Dalton wondered why the heavier and lighter gases in the atmosphere did not separate as oil and water do. He finally concluded that the constituent (61) must exist in the form of tiny particles or atoms and that these (62) be completely mixed together in the atmosphere. This threw a new light in (63) law of definite proportions. It was only necessary to suppose that the atoms (64) combine into small groups of uniform (65) and so form more complex substance; (66) the mystery of this law was (67) . Dalton suggested, for example, that carbon monoxide (68) formed by the one with one (69) of atoms of carbon and oxygen, (70) carbon dioxide results from a single (71) of carbon uniting with two atoms of oxygen. Assuming this to be true, (72) the definite proportions of Prout’s law (73) all chemical compounds the different constituents (74) enter in unvarying proportions would naturally (75) the relative weights of the many different kinds of atoms. Part Ⅳ Cloze Directions:In this part, there is a passage with 15 blanks. For each blank there are 4 choices marked A, B, G and D. Choose the best answer for each blank and mark the corresponding letter with a single bar across the square brackets on your machine-scoring ANSWER SHEET.