In 1907, Dr. Duncan MacDougall found a bunch of people who were about to die and weighed them as they expired. MacDougall claimed that at the point of death, the bodies became lighter. That lost weight, the doctor assumed, was the escaping soul. He even postulated that the souls of the sluggish in life are slow in death:
The subject was that of a man of larger physical build, with a pronounced sluggish temperament. When life ceased, as the body lay in bed upon the scales, for a full minute there appeared to be no change in weight. The physicians waiting in the room looked into each other's faces silently, shaking their heads in the conviction that out test had failed.
Then suddenly the same thing happened that had occurred in the other cases. There was a sudden diminution in weight, which was soon found to be the same as that of the preceding experiments.
I believe that in this case, that of a phlegmatic man slow of thought and action, that the soul remained suspended in the body after death, during the minute that elapsed before it came to the consciousness of its freedom. There is no other way of accounting for it, and it is what might be expected to happen in a man of the subject's temperament.
The weight lost of MacDougall's first subject at death was 3/4 of an ounce...or about 21 grams. (via radiolab)