“Yes,” replied his father; “when water is cooled below a certain point, it becomes solid. It is just so with lead. Melted lead, when it is cooled below a certain point, becomes solid. The hardening of the melted lead into solid lead, and the hardening of water into ice, as they cool, seem to be phenomena of precisely the same character, and yet the word freeze applies only to one. We say the water freezes, but we can’t say the lead freezes.”
“Why not, sir?” asked Rollo.
“Because it is not the customary use of the word. If we use the terms of common life, we must use them as they are customarily used, or we shall not be understood. Freezing, therefore, will not answer to express all cases of the hardening of a liquid by cold, because that is a term which is only applied to a few of the cases. Now, philosophers want a term which will apply to all the cases of the same kind.”
“And what is their word?” asked Rollo.
“Congelation,” replied his father.
“Congelation?” repeated Rollo.
“Yes,” said his father. “When water becomes ice, the philosophers say it congeals. So when lead hardens in cooling, they say it congeals. Different substances congeal at very different degrees of heat. If we had melted iron and melted lead, equally hot, and let them cool together, the iron would congeal first; and if they continued cooling, by and by the lead would congeal. Water would remain liquid long after lead would congeal; but if it was placed where it would grow colder and colder, the temperature would at last reach the point where water would congeal too. But whatever the liquid is, and whatever the point is at which it changes from a liquid to a solid form, it is called congealing.”
“And the word freezing, then, is only used in respect to water,” said Rollo’s mother.
“Why, yes,” said Mr. Holiday; “we speak of other things freezing beside water; but it is only such things as become solid under great degrees of cold. We say ink freezes, and oil, and if it were cold enough to freeze brandy, or mercury, we should say they were frozen. But substances that harden when they are not very cold, as lead or wax, are not said to freeze.”
“Thus you observe,” continued Rollo’s father, “in common language words are not used in a precise and definite manner. Their meaning is determined by the outward and visible effects that we see, and not by the real nature of the causes. Thus a great many different effects are called burning, in common language, because they are all effects produced in various ways by heat. But the terms used by philosophers are definite and precise, each one being confined to one specific process or phenomenon.”