in an airtight jar and the air was evacuated with a hand crank vacuum, the
unfortunate creature would almost immediately keel over, lifeless. Even
without the use of a vacuum, the importance of air was revealed by the
burning of a candle in a bell jar occupied by a rodent. As soon as the
candle flame extinguished, the rodent itself became limp. These
experiments were performed in an era before oxygen was recognized. How
to explain a burning candle or a breathing animal?
The theory among intellectuals in the early 18th century was that both
animals and inanimate objects contained phlogiston (Greek: “burned”),
and when creatures exhaled or when things burned, phlogiston was
released into the air. When the air got too saturated with phlogiston, the
theory went, the fire would spontaneously go out or an animal would
collapse inside a bell jar. It seemed like a tidy explanation, except it was
dramatically, and laughably, wrong.
Time for a quick quiz. In 1648, the Dutch physician Jan van Helmont
published the results of a clever experiment aimed at exploring the nature
of the growth of plants and trees. Some years earlier, van Helmont
carefully collected, dried, and weighed two hundred pounds of soil, and
then placed it in a large pot. He then selected a young willow sapling
which weighed five pounds, planted it, and over the next five years,
fastidiously watered the tree in its pot. At the conclusion of the
experiment, he cautiously removed the tree with its roots, and found that it
had gained 164 pounds. Van Helmont then dried the soil and weighed it,
discovering that in those five years it had only lost two ounces of weight.
His conclusion? Plants gained their weight from water alone.
Question: Do you agree with Jan van Helmont? (Answer in one
moment.)
In 1754, Joseph Black (1728–1799) presented his thesis for the
doctorate of medicine to the University of Edinburgh. As a trained
physician, Black conducted experiments (in both Glasgow and Edinburgh)
on the properties of kidney stones, testing the possibility of dissolving the
stones in acid. Black collected both kidney stones and gallstones, and then
plopped them in various acids. To his surprise, certain stones (such as
limestone), when dunked in acid, produced a fizzing effervescence of gas
bubbles. He termed this gas “fixed air,” surmising that it had been fixed as
a constituent of the solid material. Later experiments with fixed air
marcin
(Marcin)
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