114 EFFECTS OF HEAT
collect what took place when I held a cold plate, for some time, over the boiling water in the tea-urn ?"
" Yes, papa," answered Lewis, "the steam which rose from the urn was condensed; for, as you said, the particles, which had separated, again united as soon as they came in contact with the cold plate, and ran off in little streams of water."
" And what caused the separation of the particles of water?" inquired his father.
" Heat," said Lewis, " and cold, which you told us means only the absence of heat, caused them to unite again."
" Ah ! now, papa, I think I understand what you mean. Yesterday the weather was very hot; and I