The document summarizes William Barrett's 1864 experiment where he measured the amount of carbon dioxide in human breath using infrared radiation. He collected expired air in rubber bags from subjects before and after meals or exertion. The air was passed through an insulated cylinder with infrared radiation generated at one end. The heat absorbed by the breath gases was measured and used to calculate the carbon dioxide percentage, which was confirmed with chemical analysis. The experiment demonstrated measuring carbon dioxide in breath using infrared absorption and recognized the potential clinical application of this technique. It was an important early application of infrared spectroscopy.

1.  The details of this experiment, “a new and extremely delicate method of
determining the amount of carbonic acid in air expired from the
lungs”[4] were reported in July 1864 in Philosophical Magazine (Figure
3) [2] (Note 1) by Mr. W. F. Barrett (Figure 2) (Note 2), a young
assistant of Dr. Tyndall’s in the Physical Laboratory of the Royal
Institution.
 A portion of the experiment consisted of collecting from subjects
expired air in rubber bags before and after breakfast and after severe
exertion. The contents of these bags are emptied into an exhausted
brass cylinder (over 4 ft in length with its ends stopped by polished
plates of rock salt) through which infrared radiation, generated by a
flame of carbonic oxide (i.e. carbon monoxide) gas placed at one end of
the cylinder, passes. The amount of heat “intercepted by the breath” is
measured by a delicate thermo-electric pile at the opposite end of the
cylinder interfaced to a galvanometer. The deflection of galvanometer
is converted to a absorption of total radiation which in turn is mapped
to gas tension using a predetermined “calibration” table (Figure 4). The
percentage of carbonic acid is computed from this gas tension value and
compared to the values determined by chemical analysis [5] which at
that time employed potash to absorb the CO2 and measured the
remaining gas to determine the amount of CO2 absorbed.
 The potential clinical application of these measurements, characterized
as “remarkable experiments” [3] , were soon recognized by some of
the important publications of the time [3,4] (Figure 5)(Note 3) which
presented summaries of” important new facts and theories in every
department of science” intended for a wide audience and noted of these
measurements “For this purpose its application in hospitals has
already been suggested by eminent men.” [3]
“On a physical analysis of the human breath” –
The first infrared measurements of CO2 in the human breath
Michael B. Jaffe PhD, Philips-Respironics, Wallingford, CT
 John Tyndall (1820–1893), studied the radiative properties of various
gases, and constructed the first ratio spectrophotometer (Figure 1),
which he used to measure the absorption of gases, including water
vapor, “carbonic acid” (now known as CO2), ozone, and hydrocarbons.
 He also observed the large differences in the ability of “perfectly
colorless and invisible gases and vapors” to absorb and transmit radiant
heat, and noted that oxygen, nitrogen, and hydrogen are almost
transparent to radiant heat, but that water vapor, CO2, and ozone are
very good absorbers of heat radiation, and that “even in small quantities,
these gases absorb much more strongly than the atmosphere itself” [1]
Tyndall reported, in his famous Rede Lecture in May 1865 “On
Radiation” at Cambridge University, perhaps the earliest infrared
quantitative measurement of carbon dioxide in the human breath in the
context of a lecture on thermal radiation. These measurements
demonstrated sensitivity to the way different molecules absorb infrared
radiation. Since the human breath had has a significantly higher
percentage of carbon dioxide than the atmosphere, it provided an easy
source of the gas to demonstrate infrared absorption.
1. Tyndall, J. On Radiation. The “Rede” Lecture Tuesday, May 16, Longman,
Green, Longman, Roberts & Green, London. 62 pages, 1865.
2. Barrett, WF. On a physical analysis of the human breath. Phil. Mag.
XXVIII., 1864, 108-121.
3. Wells DA (editor). Year-book of facts in science and art for 1865. Gould and
Lincoln, Boston, 1865, 245-6.
4. The Intellectual Observer – Review of Natural History, Microscopic
research and recreative science, Vol. VI, Groombridge and Sons, London
1865, p 226.
5. Rose . A practical treatise of chemical analysis – Vol. II Quantitative.
William Tegg and Co., London, 1849. p. 534.
6. Noakes, R., The “Bridge Which Is Between Physical and Psychical
Research”: -William Fletcher Barrett, Sensitive Flames, and Spiritualism.
History of Science, 2004. vol. 42, 419-464.
7. Barton R. Just before Nature: The purposes of science and the purposes of
popularization in some English popular science journals of the 1860s
Annals of Science , 1998, vol 55(1), p 1-33.
ReferencesReferences
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Figure 2 - Sir William Fletcher Barrett, 2
June 1920 © National Portrait Gallery,
London
Figure 1 - Tyndall's experimental apparatus, shown here consisted of a long tube that he
filled with various gases. The ends of the tube were capped with slabs of rock salt crystal, a
substance known to be highly transparent to heat radiation. The Leslie cube was heated with a
flame and emitted radiation that traversed the tube and interacted with the gas before entering
one cone of a differential thermopile. Radiation from a second Leslie cube passed through a
screen and entered the other cone. The common apex of the two cones, containing the
differential thermopile junction, was connected in series to a galvanometer that measured
small voltage differences. The intensity of the two sources of radiation entering the two cones
could be compared by measuring the deflection of the galvanometer, which is proportional to
the temperature difference across the thermopile. Different gases in the tube would cause
varying amounts of deflection of the galvanometer needle. If the intensity of the reference
source of radiation was known, the intensity of the other source (and thus the absorptive
power of the gas in the tube) could be calculated. (4)
FootnotesFootnotes
Figure 3 - Cover of Philosophical
Magazine containing Barrett paper on
measurement of CO2 in breath
(1) The Philosophical Magazine, one of the oldest scientific journals published in
English, was first published in 1798, and has been published by Taylor & Francis
ever since.
(2) William Fletcher Barrett (1844-1925) (knighted in 1912 for his scientific work)
served as an assistant to John Tyndall at the Royal Institution (1863-1866).
Reference 2 was Barrett’s first research paper in a long career that included
experiments and publications in material science (e.g. developed stalloy-telephone,
transformeres), ophthalmology (e.g. built an early entoptoscope-instrument for
examining the media of the eyes, to ascertain their transparency) and paranormal
areas. In 1873 he became Professor of Experimental Physics at the Royal College of
Science for Ireland. Barrett became interested in psychic phenomena in the 1870s
after experimenting with hypnosis and soon sought to make psychical research a
branch of physics. [6] He was a founding member of the Society for Psychical
Research (1882) and .the American Society of Psychical Research (1884). Barrett
after experimentation with divining rods (and dowsing) in the late 1890s concluded
that the ideomotor response was responsible for the rod's movements. [6]
(3) The Intellectual Observer, a popular science journal of the 1860’s, under its Notes
and Memoranda section summarized Barrett’s experiment and speculated on its use
in the hospital. Under a different name it ceased publication in 1871.
Figure 5- Publications summarizing Bartlett’s 1864 paper on CO2 measurement in the
human breath .
(a) – Mapping between gas tension and
absorption of CO2 in dried air
(b) Table of measured absorptions of 8 bags of breath air, gas tension
(determined using Table XIII) and percentage by absorption and by
independent chemical analysis (for bags 1-4).
Figure 4 – Estimation from of percent CO2 in breath (Tables from Barrett)[2]