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 1 must exist in the form of 2 particles or atoms and that these 3 be completely mixed together in the 4.This threw a new light in 5 laws of definite proportions.It was 6 necessary to suppose that the atoms 7 combinesintossmall groups of uniform 8 and so form more complex substance; 9 the mystery of this law was 10. Dalton suggested, for example, that carbon monoxide 11 formed by the one with one 12 of atoms of carbon and oxygen, 13 carbon dioxide results from a single 14 of carbon uniting with two atoms 15 oxygen.Assuming this to be true, 16 the definite proportions of Prout's law ( 17 all chemical compounds the different constituents 18 enter in unvarying proportions) would naturally 19 the relative weights of the many 20 kinds of atoms.

　　1.A.gases B.pieces C.things D.gas

　　2.A.timely B.trimmed C.tiny D.topical

　　3.A.ought B.could C.will D.must

　　4.A.sky B.atmosphere C.oxygen D.environment

　　5.A.various B.many C.the D.those

　　6.A.only B.too C.that D.extremely

　　7.A.should B.were C.get D.could

　　8.A.construct B.structure C.piece D.feature

　　9.A.but B.also C.thus D.nevertheless

　　10.A.solved B.discovered C.broked D.told

　　11.A.must B.been C.were D.is

　　12.A.putting B.pairing C.placing D.preparing

　　13.A.while B.when C.therefore D.also

　　14.A.piece B.type C.atom D.measure

　　15.A.from B.in C.of D.for

　　16.A.then B.so C.by D.that

　　17.A.from B.in C.to D.for

　　18.A.are B.wouldn't C.always D.never

　　19.A.describe B.unit C.mix D.reveal

　　20.A.different B.usual C.important D.chemical

　　1.【答案】A

　　【解析】根据上文中的heavier and lighter gases可以确定，本空中应填入“组成的气体”，即constituent gases.

　　2.【答案】C

　　【解析】根据句子意思判断，句中的词意为微粒的意思，选项C.tiny合乎题意。

　　3.【答案】D

　　【解析】情态动词must在这里是“必定、必须”的意思。

　　4.【答案】B

　　【解析】sky意为“天空”，atmosphere意为“大气”， oxygen意为“氧气”， environment意为“环境”，只有B项正确。