66.You should also learn as much about chemistry as possible from your own experience in the laboramry and from your observations of chemical substances and chemical reactions in everyday life. 
  你们也应该从你们自己的实验室经验中,从你们日常对化学物质和化学反应的观察中学习更多的化学方面的知识(1earn...from.., “从……学……”。句中两个from均为learn要求的。) 

  67.You will notice that specific gravity is always the ratio of two weights,and so it is a pure number,such as 3 or 5.25,with out any unit associated with it. 

  68.With the increased productionmade possible by the assembly line,automobiles became much cheaper,and more and more people were able to offord them. 
  由于装配线所带来的增产,汽车价格便宜多了,买得起汽车的人越来越多。 (介词短语with…用作原因状语。) 

  69.Today physicists find that light has the characteristics of both waves and particles with one aspect sometimes predominating and sometimes the other. 
  物理学家现在发现,光具有波粒二重性,有时波动性起主要作用,有时粒子性起主要作用。(with。..the other是分词复合结构,在句中作补充说明。) 

  70.It is amatter of common observation that a body A cannot exert force on a second body B without B at the same time exerting force on A. 
  这是我们通常都能观察到的情况:如果物体B没有同时对物体A施加作用力,那么物体A也不能对B施加作用力。 (without引导的介词复合结构为条件状语,用来说明谓语cannot exert。) 

  71.Without the sun’s light warming the earth’s surface,it would be so terribly cold that life could not exist. 

  72.Just as the length and cross-section of the pipe affect the flow of water so it is with conductors of electric current. 

  73.Water vapor may vary from almost nothing to as much as 4% and is not usually included with the other constituents which,in terms of global averages,do not vary significantly. 

  74.This atom,with 92 electrons spinning about the closely packed and complex nucleus,is the element uranium,which seems to be the most complicated of the natural atoms.