The 21 Grams Theory

On 10 April 1901, an unusual experiment was conducted in Dorchester, Massachusetts by Dr. Duncan MacDougall. He was going to prove that the human soul had mass, and was therefore, measurable.

The experiment was conducted on six dying patients who were placed on specially made Fairbanks weight scales just prior to their deaths. Out of the six subjects, Five were men and one was woman. The doctor used very precise scales in order to determine any minute difference in the weight of the subjects. The patients were selected based upon their imminent death in which two patients were suffering from tuberculosis.

After measuring the first subject weight, an unusual result came out. “Suddenly, coincident with death,” wrote Macdougall, “the beam end dropped with an audible stroke hitting against the lower limiting bar and remaining there with no rebound. The loss was ascertained to be three-fourths of an ounce.”
After the second result, Dr MacFougall felt again the same thing. This was extraordinary to get this result. 

A quote from the 11 March 1907 New york Times article captures the historic moment :
“ The instant life ceased the opposite scale pan fell with a suddenness that was astonishing – as if something had been suddenly lifted from the body.  Immediately all the usual deductions were made for physical loss of weight, and it was discovered that there was still a full ounce of weight unaccounted for”.
All the five doctors took their results. Not all the patients lost the same weight but they did lose something. Only four out of six patients were experimented due to mechanical failures.

A shocking result was observed in the third patient, who maintained his same weight immediately after his death. But after one minute, he lost about an ounce of weight.  Dr. MacDougall explained this as follows :
“ 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 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”.
By consulting with the other attending physicians, it was determined that the average weight loss was 3/4 of an ounce of each person. Dr. MacDougall concluded that a human soul weighed 21 grams.

He also experimented the same on 15 dogs but the same result didn't come up. They showed no change after their death. MacDougall concluded that this may signify only humans have souls.

Dr. MacDougall admitted that more research needed to be done and he was passed away in 1920.
Source : [1][2]