While those atoms are hustling about, lengthening the wire and getting a better grip on one another, they grow warm with the exercise.  Hold a thick rubber band against your lip—­suddenly stretch it.  The lip easily perceives the greater heat.  After a few moments let it contract.  The greater coldness is equally perceptible.

A wire suspending thirty-nine pounds being twisted ninety-five full turns lengthened itself one sixteen-hundredth of its length.  Being further twisted by twenty-five turns it shortened itself one fourth of its previous elongation.  During the twisting some sections took far more torsion than others.  A steel wire supporting thirty-nine pounds was twisted one hundred and twenty times and then allowed to untwist at will.  It let out only thirty-eight turns and retained eighty-two in the new permanent relation of particles.  A wire has been known to accommodate itself to nearly fourteen hundred twists, and still the atoms did not let go of each other.  They slid about on each other as freely as the atoms of water, but they still held on.  It is easier to conceive of these atoms sliding about, making the wire thinner and longer, when we consider that it is the opinion of our best physicists that molecules made of atoms are never still.  Masses of matter may be still, but not the constituent elements.  They are always in intensest activity, like a mass of bees—­those inside coming out, outside ones going in—­but the mass remains the same.

The atoms of water behave extraordinarily.  I know of a boiler and pipes for heating a house.  When the fire was applied and the temperature was changed from that of the street to two hundred degrees, it was easy to see that there was a whole barrel more of it than when it was let into the boiler.  It had been swollen by the heat, but it was nothing but water.

Mobile, flexible, and yielding as water seems to be, it has an obstinacy quite remarkable.  It was for a long time supposed to be absolutely incompressible.  It is nearly so.  A pressure that would reduce air to one hundredth of its bulk would not discernibly affect water.  Put a ton weight on a cubic inch of water; it does not flinch nor perceptibly shrink, yet the atoms of water do not fill the space they occupy.  They object to being crowded.  They make no objection to having other matter come in and possess the space unoccupied by them.

Air so much enjoys its free, agile state, leaping over hills and plains, kissing a thousand flowers, that it greatly objects to being condensed to a liquid.  First we must take away all the heat.  Two hundred and ten degrees of heat changes water to steam filling 1,728 times as much space.  No amount of pressure will condense steam to water unless the heat is removed.  So take heat away from air till it is more than two hundred degrees below zero, and then a pressure of about two hundred atmospheres (14.7 pounds each) changes common air to fluid.  It fights desperately against condensation, growing hot with the effort, and it maintains its resilience for years at any point of pressure short of the final surrender that gives up to become liquid.