Transcript The History of Science

The History of Science
Unit 2 continued…
Think and Discuss
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Over the years, can you think of any
scientific theories or knowledge that
has changed?
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What does this tell you about science?
Where do living things come
from?
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Around 2,000 years ago, people
believed that life came from non-living
matter.
This is called “spontaneous
generation”
Observations seemed to show that
some living things could just suddenly
appear.
Where do living things come
from?
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Maggots showed up on meat.
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Mice were found in grain.
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Beetles turned up in cow dung.
A Recipe for Making Bees:
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Kill a bull during the first thaw of
winter.
Build a shed.
Place the dead bull on branches and
herbs inside the shed.
Wait for summer. The decaying body
of the bull will produce bees.
Maggots appear on food that is
left out in an open trash can
during the summer.
Where do the maggots come
from?
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About 400 years ago, people began to
question the idea of “spontaneous
generation”.
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In 1668, Francesco Redi proposed a
different hypothesis for where the
maggots came from.
Francesco Redi
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Observed that maggots appeared on
meat a few days after flies landed on
the meat.
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His hypothesis was that the flies laid
eggs that were too small to see. The
eggs hatched into maggots.
How would you test this
hypothesis?
How to Design an Experiment
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Variables are the factors in an
experiment that can change.
Examples:
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equipment used
type of material
amount of material
temperature
light
A Controlled Experiment
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Only one variable is changed at a time.
All other variables are kept the same (they
are controlled)
Manipulated (independent) variable: is
changed on purpose
Responding (dependent) variable: changes
in response to the manipulated variable
Control Group: the group that shows what
would happen “normally” without treatment
Let’s talk about variables…
Manipulated
Responding
(Independent
Variable)
(Dependent Variable)
Francesco Redi’s Experiment
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The Hypothesis: Keeping flies away
from the meat will prevent maggots
from appearing.
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The Independent (Manipulated)
Variable: Whether or not the jars
were covered with gauze.
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(diagram on page 9)
Francesco Redi’s Results
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Maggots appeared on the meat in the
uncovered jars (the control group jars)
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No maggots appeared on the meat in the
jars covered with gauze.
Francesco Redi’s
Conclusion
Experiments must be Repeated!
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Basic assumptions in science:
Nature is consistent.
Results can be reproduced.
Scientists test each other’s results.
Today, results must be published in a
Scientific Journal and peer-reviewed
to be considered valid (accepted).
John Needham’s experiment
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Mid-1700s, John Needham attacked
Redi’s work.
He claimed that spontaneous
generation could happen in the right
conditions.
He claimed that tiny “animalcules”
came from gravy.
Needham’s gravy experiment
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Experiment: He sealed a bottle of gravy,
briefly heated it to kill any living things in the
gravy, and then left it open in room
temperature.
Results: After a few days, he examined the
gravy with a microscope and found it
swarming with activity.
His Conclusion: “These little animals can
only have come from the juice of the gravy.”
Lazzaro Spallanzani’s
Experiment
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He repeated Needham’s gravy
experiment.
BUT… Spallanzani boiled 2 flasks of
gravy , one was sealed, the other was
left open.
After a few days, the open flask had
living microorganisms, but the sealed
flask contained no microorganisms.
What about Spontaneous
Generation?
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During the 1800’s, some scientists
continued to support the spontaneous
generation hypothesis.
They argued that air contained the “life
force” needed to produce new life.
They thought Spallanzani’s results
weren’t accurate (should not be
accepted) because air had been
excluded from the sealed jar.
Pasteur’s test of Spontaneous
Generation
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During the 1800’s, some scientists argued that
air contained the “life force” needed to
produce new life.
In 1864, a French scientist named Louis
Pasteur figured out how to solve the argument.
He designed a flask with a long, curved neck
that would allow air into the flask, but would
protect the flask from microorganisms in the
air.
http://bcs.whfreeman.com/thelifewire/content/chp03/0302003.html
Pasteur’s discoveries were
VERY important!
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He saved the French wine industry
from unexplained souring of wine.
He saved the silk industry, which was
being harmed by a silkworm disease.
He began to uncover the nature of
infectious diseases by showing that
they were caused by microorganisms.
Robert Koch: Germ Theory of
Disease
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Previously, people thought that bacteria arose in sick animals
AFTER infection.
However, Robert Koch (1843 - 1910) proved that microscopic
pathogens cause disease.
Steps of Experiment:
1. Isolated bacteria from sheep infected with anthrax
2. Placed bacteria on agar plates in Petri dishes so
the colonies (groups of organisms) could
reproduce.
3. Injected bacteria into healthy sheep
4. Result: Previously healthy sheep now had anthrax
Robert Koch: Germ Theory of
Disease
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After his experiments with anthrax,
Koch came up with a series of steps
that are needed to identify the microorganism responsible for a particular
disease.
These steps (Koch's postulates) can
be used to identify the causative
organism of an infectious disease.
Summarize:
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Discuss and summarize how the work
of Pasteur and Koch contributed to our
understanding of the infectious nature
of disease (germ theory of disease).
Discuss with a partner and write 5-7
sentences using your notes
(*to be turned in)