he class is again promptly in place and ready for work.
“As I announced a week ago,” said Mr. Wilton, “we will to-day take a rapid review of the effects and laws of heat. Will you tell us, Peter, the first and chief of these effects?”
“Yes, sir: combustion.”
“What is combustion?”
“Commonly the rapid union of oxygen with some combustible substance, attended with the evolution of heat.”
“Was your answer correct, then?”
“No, sir,” said Peter, blushing; “I spoke before I thought.”
“Will you correct your answer?”
“The first and chief effect of heat is expansion.”
“That is right. Our sensation of heat is of course only a sensation—merely the feeling which results from the effects of heat upon our nerves—but the chief physical effect of heat is the expansion of bodies. The chemical qualities of bodies are not changed: they are not made either heavier or lighter. A sufficiently high temperature renders bodies luminous, and then we call them red hot or white hot. Solid bodies begin to be luminous at a temperature of about one thousand degrees. But the one invariable effect of heat, with two or three apparent exceptions, is expansion. You may mention, Samuel, some familiar illustrations of the effect of heat in expanding bodies.”