ULES gave a lengthy account of the day to his brother and
sister. At the part relating to the thunderbolt Claire
trembled like a leaf. "I should have died of fright," said
she, "if I had seen the lightning strike the pine." After
the deeper emotion came curiosity, and they all agreed to
beg their uncle for a talk on the subject of thunder. And so
the next day Jules, Emile, and Claire gathered around their
Uncle Paul to hear him tell them all about it. Jules
broached the subject.
"Now that I am no longer afraid, will you please tell us,
Uncle, why we should not take refuge under trees during a
storm? Emile, I am sure, would like to know."
"I should first of all like to know what thunder is," said
"I too," said Claire. "When we know a little what thunder
is, it will be much easier to understand the danger from
"Quite right," commented their uncle, approvingly. "First
let us see whether any one of you knows anything about
"When I was very small," Emile volunteered, "I used to think
it was produced by rolling a large ball of iron made of
resounding metal over the vault of
 the sky. If the vault
broke anywhere, the ball was dashed to the ground and the
thunder fell. But I don't believe that now. I am too big."
"Too big—a little fellow not so high as the first button
on my vest! Say rather that your little reasoning powers are
awakening and that the simple explanation of the iron ball
no longer satisfies them."
Then Claire spoke. "I am not satisfied either with the
explanations I used to give myself a while ago. With me,
thunder was a wagon heavily loaded with old iron. It rolled
on top of a sonorous vault. Sometimes a spark would flash
out from under the wheels, the same as from a horse's hoof
when it strikes a stone: that was the lightning. The vault
was slippery and bordered with precipices. If the wagon
happened to tip over, the load of old iron would fall to the
ground, crushing people, trees, and houses. I laughed
yesterday at my explanation, but I am no farther advanced
now: I still know nothing at all about thunder."
"Your two thunders, varying to suit your infant
imaginations, are based on the same idea, the idea of a
sonorous vault. Well, know once for all that the blue vault
of the sky is only an appearance due to the air which
envelops us, and which, owing to the thickness of the
envelope, has a beautiful blue color. Around us there is no
vault, only a thick layer of air; and beyond that there is
nothing for a vast distance until you come to the region of
"We will give up the blue vault," said Jules. "Emile,
Claire, and I are persuaded there isn't any. Please go on."
 "Go on? Here is where difficulty begins. Do you know, my
children, that your questions are sometimes very
embarrassing? 'Go on' is soon said; and, filled with
unbounded faith in your Uncle Paul's knowledge, you expect
an answer which, you feel sure, will satisfy your curiosity.
You must, however, understand that there are innumerable
things beyond your intelligence, and before you can grasp
them you must attain to riper reason. With age and study
many things will become clear that now are dark to you. In
this number is the cause of thunder. I am very willing to
tell you something about it; but if you do not understand
all that I say you must blame your own premature curiosity.
It is a difficult subject for you, very difficult."
"Only tell us about it," Jules persisted; "we will listen
"So be it. Air is not visible, one cannot take hold of it;
if it were always at rest you would not, perhaps, suspect
its existence. But when a violent wind bends tall poplars
and scatters the leaves in eddies, when it uproots trees and
carries off the roofs of buildings, who can doubt the
existence of air? For wind is only air streaming
irresistibly from place to place. Air, so subtle, so
invisible, so peaceful in repose, is therefore in very truth
a material substance, even a very brutal one when in violent
motion. That is to say, a substance can exist, although at
times nothing betrays its presence. We do not see it or
touch it, are not sensible of it, and yet it is there, all
about us; we are surrounded by it, live in the midst of it.
 "Well, there is something still more hidden than air, more
invisible, more difficult to detect. It is everywhere,
absolutely everywhere, even in us; but it keeps itself so
quiet that until now you have never heard of it."
Emile, Claire, and Jules exchanged glances full of meaning,
trying to guess what it could be that was found everywhere
and that they did not yet know of. They were a hundred
leagues from guessing what their uncle meant.
"You might seek in vain by yourselves all day, all the year,
perhaps all your life; you would not find it. The thing I am
speaking of, you understand, is singularly well hidden;
scholars had to make very delicate researches to learn
anything about it. Let us make use of the means they have
taught us to bring it to light."
Uncle Paul took from his desk a stick of sealing-wax and
rubbed it rapidly over his cloth sleeve; then he put it near
a small piece of paper. The children were all eyes. Behold,
the paper flies up and sticks to the sealing-wax. The
experiment is repeated several times. Each time the paper
rises unaided, starts off, and fastens on to the stick.
"The piece of sealing-wax, which formerly did not attract
the paper, now does. The rubbing on the cloth has, then,
developed in it something that cannot be seen, for the stick
has not changed in appearance; and this invisible thing is
nevertheless very real, since it can lift up the paper, draw
it to the wax, and hold it glued there. This thing is called
electricity. You can easily produce it by rubbing on
 cloth either a piece of glass or a stick of sulphur, resin, or
sealing-wax. All these substances, when rubbed, will acquire
the property of drawing to themselves very light objects,
like small pieces of straw, little bits of paper, or
particles of dust. This evening the cat shall teach us more
about it, if it will be good."