Here is the original paper: https://www.newdualism.org/papers/D.MacDougall/soul-substance.htm

In short, taking into account every eventually and using very precise scales (for 1907) a man named MacDougall measured the body's weight for 6 people that died of conditions (like TB) where they would be non-moving and stable. The results indicate the human body weight about as much as a slice of bread less when the soul departs.

No good naturalistic/materialistic explanations for his results has not been found. MacDougall wanted to perform many more experiments, but opposition to the project made this impossible. The experiment has never been replicated on humans. Replications on animals show they gain weight at the moment of death. Strange.

Here is a decent summary of the issue, with the theory that perhaps the soul has some sort of psycho-kinetic effect on the weight of the body.

http://jimmyakin.com/2021/12/can-the-soul-be-weighed.html