"For that same reason you are making the heat in drawing the copper through the die."
"But I notice that if I hammer a piece of cold iron it will get hot. There is not any rubbing motion there to make friction."
"Do you think not? You have by that means made the most intense friction. The iron is composed of tiny particles, called atoms, and molecules. When you strike a piece of iron you force these particles in among themselves, and the friction caused by this movement produces the heat."
"Is that true of all substances?"
"Yes."
"Well, if air is forced together will it heat in the same way?"
"Yes, and for the same reason. The tiny particles, of which air is composed, move among each other with such rapidity, under compression, that the heat their frictional contact develops is dependent on the pressure exerted."
"You used the terms 'atom' and 'molecules' a moment ago. What is the difference between them?"
"A molecule is always composed of two or more atoms. An atom is smaller than a molecule, for this reason. Furthermore, an atom comprises only one substance. A molecule has two or more substances in its make-up. For instance, water is composed of two parts of hydrogen and one part of oxygen. One molecule of water, therefore, has three atoms, two of the atoms being hydrogen, and one atom oxygen."
"Baby," the infant orang-outan, had now grown to be a pretty good-sized boy. He would sit at the table and gravely eat with a knife and fork, which he had learned to handle most intelligently. In the various trips which had been made from time to time, the Baby was kept at home, but on more than one occasion he would follow up the wagon, and would as often be welcomed when he did come.