absorbs only Kedei Klipah. Why did he totally forbid in Yam Shel Shlomo (8:71)? The Bach is stringent to say that a Kli Sheni emits and absorbs at once, and totally forbids. This is astounding. We are stringent to say that Iruy Kli Rishon forbids Kedei Klipah! Therefore, regarding Shabbos and meat and milk it is forbidden. Even if the flow was interrupted, Kedei Klipah is forbidden (Pesachim 76a). Even though the Rema (76:10) does not require Kedei Klipah for Iruy Kli Rishon over a chicken, that is because it does not emit and absorb at the same time. However, it absorbs. Tosfos in Zevachim, the Mordechai, Hagahos Ashri and the Ran say so. Kedei Klipah suffices. This is clear from the Poskim, Tur and Shulchan Aruch (91:4). If boiling milk fell on meat, one peels the meat, and the rest of the meat is permitted. Any heat less than Yad Soledes Bo does not forbid at all, even in a Kli Rishon on the fire. All the Poskim say so, just Maharshal was hesitant to be lenient about a hot Kli Rishon on the fire even if it is not Yad Soledes Bo, for Chachamim made fences for a Kli Rishon, like the Yerushalmi says. I do not understand. The Yerushalmi is stringent about a Kli Rishon she'Ein ha'Yad Sholetes Bo', i.e. it is too hot to touch, i.e. Yad Soledes Bo! All the Poskim say so, including Toras Chatas 57:12. However, Toras Chatas 23:3 rules like the Rashba and Tur who obligate Kedei Klipah when cold falls into hot that is not Yad Soledes Bo. One should be stringent like this, even though it seems that the other Poskim disagree.

Gra (13): The Yerushalmi says that Chachamim made a fence for a Kli Rishon, but not for a Kli Sheni. A Kli Sheni does not cook even if it is Yad Soledes Bo.

Gra (68:33): If it is not Yad Soledes Bo, even a Kli Rishon does not cook (Shabbos 40b). Chulin 104b proves that a Kli Sheni does not cook even if it is Yad Soledes Bo.

Pischei Teshuvah (6, on Shach 5): Teshuvas Beis Yakov (20) rules like Maharshal, because the Yerushalmi supports him. According to the Shach, the Yerushalmi is not a stringency. It is letter of the law! The Shach himself admitted to this! Also Teshu'as Chen 6 said that one should be stringent like the Maharshal if it is not a big loss. If it was noit on the fire, one need not be stringent at all.