Continuing down this research hole, I checked out Patricia Noel's Benjamin Rush’s Lectures on the Mind. For context, the book says Benjamin Rush has been called "the father of American psychiatry." The anecdote in question was part of Rush's regular scripted lectures on psychology that he gave his students from 1791 to 1811, expanding it over the years, and publishing them as books. Patricia Noel's book is Rush's lectures with annotations. The 18-year-old was not Rush's own patient, but one he'd heard about. He brought it up in a part of his lecture about somnambulism, which usually means sleepwalking, but he's using it for altered states like this too. This is the part of Rush's lecture where he brings up that anecdote:
"I [should] have been much puzzled to find out the cause of this curious phenomenon [somnambulism and lack of memory of it] in the operation of the mind, had I not met with the following history of a case in Connecticut in a letter from Dr. [Joseph] Lathrop of Springfield [Massachusetts, not Connecticut], to the late Dr. [Ezra] Stiles, President of Yale College in New Haven. [...] The letter is dated July 18, 1791.
" 'A young man in this town [...] some years since, was in consequence of bathing water, visited with a peculiar kind of disorder, which operated by paroxisms [sic]. When a fit seized him he would at first fall down; but in a moment or two rise, possessed of an agility far superior to what was natural. In two or three hours, and sometimes sooner, the fit would pass off and leave him in his usual state, and, to appearance, in health. But what was most remarkable in his case, was the state of his mind. While he was in a fit, he perfectly remembered things which had occured [sic] in the preceding fits, but nothing which had happened in the intervals, or in the time prior to his disorder. In the intervals, all his fits, and everything which had passed in them, were totally obliterated; but he could distinctly recollect the occurrences of the former intervals. The time of his fits appeared to him in continuity, as did also his healthful periods--when one was perfect the other was lost. If in the time of a fit, he took up any business, he would drop it when the fit ceased, without any recollection of the matter; and when the fit returned he would resume the business without any idea of his having discontinued it. The case was the same, if he undertook anything in the intervals of his disorder. In short, he seemed to have two distinct minds which acted by turns independently of each other. In the space, I think, of about two years, in the use of a particular remedy, his fits left him, and he was reduced to a simple consciousness. The remedy which cured him, or deprived him of one of his souls, I have not been able to learn, the family having lost the recipe.
" 'The above account I received from his father, and from others of his family.' [A footnote here says that this quotation of the letter from Dr. Lathrop had been "clipped from an unidentified newspaper."]
"I shall hereafter mention the case of a lady, nearly similar to the one I have read, induced by derangement. [...] The Countess of Laval, a French lady, was ill of a fever attended by delirium, in which she spoke a language with great fluency, which none of her attendants understood. An old Welch [sic] woman who had nursed her when she was a child, was sent for to see her. She understood every word she said. It was the Welch language, which she had learned when a child, but of which she could not speak a word before nor after her recovery. [Footnote: James Burnett (1714-1799), Lord Monboddo, Ancient Metaphysics (London, 1782), II pp. 217-224. Non, RC. Communicated to Monboddo in a letter from Hans Stanley (?1720-1780). Stanley was a British politician who served in Paris. The Countess is unidentified.]
"During my residence in Edinburgh, I conducted a fellow student who was much too intoxicated to walk alone, to his lodgings [...] On our way, my fellow student spoke to me only in the French language. The next morning, [...] I mentioned to him his uncommon fluency in speaking French. He was much surprised at this information, for although he had learned to speak French when a boy, he had entirely forgotten it for many years.
" [...] madmen frequently remember nothing of what passed when they were deranged; but when they relapse, they distinctly remember the former subjects of their derangement. We had an insane sailor in our hospital some years ago, who believed himself an admiral, and assumed the authority and consequence connected with that rank in a navy. This man recovered, but relapsed some time afterwards, and was again brought in the hospital when he resumed the conversation and manners of his former paroxysm of insanity." (pp. 668-672)
I was hoping to track down some more information about the 18-year-old American case, but it looks like that will be harder to find. We still don't know his name, and now there's some confusion about whether he was located in Massachusetts or Connecticut, or how many years previous to 1791 his case had happened. Even if we did find the unidentified newspaper that the story had originally appeared in, it seems unlikely to shed any more light on it.
no subject
"I [should] have been much puzzled to find out the cause of this curious phenomenon [somnambulism and lack of memory of it] in the operation of the mind, had I not met with the following history of a case in Connecticut in a letter from Dr. [Joseph] Lathrop of Springfield [Massachusetts, not Connecticut], to the late Dr. [Ezra] Stiles, President of Yale College in New Haven. [...] The letter is dated July 18, 1791.
" 'A young man in this town [...] some years since, was in consequence of bathing water, visited with a peculiar kind of disorder, which operated by paroxisms [sic]. When a fit seized him he would at first fall down; but in a moment or two rise, possessed of an agility far superior to what was natural. In two or three hours, and sometimes sooner, the fit would pass off and leave him in his usual state, and, to appearance, in health. But what was most remarkable in his case, was the state of his mind. While he was in a fit, he perfectly remembered things which had occured [sic] in the preceding fits, but nothing which had happened in the intervals, or in the time prior to his disorder. In the intervals, all his fits, and everything which had passed in them, were totally obliterated; but he could distinctly recollect the occurrences of the former intervals. The time of his fits appeared to him in continuity, as did also his healthful periods--when one was perfect the other was lost. If in the time of a fit, he took up any business, he would drop it when the fit ceased, without any recollection of the matter; and when the fit returned he would resume the business without any idea of his having discontinued it. The case was the same, if he undertook anything in the intervals of his disorder. In short, he seemed to have two distinct minds which acted by turns independently of each other. In the space, I think, of about two years, in the use of a particular remedy, his fits left him, and he was reduced to a simple consciousness. The remedy which cured him, or deprived him of one of his souls, I have not been able to learn, the family having lost the recipe.
" 'The above account I received from his father, and from others of his family.' [A footnote here says that this quotation of the letter from Dr. Lathrop had been "clipped from an unidentified newspaper."]
"I shall hereafter mention the case of a lady, nearly similar to the one I have read, induced by derangement. [...] The Countess of Laval, a French lady, was ill of a fever attended by delirium, in which she spoke a language with great fluency, which none of her attendants understood. An old Welch [sic] woman who had nursed her when she was a child, was sent for to see her. She understood every word she said. It was the Welch language, which she had learned when a child, but of which she could not speak a word before nor after her recovery. [Footnote: James Burnett (1714-1799), Lord Monboddo, Ancient Metaphysics (London, 1782), II pp. 217-224. Non, RC. Communicated to Monboddo in a letter from Hans Stanley (?1720-1780). Stanley was a British politician who served in Paris. The Countess is unidentified.]
"During my residence in Edinburgh, I conducted a fellow student who was much too intoxicated to walk alone, to his lodgings [...] On our way, my fellow student spoke to me only in the French language. The next morning, [...] I mentioned to him his uncommon fluency in speaking French. He was much surprised at this information, for although he had learned to speak French when a boy, he had entirely forgotten it for many years.
" [...] madmen frequently remember nothing of what passed when they were deranged; but when they relapse, they distinctly remember the former subjects of their derangement. We had an insane sailor in our hospital some years ago, who believed himself an admiral, and assumed the authority and consequence connected with that rank in a navy. This man recovered, but relapsed some time afterwards, and was again brought in the hospital when he resumed the conversation and manners of his former paroxysm of insanity." (pp. 668-672)
I was hoping to track down some more information about the 18-year-old American case, but it looks like that will be harder to find. We still don't know his name, and now there's some confusion about whether he was located in Massachusetts or Connecticut, or how many years previous to 1791 his case had happened. Even if we did find the unidentified newspaper that the story had originally appeared in, it seems unlikely to shed any more light on it.