ust be replied that there do remain classes of organic phenomena
unaccounted for. It may, I believe, be shown that certain cardinal
traits of animals and plants at large are still unexplained; and that a
further factor must be recognized. To show this, however, will require
another paper.

II.

Ask a plumber who is repairing your pump, how the water is raised in it,
and he replies--"By suction." Recalling the ability which he has to suck
up water into his mouth through a tube, he is certain that he
understands the pump's action. To inquire what he means by suction,
seems to him absurd. He says you know as well as he does, what he means;
and he cannot see that there is any need for asking how it happens that
the water rises in the tube when he strains his mouth in a particular
way. To the question why the pump, acting by suction, will not make the
water rise above 32 feet, and practically not so much, he can give no
answer; but this does not shake his confidence in his explanation.

On the other hand an inquirer who insists on knowing what suction is,
may obtain from the physicist answers which give him clear ideas, not
only about it but about many other things. He learns that on ourselves
and all things around, there is an atmospheric pressure amounting to
about 15 pounds on the square inch: 15 pounds being the average weight
of a column of air having a square inch for its base and extending
upwards from the sea-level to the limit of the Earth's atmosphere. He is
made to observe that when he puts one end of a tube into water and the
other end into his mouth, and then draws back his tongue, so leaving a
vacant space, two things happen. One is that the pressure of air outside
his cheeks, no longer balanced by an equal pressure of air inside,
thrusts his cheeks inwards; and the other is that the pressure of air on
the surface of the water, no longer balanced by an equal pressure of air
within the tube and his mouth (into which part of the air from the tube
has gone) the water is forced up the tube in consequence of the unequal
pressure. Once understanding thus the nature of the so-called suction,
he sees how it happens that when the plunger of the pump is raised and
relieves from atmospheric pressure the water below it, the atmospheric
pressure on the water in the well, not being balanced by that on the
water in the tube, forces the water higher up the tube, so that it
follows the plunger. And now he sees why the water