Some of you may know that I've been spending a bit of time trying to antedate OED's citations -- not checking all of them, of course, but just those that seem to be the best candidates.

OED's first cite for mad scientist is from 1908, but I uncovered it in a tall tale that appeared in several newspapers from 1893 to 1895. It's a long tall tale but a fun one, and those interested will find it below, complete with op-scan errors.

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NOT MURDER
. . .Imagine a mob of men as hungry as a pack of wolves whose frenzy has been fired by the scent of blood and in its midst a fellow being, bound hands and feet, with the noose of the lynchmen around his neck. I came suddenly upon such a scene in one of the settlements on the Rio Grande, whither I had journeyed from England to find a brother whom I had not seen for many years.
. . .As I reined up my horse near the crowd and glanced over the exciting spectacle an involuntary cry left my lips as I recognized in the victim him whom I had come so far to see.
. . .He saw and knew me instantly, and though no word came from him his pale, mute lips appealed to my heart for help. My appearance caused a hesitation on the part of the leaders.
. . .I quickly learned that a stranger in those parts had been killed that morning near a small body of water a short distance from the settlement and that my brother had been caught in the very act of striking the deathblow. Not one of the spectators seemed to have any doubt of his guilt, but who is so hardened as to condemn his own brother?
. . .I shuddered as I realized Almont's peril and in my own heart judged him innocent of any crime.
. . ."He is my brother,'' I cried.
. . .•'Hark!" shouted one of the onlookers, "the brother of the murdered man is here for justice. Up with Mm, boys!"
. . ."Hold," I commanded, rising in my stirrups as I spoke;' 'blood calls for blood. Let me have hold of that rope."
. . .The mistake in my identity had suggested a way in which I might enable my brother to escape, and I resolved to attempt it at whatever risk. Fortunately there was little family resemblance between us.
. . .Urging my horse forward, the crowd parted, allowing me to reach his side, when I dismounted, ostensibly to examine the slipping noose.
. . ."It will do," I said loud enough to be heard by all "Now help me to lift him on the back of my horse. We want to do this job in some shape."
. . .Willing ones sprang to my assistance, but in the brief interval I cut the prisoner's bonds so that they held only by a thread and arranged the noose so that it could be thrown off as soon as his arms were free.
. . .I knew few horses could match mine in speed, and once he had cleared the throng my brother would be comparatively safe.
. . .He understood my intentions, and the moment we lifted him upon the horse he wrenched his arm free, threw off the noose, dropped into the saddle, and giving the animal a smart blow dashed through the crowd like a whirlwind, and in a few moments was beyond pursuit.
. . .Of course there was loud reviling over his escape, but I appeared so anxious for his recapture that no blame was attached to me. To carry out the deception I had the body of the stranger carefully buried and remained in the place until I deemed it safe to depart.
. . .It was nearly three months before I met my brother in London, whither he had fled, and then he thanked me with tears in his eyes for my daring assistance in his escape from the lyncliers. To my surprise, however, he evaded the subject of the murder, saying simply that no crime had been committed. I did not feel like pressing the matter, so the affair was not mentioned again, though it has haunted my mind ever since. Last week my brother died with no kindred near him, and today's post has brought me a manuscript containing a startling 'revelation.
. . .In justice to my brother's name, as well as my own satisfaction, I am prompted to give to the world the strangest confessions ever made. The following is his account as he wrote it for me:
. . ."When this is read, I shall have passed beyond the tribunal of man, so I wish to impress upon you that I am about to record faithfully an experience which I sincerely hope will fall to the lot of no other person.
. . ."I was alone in my room late one dark, stormy night when I heard a rap on the door, which I fancied at first was but the wind shaking it on its hinges. But it was repeated louder than before. I bade the applicant, whoever he might be. to come in, without looking up from the book which held my attention.
. . ."A momept later the door was opened and with the gust of wind which sent every light object in the room flying topsy turvy a man entered the apartment with quick, catlike steps.
. . ." 'Pardon me for the unreasonable hour at which I call,' he said in a clear, crisp tone, 'but I suppose doctors get used to all sorts of calls.'
. . ." 'Certainly,' I replied, I fear somewhat impatiently, as he had interrupted me at a time when I did not like to be disturbed. 'What can I do for yon?
. . ." 'Oh, I do not come for professional assistance,' he hastened to say, evidently reading my thoughts. 'Mine is strictly a business calL Are you at liberty for a few minutes?' w
. . ." 'Yes, but the hour is late, so I trust you will be as brief as possible.' " 'Dr. Barlow, how much are yon worth?
. . ." 'Enough to made life comfortable for myself,' I replied. 'If you have no more important question than that, our interview might as well come to an end at once.'
. . ." 'Pardon me, I will come to business. As I told you, I am a professor of science, and I have made a discovery which is worth millions—yes, sir, millions.
. . ." 'I need not tell you of the anxious days and sleepless nights it has cost me. No matter; I have succeeded at last, and you are the first man I have ever approached with my secret. I did not do that until I was satisfied you were the safest one I could find.'
. . ."As be spoke he opened a small bag which he carried and took out three or four vials to place upon the table.
. . ." 'Education based upon scientific research,' he remarked, 'has made a startling advance within the past few years. But no man has gone further into the unfathomable depths than myself. You have a basin of water here. Pardon me if I appropriate it to my own use.'
. . ."I bowed in acquiescence, too much surprised to speak.
. . ."He quickly unsealed one of the vials and poured its contents into the basin of water. Then from another he sifted a bluish colored powder upon the surface of the liquid, which no sooner had touched the other than it began to hiss, foam and sparkle until there came a report like a pistol shot, and a column of lurid flame leaped up to the ceiling. I started back with a cry of terror.
. . ." 'Don't be alarmed,' he assured me, with a smile; "the water will soon burn out.'
. . ."The fire soon began to grow pale and to diminish in height, when it finally died out altogether, and I saw that the basin was empty.
. . ." 'How much do you think that secret is worth?' asked my visitor, still showing his white teeth between his parted lips.
. . ." 'What do you mean?' I cried.
. . ." 'Sit down and be composed, and I will quickly explain.' Then as I sank into the nearest seat, at a loss what to do or say, he continued:
. . ." 'Seeing is believing, so I have shown to you what I can do to impress upon you more deeply the power that I possess. You have seen that basin of water burn like so much oil, and now you will believe me when I tell you that I have unlocked one of nature's great secrets and that the key lies in that small •vial!'
. . ."His demoniacal smile as he spoke made me shudder.
. . ." 'I do not understand you,' I faltered. 'If you mean that you can burn water'"
. . .'Haven't I done it?' he cried. 'Why, man alive! don't you realize the importance of that secret? In those vials are held the component agents able to separate the constituent parts of water and, freeing the same, set them at war with each other, which must result in combustion and total annihilation.
. . ." 'Think of that and realize that I hold in my hand the destiny of the world. Let me throw ever so little of those wonderful properties into the Atlantic and dare you contemplate the result? In one instant a nucleus of fire would be formed to grow swiftly in size, separating the gases of water and feeding upon them until the shores of Europe and America would be wrapped in a sheet of flame.
. . .•' 'No deluge that ever" drowned the world could extinguish the conflagration, but would rather transport the fiery legions to the very pillars of the heavens, and it would spread from shore to shore and from ocean to ocean, until it had infolded the globe in its seething embrace. Every creature of the sea, the air and the land would perish—ay, the earth itself would melt into fervent heat.'
. . ."During this startling speech he had worked himself into a fearful frenzy to fix his intent gaze upon me as he concluded" with a light that burned into my inmost being. I felt I was in the presence of a madman.
. . ." 'Oh, well,' I said, with what calmness I could command, 'we won't anticipate so dreadful a catastrophe as you so vividly describe. But it is evident you have made a remarkable discovery. I am anxious to know just how you accomplished it.'
. . ." 'Which is my secret,' he said, with another smile, and I saw that my dispassionate speech had had a soothing effect upon him. The man was evidently sane except on that one subject.
. . ." 'You are the most sensible man I have met,' he soon resumed, 'and I am going to impart enough of my secret to you so you will act with good faith in assisting me in a direction where I am powerless.
. . ." 'It needs not my words to tell you that water is composed of two gases, hydrogen and oxygen, in parts as 2 to 1, United in that proportion these elements are impervious to fire. Every schoolboy knows that. But mix them in any other proportion, and heat, flame, combustion, are the immediate consequence.
. . ." 'Now, I have discovered the key which unlocks the affinity holding together the constituent parts of water. A few grains of this powder are sufficient to dismember its warlike elements, when the funeral pyre of the human race is kindled as far as this planet is concerned.'
. . ." 'Impossible!' I could not help exclaiming. 'God in his infinite wisdom never created a world so beautiful as this and then placed in the hands of its subjects the means of its destruction.'
. . ." 'Poor fool!' he said, compassionately. 'You forget that the moon is but a fire extinguished world; that planets without number are the charred remains of what were once scenes of life and beauty: that the sun is a molten mass of heat; that he has said in his own word, in the end "the heavens shall be folded together like a scroll, the elements to melt with fervent heat."'
. . ." 'You see this vial. It contains potassium. It needs not me to tell a man of your information the result when this is brought into contact with oxygen. It ignites instantly. This powder here, the secret of whose compound is known only to me, contains properties which iastantly decompose the watery elements. The moment the oxygen is free the potassium ignites it, and the work of fiery destruction is begun.
. . ." 'You betray a look of doubt. Perhaps you think that this action will be merely local—that the properties will quickly burn out, and in consequence the fire die for want of sustenance. If so, you err. The properties of this powder are self generating, and as long as the water lasts must of necessity continue their work of decomposition, the oxygen continually feeding the flames.
. . ." 'Get me another basin of water. I want to demonstrate it more clearly to you.'
. . ."As he had done before, he turned the potassium into the basin and then sifted in a certain amount of the powder. The hissing and fuming quickly began, followed by a sharp report, when a column of fire again sprang up, which lasted until the water was consumed.
. . ." 'You see, my first trial was no illusion,' he said, turning to me. 'What I have done once I can do every time.'
. . ." 'It is a terrible thing!' I exclaimed, with a shudder. 'But why have you come to me?'
. . ." 'Because the secret is worth much to me. But when I approach men they call me mad and will not listen. They will believe you, and when you have proved what I can do they will gladly pay my price. Then I will divide with yon, and we both shall be rich, you to live at your ease and I to continue my investigations. Will you help me?' and he caught me by the arm with a clutch I seem to feel now.
. . ." 'Help you? I asked in a husky voice. 'Would you jeopardize the lives of the whole human race for a few paltry thousands? A man of your great intellect and research should be above'
. . ." 'You still doubt my ability to do what I claim?' he interrupted. 'Perhaps in the open air you think I would fail? Come with me and I will astonish even you. See, the storm has cleared away, and the day is breaking.'
. . ."I was puzzled what to do. There was no one in the house upon whom I could call for assistance, but outdoors I might escape the man, whom, I confess, I feared. So I consented to accompany him.
. . ."The morning light was fast dispelling the shadows of night and storm, and we had no difficulty in making our way to a little body of water quite hemmed in by the mountains and the forest. My companion, as if fearing I would attempt to escape, had not taken his gaze from me since we had left the house.
. . ." 'There is a good place to test our work,' he declared, pointing to a small pool of water formed in a depression of, the earth by the late storm.
. . ."Without waiting for my reply he threw some of the potassium and powder into the water. The result was startling to me, though I had anticipated the consequence.
. . ."The report was deafening, and the flames seemed to leap to the sky, illuminating the night scene with a ghastly light, but startling as was the light of the burning water the appearance of my companion, who had seemed to be suddenly transformed into a demon, was more terrible.
. . ." 'See, see!' he cried, dancing to and fro with fiendish glee, 'it burns—will burn till the pool is dry. What do you think of my secret now? Do not I hold the key to all life? Oh, I feel like a god, and all men are but worms crawling at my feet! See, the flames leap higher and higher!'
. . ." 'Now, let me drop the same agents which set that pool on fire into this lake, and the result will be the same. Aye, the same, only a million times more grand, for the fire will follow the river to the gulf and thence to the ocean, to envelop the entire w.orld in its blazing sheet. What a sight for the gods to witness!"
. . ."He gesticulated fiercely and reached one arm over the water, as if to drop the infernal powder upon its placid bosom, his wild looking figure lit up by the transplendent glow of the burning pool. I gazed with awe upon him, realizing only too well the terrible earnestness of his tone.
. . ." 'Wait!' I cried hoarsely, 'you forget the money. Your secret is worth —'
. . ." 'Bah! Who prates of money with a cringing world at his feet?' he shrieked. 'They laugh at me. Now let their tears put out the flames my hand has kindled. See! The potassium, it fumes, hisses, dances upon the water! Now the pow'—
. . ."Imagine who can the horror of my situation. The blood seemed to freeze in my veins. My limbs seemed paralyzed, but I quickly overcame my lethargy. The life of every being in the world was in my hands. Nerving myself for the blow, I felled the mad scientist dead at my feet. At that moment the fire behind me expired. The world was saved.
. . ."You know the rest. I was discovered in the act of dealing the fatal blow by men who could not understand the immeasurable deed I had done. You saved my life. In the sight of God I feel that I have committed no crime, but I shall die easier knowing that when I am gone the truth will be known to the world. My conscience is clear, and yet the secret has pointed in my every action like a finger of fire."—Tit-Bits.

I doubt that that would get past the OED's panel, judging from the BBC program earlier this week. The guy is a scientist and he's mad.

Sort of like the Disraeli citation they have for "dark horse," huh?