soul scale (1988)
An image from the infamous Weekly World News hoax article (Ѻ)(Ѻ) purporting that two German physicians had measured 200 terminal patients and found their soul to weight 1/3000th of an ounce.
In experiments, soul experiments refers to any number of experiments, conducted historically, to quantitatively measure the soul, all generally based on the theory of soul weight as the measure of morals.

Overview
In circa 1230, Roman emperor Frederick II, in one of his many experiments on people, to test to see of the soul exists, constructed a crude soul detection experimental device, in which, as recorded by the monk Salimbene di Adam, in his Chronicles (Ѻ), he shut a prisoner up in a cask to see if the ‘soul’ could be observed escaping though a hole in the cask when the prisoner died.

In c.1540, French princess Margaret of Navarre conducted a soul experiment of sorts; specifically, to test the religious theory, that she had been taught, namely that the “soul left the body at the moment of death”, she stayed by the deathbed of one of her maids for days on end until the last breath to see if she could see or hear the soul departing; this was recounted as follows: [3]

“She never stirred from her bedside, as long as she was agonizing, looking her earnestly in the face, without interruption, till she was dead. Some of her ladies, who were most familiar with her, asked why she looked with so much attention on that poor dying creature: she answered, that having often heard many learned men assert that the soul left the body the moment it died, she was willing to see if there came from it any wind or noise, or sound on the removal and going out of the soul, but that she could perceive nothing like it ... she added, that if she were not well settled in her faith, she should not know what to think of that removal of the soul and its separation from the body, but that she would believe what her god and her church commanded her to believe, without any further inquiry.”

In 1901, American physician Duncan MacDougall weighed six patients while they were in the process of dying from tuberculosis in an old age home. It was relatively easy to determine when death was only a few hours away, and at this point the entire bed was placed on an industrial sized scale which was reported to be sensitive to "two-tenths of an ounce". He took his results (a varying amount of unaccounted for mass loss in four of the six cases) to support his hypothesis that the 'soul' had mass, and when the 'soul' departed the body, so did this mass. The determination of the 'soul' weighing 21-grams was based on the loss of mass in the first subject at the moment of death. [1]

In 1915, American physicist Harry Twining (1863-1947), in his The Physical Basis of the Soul (Ѻ), reported experiments conducted on mice and cats, into the nature of the soul, according to which they revealed it to have a natural basis and had not proven any supernatural soul. He explained that all of his experiments showed a loss of weight due to the natural cause of moisture loss. (Ѻ)

In the 1930s, American X-ray technician R.A. Watters, at the time the director of the William Bernard Johnston Foundation for Psychological Research in Reno, Nevada, supposedly, developed an atomic hypothesis of the soul, according to which he posted that souls (whether human or animal) existed in a limbonic state within the “intra-atomic space between the atoms of human cells.” (Ѻ) In his pamphlet “The Intra-Atomic Quantity” (Ѻ) , supposedly, he argues that soul is a type of energy. (Ѻ) To test this theory, Watters, supposedly, conducted some 40+ experiments wherein he put various insects and animals (grasshoppers, frogs, and mice) in some type of cloud chamber, and photographed “expansion of water vapor” at their time of death, or something along these lines. (Ѻ) Watters reported on his findings as follows: (Ѻ)

“New experimental evidence that identifies a form of energy which is lost to the physical body at the moment of death; not only a form of energy which we have been able to predict because of physical evidence, but an ‘immaterial body’ which at the moment of death, makes its escape from the physical body, and is made visible by the medium of water vapor.”

The 1934 Nature review of Watters’ findings are as follows: (Ѻ)

“There has recently been published by the Dr. William Bernard Johnston Foundation for Psychological Research, Reno, Nevada, a pamphlet by R. A. Watters entitled “The Intra-Atomic Quantity”. Mr. Watters describes a series of experiments in which grasshoppers, frogs and mice were killed in a Wilson expansion chamber, a cloud produced at the moment of death, and the resulting ‘track’s photographed. It is alleged that these photographs reveal forms corresponding in shape to the dead bodies, and it is claimed that this result demonstrates the existence of an “intra-atomic Quantity” which is an “immaterial body” and an “exact counterpart of the physical body to which it belongs”. It is further claimed that when the subjects of the experiments were removed from the Wilson chamber and gave any signs of life, the photographs never showed anything unexpected; but that when the photographs showed ‘intra-atomic’ tracks, the subjects were unquestionably dead. Unfortunately, the few photographs reproduced in the bulletin before us reveal the alleged markings only to the eye of faith; for the rest, the essential experimental details are almost wholly wanting. If Mr. Watters wishes his work to receive attention, he should publish a more adequate and a more fully illustrated report.”

In 1988, a hoax article appeared, as reported by Weekly World News, reporting that Germans Becker Mertens and Elke Fisher had weight some 200 terminally ill patients, and the weight of the would was 1/3000th of an ounce. (Ѻ)
Nahum soul detection device
American physician Gerard Nahum's 1998 diagram for an electromagnetic soul weighting device.

In 1998, American chemical engineer and physician Gerard Nahum, based on his information entropy model of soul, devised an electromagnetic spectrum soul detection device, and attempted to get funding to build the device. In this year, Nahum began to promulgate, in various scientific circles, a 25-page presentation entitled “A Proposal for Testing the Energetics of Consciousness and its Physical Foundation”, a cessation thermodynamics proposal to conduct a consciousness-weighing project to quantify the energy-information of consciousness (the “weight of the soul”), at the point of death using a negative entropy theory, and estimated that the electromagnetic field soul detection device would cost approximately $100,000; defined the soul or rather 'residual energy/information' after the complete dissolution of an organism after death as:

Soul is the (obligatory) negative entropy (i.e., energy/weight equivalent) that is necessary to allow for the nonequilibrium meta-stable physical 'quasi-steady-state' of a living/conscious biological system.”

A diagram of Nahum's electromagnetic field soul detection device is shown adjacent.

Quotes
The following are related quotes:

“The careful observations and many experiments of Verworn, together with those of Wilhelm Engelmann, Wilhelm Preyer, Richard Hertwig, and other more recent students of the protists, afford conclusive evidence for my ‘theory of the cell-soul’ (1866).”
Ernst Haeckel (1899), The Riddle of the Universe [2]

See also
Soul theorist

References
1. (a) MacDougall, Duncan. (1907). “The Soul: Hypothesis Concerning the Soul Substance Together with Experimental Evidence of Such Substance”, American Medicine, New Series, 2:240-43.
(b) Fisher, Len. (2004). Weighing the Soul: the Evolution of Scientific Beliefs. London: Weidenfeld & Nicolson.
2. Haeckel, Ernst. (1899). The Riddle of the Universe: at the Close of the Nineteenth Century (translator: Joseph McCabe) (pg. 152). Harper & Brother, 1900.
3. (a) Brantome. (c.1600). The Lives of Illustrious Women (Vies des Dames Illustres) (Ѻ). Publisher.
(b) Bayle, Pierre. (1734). Dictionaire Historique et Critique, Volume IV (5th ed; 5 vols) (pg. 318); in: A General Dictionary, Historical and Critical, Volume VII (10 vols) (pg. 733). Publisher, 1741.
(c) Febvre, Lucien. (1947). The Problem of Unbelief in the Sixteenth Century: the Religion of Rabelias (translator: Beatrice Gottlieb) (Navarre, soul, 5+ pgs; esp. pg. 191-92). Harvard University Press.
(d) Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (pg. 278-79). HarperOne.

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