What does a human soul weigh?

Does the human soul exist, and if so, how much does it weigh ? To find out, surgeon Duncan Macdougall devised a grief experiment in Dorchester, Massachusetts, in April 1901. He placed a dying tuberculosis patient on a platform scale rigged with a cot. At the moment of death, Duncan Macdougall hypothesized, the scale should register a drop in weight when ghost shelled off its earthly body. Their first patient in place, Macdougall and his team took up their grim posts, assumed an air of breathless, intent expectancy. The first patient's soul clocked in, or rather, out, at 21 grams.
Sadly, Macdougall never managed to duplicate that data later.Source: "Spook: Science Tackles the Afterlife" by Mary Roach