Stephen Hales, Joseph Priestley
By: Monica Heney-Racine and Razan Ali
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Jean-Baptiste Van Helmont, a 15th century
physician, strived to discover whether or not
plants obtained all their food from soil.
In order to prove this hypothesis, he performed
an experiment that consisted of watching a plant
grow for five years.
Independent variable: Amount of time taken for
the plant to grow
Dependent variable: Mass of tree, mass of soil
Controlled variable: Amount of water given,
amount of air exposed to
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After five years, Helmont observed that the
mass of the tree had increased by 74.4 kg,
but that the soil’s mass had only increased by
60 g.
He therefore concluded that soil was not
responsible for the tree’s increased in mass,
the absorption of water was. (1)
An image from Nelson Biology 12
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Known best for his Statical Essays.
Speculated that plants draw in nourishment
from the air, and that light may contribute to
a plant’s growth.
According to www.biologyreference.com,
Stephen Hales conducted pioneer
experiments on the transpiration, growth,
and gas exchanges of plants.
Hales explained how water from the soil is
transported up the stems and into the leaves,
where it gets lost as a water vapour.
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This process was called transpiration.
Stephen Hales proposed that plants extract some of
their matter from air.
In evaluating the loss of water from leaves, he had to
estimate the total surface area of the leaves on the
plant.
He cut off all the leave of this plant, and laid them in
five several parcels, according to their sizes.
He then measured the surface of the leaves in each
parcel by laying over it a large lattice made with
threads, in which the little squares were ¼ of an inch
each.
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He multiplied the surface of the leaves by the
number of the leaves in the corresponding
parcel, which then gave him the area of all
the leaves.
Then, he estimated the length of the root
system and calculated the corresponding
surface area.
Using the values he collected, he compared
the flux of water into the plant through the
roots with the transpiration from the leaves.
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In one of his works, Hales concluded that
plants get some part of their nourishments
from the air.
In Hales work Vegetable statics he explains
that “… air freely enters plants, not only with
the principal fund of nourishment by the
roots, but also thro’ the surface of their
trunks and leaves…” [10: 153]
Independent variable: amount of lost water
from the leaves
Dependent variable: the measured surface of
the leaves
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Controlled variables: Size of leaves, size of
the squares that have been cut.
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Often given credit for the discovery of
oxygen(dephlogisticated air) and its role in
photosynthesis. (2)
He lit a candle and placed it in a transparent
closed space with a mint plant. He did the same
thing with another candle, but did not place a
mint plant in the transparent closed space.
After some time, both candles went out.
However, the candle that was placed alongside
the mint plant was able to burn longer. (3)
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10 days later, he was able to relight the candle
that was placed alongside the mint plant(using
light from the sun) and it was able to burn
perfectly in the air that did not previously
support it.
In another experiment, Priestley placed a mouse
in one transparent chamber, and a mouse and a
mint plant in another transparent chamber.
He observed that the mouse in the chamber that
contained the mint plant was able to survive
whereas the mouse in the chamber that did not
contain a mint plant was unable to survive. (3)
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As a result, Priestley concluded from these
experiments that plants restore to the air
whatever breathing animals and burning candles
remove(OXYGEN). (3)
Independent variable: The presence of a mint
plant
Dependent variables: Duration of time the candle
can stay lit for, Duration of time the mice can
remain alive
Controlled variables: Equally sized transparent
chambers, temperature of chambers, type of
candle(4)
Photo from the internet
(http://employees.csbsju.edu/SSAUPE/biol115/photo
synthesis%20-%20classics.htm)
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1. Di Giuseppe, Maurice. Vavitsas, Angela. Ritter,
Bob. Fraser, Douglas. Arora, Anu. Lisser, Beth.
2003 photosynthesis p.149, Nelson Biology 12
2. Unknown author, 2012, Oxygen. Received on
April 1
http://en.wikipedia.org/wiki/Oxygen#Discovery
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3. Ruban, Julien. 2011, The Discovery of
Photosynthesis. Received on April 1
http://www.juliantrubin.com/bigten/photosynthesi
sexperiments.html
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4. Unknown author, 2010, Joseph Priestley:
Variables, Controls, Errors. Received on April
1
 https://sites.google.com/site/experimentsin
photosynthesis3b/josephpriestley/variables-and-controls-and-error
5. Unknown author, 2011, Stephen Hales and
the practice of science. Received on April 1
http://medicalphysiologyonline.wordpress.com
/2011/10/18/stephen-hales/
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6. Unknown author, 2002, Hales, Stephen 1671-
1761
http://www.hcs.ohiostate.edu/hort/history/108.htm
7. Unknown author, 2012, Stephen Hales
http://www.nndb.com/people/146/000085888/
8. Unknown author, Unknown year, History of Plant
Physiology
http://www.biologyreference.com/Gr-Hi/History-ofPlant-Physiology.html#b