Laudator Temporis Acti

In the chapter on Visitors in Walden, Thoreau describes but does not name Alex Therien:

His only books were an almanac and an arithmetic, in which last he was considerably expert. The former was a sort of cyclopaedia to him, which he supposed to contain an abstract of human knowledge, as indeed it does to a considerable extent. I loved to sound him on the various reforms of the day, and he never failed to look at them in the most simple and practical light. He had never heard of such things before. Could he do without factories? I asked. He had worn the home-made Vermont gray, he said, and that was good. Could he dispense with tea and coffee? Did this country afford any beverage beside water? He had soaked hemlock leaves in water and drank it, and thought that was better than water in warm weather.

So great is my ignorance that I wondered why drinking hemlock didn't kill Therien the way it did Socrates. I didn't realize that the North American evergreen tree called hemlock (genus Tsuga) has nothing in common with the poisonous hemlock plant of Europe (species Conium maculatum).

Danielle S. Allen, The World of Prometheus: The Politics of Punishing in Democratic Athens (Princeton: Princeton, University Press, 2000), p. 233 (via Google Book Search), writes:

In contrast, discussions of execution by hemlock that occur outside of oratory appear primarily in the specialist literature of the philosophers and in texts where death by hemlock takes on an especial thematic prominence. It is Xenophon who describes the death of Theramenes by hemlock in 404 (Xen. Hell. 2.3.54-56), Xenophon and Plato who mention hemlock in describing the death of Socrates in 399 (Xen. Apol. 7; Plat. Phaed. 1187a7-8), and Plutarch who describes the executions of Phocion, Thudippus, Hegemon, Nicocles, and Pythocles in 318 (Plut. Phoc. 37-38).

The reference to Plato's Phaedo seems to be garbled. I think it should be 117a - 118a, the famous account of the execution of Socrates, which in H.N. Fowler's translation reads:

[117a] The boy went out and stayed a long time, then came back with the man who was to administer the poison, which he brought with him in a cup ready for use. And when Socrates saw him, he said: "Well, my good man, you know about these things; what must I do?" "Nothing," he replied, "except drink the poison and walk about [117b] till your legs feel heavy; then lie down, and the poison will take effect of itself."

At the same time he held out the cup to Socrates. He took it, and very gently, Echecrates, without trembling or changing color or expression, but looking up at the man with wide open eyes, as was his custom, said: "What do you say about pouring a libation to some deity from this cup? May I, or not?" "Socrates," said he, "we prepare only as much as we think is enough." "I understand," said Socrates; [117c] "but I may and must pray to the gods that my departure hence be a fortunate one; so I offer this prayer, and may it be granted." With these words he raised the cup to his lips and very cheerfully and quietly drained it.

Up to that time most of us had been able to restrain our tears fairly well, but when we watched him drinking and saw that he had drunk the poison, we could do so no longer, but in spite of myself my tears rolled down in floods, so that I wrapped my face in my cloak and wept for myself; for it was not for him that I wept, [117d] but for my own misfortune in being deprived of such a friend. Crito had got up and gone away even before I did, because he could not restrain his tears. But Apollodorus, who had been weeping all the time before, then wailed aloud in his grief and made us all break down, except Socrates himself. But he said, "What conduct is this, you strange men! I sent the women away chiefly for this very reason, that they might not behave in this absurd way; for I have heard that [117e] it is best to die in silence. Keep quiet and be brave."

Then we were ashamed and controlled our tears. He walked about and, when he said his legs were heavy, lay down on his back, for such was the advice of the attendant. The man who had administered the poison laid his hands on him and after a while examined his feet and legs, then pinched his foot hard and asked if he felt it. He said "No"; then after that, [118a] his thighs; and passing upwards in this way he showed us that he was growing cold and rigid. And again he touched him and said that when it reached his heart, he would be gone. The chill had now reached the region about the groin, and uncovering his face, which had been covered, he said—and these were his last words—"Crito, we owe a cock to Aesculapius. Pay it and do not neglect it." "That," said Crito, "shall be done; but see if you have anything else to say." To this question he made no reply, but after a little while he moved; the attendant uncovered him; his eyes were fixed. And Crito when he saw it, closed his mouth and eyes.

Note that the word hemlock (κώνειον) does not actually occur in this passage from the Phaedo. Unless I'm mistaken, it occurs nowhere in the Phaedo. I also don't find hemlock mentioned in Xenophon's Apology 7, or anywhere else in that work. Either Allen is wrong in her assertion that "Xenophon and Plato ... mention hemlock in describing the death of Socrates in 399," or I'm missing something.

Later authors (e.g. Lucian, Dialogues of the Dead 4.1) of course often explicitly refer to hemlock in connection with Socrates' death, but I don't know who was the first to do so.

Mention of hemlock as a means of execution does occur in the orators:

• Andocides 3.10 (tr. E.J. Maidment): However, I have heard people saying before now that the result of our last peace with Sparta was the installation of the Thirty, the death of many people by the hemlock-cup, and the exile of others.
  
• Lysias 12.17 (tr. W.R.M. Lamb): Polemarchus received from the Thirty their accustomed order to drink hemlock, with no statement made as to the reason for his execution: so far did he come short of being tried and defending himself.
  
• Lysias 18.24-25 (tr. W.R.M. Lamb): I have nobody, gentlemen, whom I can put up here to plead on our behalf: for some of my kinsmen, after giving proof of their valour in promoting the greatness of the city, have perished in the war; others, in the defense of the democracy and of your freedom, have drunk hemlock under the Thirty.

I haven't seen Robert J. Bonner, "The use of hemlock for capital punishment" in Athenian Studies (Cambridge: Harvard University Press, 1940; rpt. New York: Arno Press, 1973), pp. 299-302.