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נחש נחשת A SERPENT OF COPPER — He had not been told to make it of copper, but Moses said, “The Holy One, blessed be He, terms it נחש; I will therefore make it of נחשת — one term fitting the other term” (Genesis Rabbah 31:8).

AND MOSES MADE ‘NECHASH NECHOSHETH’ (A SERPENT OF BRASS). “He was not told to make it of brass. But Moses said: ‘The Holy One, blessed be He, called it a nachash (serpent); therefore I will make it of nechosheth (brass) — since the one word resembles the other [in sound].” This is Rashi’s language, based upon the words of our Rabbis.²⁵¹Bereshith Rabbah 31:8. But I do not understand this, for the Holy One, blessed be He, did not mention to Moses [the word] nachash, but said to him, Make thee ‘saraf’ (a fiery serpent)!²⁵²Verse 8. But the intention of the Sages is that Moses was guided by the substantive name thereof [which is nachash; and saraf is merely a particular description of a certain kind of serpent — a “fiery” serpent — hence he made the saraf which G-d mentioned to him from nechosheth (brass), corresponding with its substantive name].
It appears to me that the secret of this matter is that this is one of the ways of the Torah, every deed of which is a miracle within a miracle. Thus [the Torah] removes injury by means of the cause of the injury, and heals illness by means of the cause of the sickness, as the Rabbis have mentioned in connection with [the verse], and the Eternal showed him a tree,²⁵³Exodus 15:25. See Vol. II, p. 211. and as occurred with the salt that Elisha [cast] into the water.²⁵⁴II Kings 2:21. See Vol. II, p. 211, Note 259. Now it is a well-known medical principle that all people bitten by poisonous creatures become dangerously ill when they see them, or [even] when they [only] see their likeness, so that if people who have been bitten by a mad dog or other mad animals look into water,²⁵⁵“Or any mirror.” See Vol. I, p. 259. they see there the image of the dog or the attacker, and [this] can [cause them to] die, as is written in medical books and mentioned in the Gemara of Tractate Yoma.²⁵⁶Yoma 84a. Similarly doctors protect them from [people] mentioning in their presence the name of the animal that bit them, [and they forbid people] to mention it at all, because their minds cling to this thought and do not turn away from it altogether until it causes their death. [The doctors] have already mentioned that it is an empirical fact, amongst the wonders of reproduction, that if the urine of a person bitten by a mad dog is put in a glass receptacle after he has become rabidly sick, there will appear in that urine the likeness of the young of small dogs. And if you pass the liquid through a cloth and filter it, you will not find any trace of them at all; but if you return the liquid to the glass bottle, and it remains there for about an hour, you will again clearly see in it the small dogs. This is a true fact, and of the wonders of the powers of the soul. Now in view of all this, it would have been correct that the Israelites, who had been bitten by the fiery serpents, should not look upon a serpent, and should not mention it or bring it to mind at all. But the Holy One, blessed be He, commanded Moses to make for them the likeness of a fiery serpent, which was [the creature] that killed them. And it is well-known that these fiery serpents have red eyes and wide heads, and their bodies at their necks are like brass. Therefore Moses could not fulfill His command to make a fiery serpent except by making a serpent of brass, which appears similar to a fiery serpent. For had he made it of any other material, it would have had the likeness of a serpent, but not that of a saraf (a fiery serpent). And the Rabbis²⁵¹Bereshith Rabbah 31:8. who said [that Moses was to make a brass serpent] because “one word corresponds [in sound] to the other,” [meant to imply] that the mere mention of the name [saraf, which is a particular form of nachash, as explained above] was harmful [for the people who had been bitten, and yet G-d wanted to heal them through it]. The general intention [of this section, then] is that G-d commanded that they should be healed by the harmful agent whose nature is to kill; therefore they made its likeness in form and name, and when a person concentrated his gaze upon the brass serpent which resembled totally the offending agent, he lived. This was to make them realize that it is G-d [alone] Who sendeth death and maketh alive.²⁵⁷I Samuel 2:6.

נחש נחושת, after Moses had understood what G’d had intended with His command, he agreed to make this replica of a snake out of copper and not out of gold. Copper would be more likely to induce thoughts of remorse as they would associate its colour with the sensation of burning.