填空题

Substances which are made up of only one kind of atom are called S36 ______ and there are about a hundred of these -- the same S37 ______as there are kinds of atom. These are the substances from which everything else is made. Aristotle taught that there were four elements -- earth, air, fire, and water -- but he was very wrong. None of these is an element in any sense of the word and S38 ______ There is nothing obvious about the appearance of an element which makes it different from other substances, and this explains why man take so long to discover the building blocks from which his world is made, 0nly patient and S39 ______ were able to show that sulfur, copper, and diamond were elements, but air, water, and sand were not. S40 ______ but have to be prepared in the laboratory. For example, the two elements of which water is made are the gases oxygen and hydrogen; oxygen occurs in the air, but it is mixed with a large amount of other gases, and hydrogen is not even present in the air. S37