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nearer in size to whale or to bacterium? How many electrons are
there in a man? And how does this number compare with the
number of men it would take to weigh down the earth?—the sun?—
the entire universe?
Let us begin with a foundation of hard fact, giving the weights in
grams. A gram is about -^ of an ounce; a thousand grams make a
kilogram, close to 2| pounds: a thousand kilograms make a metric
ton, almost identical with an English ton. A milligram is a thous-
andth part of a gram. But both upward and downward the weights
prolong themselves to regions where we have no units to deal with
them. The simplest way to bring them home is to express them all in
grams, but in powers often. The exponent, or little number after and
above the ten, represents the number of ciphers to put into the figure
for grams. When, for instance, the weight of the moon is given as
7X io24 g.5 this means 7X 1,000,000,000,000,000,000,000,000 grams,
or, since there are one million grams to the ton, seven million million
million tons—that is, seven trillion tons. When the exponent has a
minus sign in front of it, it denotes a fraction of a gram, and again the
number of ciphers in the denominator of the fraction is given by the
exponent. Thus one of the insulin-secreting cells of our pancreas
weighs about io~9 gram. This is ^^^ gram, or one millionth of
a milligram.
In most cases, since the specific gravity of protoplasm is very close
to that of water, the weight in grams is close to the volume in cubic
centimetres. With trees, this volume will be considerably greater
than the weight; while with armoured creatures like crabs or some
dinosaurs the weight in grams will exceed the volume in cubic centi-
metres. Let us also remember that volumes go up as the cube of the
linear dimensions. An animal weighing a ton, for instance, would be
just balanced by a cubic vessel full of water measuring one metre each
way. The corresponding cube of water wiiich would balance a
human insulin-producing cell would measure IQ-3 centimetre along
each side, which is 1 0*0 0 centimetre, or y^j- millimetre, or 10/4, one p
being i 0\ 6 millimetre.
Since the weights of animals and plants are variable, since many
are not very accurately known, and others have to be calculated, with
a certain unavoidable margin of error, from their linear dimensions,
we do not pretend to give precise weights, but only put organisms
between certain limits of weight, the upper limit of each pigeon-hole
being ten times as heavy as the lower. Thus most men come in the
class between io4 and io5 grams—between ten and a hundred kilo-
grams. Men are near the upper limit of the class; in the same class,