In 1907, Duncan MacDougall of Haverhill, Massachusetts decided to try and prove the existence of the soul. All he needed was some people willing to die under his supervision, and some dogs.
The Scottish doctor, for reasons he didn't take time to note down, believed that the soul had physical mass and thus it should be possible to weigh it. He had the imagination to believe that a piece of soul magically departs your body when you die, possibly to live in a heaven-like place for eternity, but thought it unrealistic that this couldn't be weighed like a bag of flour. He decided the best way to prove his theory would be to weigh somebody at the precise moment they snuffed it, and then immediately after. The weight disparity would be the weight of the departed soul.
MacDougall looked for volunteers that would be up for the job of dying, and, shortly before that, having their last act on Earth be a weigh-in. He also needed patients that would keep themselves nice and still while they were in the act of snuffing it, so as not to jolt his scales and render his experiment – aimed at weighing somebody's ghost – invalid. He chose terminally ill patients dying from tuberculosis or similar diseases that kept them exhausted and still, lest he had to ask a dying patient, "Would you mind staying still, Sheila, you don't want to wobble around your ghost."
He constructed a bed in his office for the patients that he placed on large balance scales. In a version of events that goes around the Internet every now and then, the patients had their "bodily apertures blocked" (a polite way of saying a cork up the anus). This isn't true, but given MacDougall's scientific rigor, that's possibly only because he wasn't worried souls might escape through the anus. He instead insisted that any fluids and solids lost from the body would be captured by the bed and included in the total weight.
When it came time for the weigh-in, things didn't go quite to plan.
"Unfortunately our scales were not finely adjusted and there was a good deal of interference by people opposed to our work," he wrote of one dismissed test, which sounds suspiciously like someone had questioned why on Earth he was weighing patients as they died rather than providing palliative care.
Another patient died whilst he was still setting up his scales.
One patient did appear to lose weight at the exact time of death, the famous 21.3 grams (0.75 ounces), while another lost 14 grams (0.5 ounces) before having their pulse checked and death confirmed, after which they lost 42.5 grams (1.5 ounces). A third also lost weight and then more weight (but not as much as the other) shortly afterward. While most would conclude that what he discovered was some bad scales, MacDougall concluded that he had proved the existence of the soul.
But, being a "scientific" weigher of spirits, the doctor realized he would need to carry out a control experiment. So, he got 15 dogs and murdered them on some scales. The thing about dogs is they don't really stay still. “[I]t was not my fortune to get dogs dying from such sickness,” he wrote, so he killed some healthy dogs instead.
He posited that animals didn't have souls to lose, so they shouldn't get lighter as they die. The human experiments were bad, but imagine being the dog lying on the scales waiting to snuff it as the doctor leans in excitedly and whispers, "You have no soul, after I've killed you there is nothing."
The dogs did not lose any weight according to his scales, and MacDougall published his results, whereupon they were torn apart instantly.
MacDougal's results were flawed from the beginning due to his suspect methodology. He himself admitted it was difficult to measure the exact precise moment of death. Any weight change noted in his very small sample of patients could be put down to an increase of sweat and moisture evaporation that occurs when body temperature rises shortly after death, as the blood is no longer cooled as it circulates. Dogs meanwhile do sweat, but mainly through their paws, meaning any weight loss would be minimal. Or, and it's a big possibility here, dying people don't keep deadly still and the scales just weren't that accurate. Either way, he was ridiculed by his peers for months.
He hadn't proved the existence of a soul, he was just your bog-standard dog murderer.