农药对环境的影响及其防治措施

农药在防治农作物病虫害,保证农作物正常生长,提高单位面积产量上起到了举足轻重的作用.但其对环境的污染和生态的破坏也十分严重.文章通过概述我国农药生产发展的基本现状及其在国民经济中的作用,表明农药给人类和环境带来的正、负两方面的影响.通过对我国农药污染防治的探讨,突出介绍了现代农药的发展及生物农药在环境可续发展中的作用.     

Structures and moelcular motions in alkaline earth hexammines

The alkaline earth hexammines have novel structures and molecular motions. The ammonia molecules in these compounds adopt an entirely different geometry from that of normal ammonia. There are two motional transitions as the temperature is increased. The first transition is due to ammonia rotation, and the second results from ammonia diffusion.     

Reconstructing galaxy histories from globular clusters

Nearly a century after the true nature of galaxies as distant 'island universes' was established, their origin and evolution remain great unsolved problems of modern astrophysics. One of the most promising ways to investigate galaxy formation is to study the ubiquitous globular star clusters that surround most galaxies. Globular clusters are compact groups of up to a few million stars. They generally formed early in the history of the Universe, but have survived the interactions and mergers that alter substantially their parent galaxies. Recent advances in our understanding of the globular cluster systems of the Milky Way and other galaxies point to a complex picture of galaxy genesis driven by cannibalism, collisions, bursts of star formation and other tumultuous events.     

A high abundance of massive galaxies 3-6 billion years after the Big Bang

Hierarchical galaxy formation is the model whereby massive galaxies form from an assembly of smaller units. The most massive objects therefore form last. The model succeeds in describing the clustering of galaxies, but the evolutionary history of massive galaxies, as revealed by their visible stars and gas, is not accurately predicted. Near-infrared observations (which allow us to measure the stellar masses of high-redshift galaxies) and deep multi-colour images indicate that a large fraction of the stars in massive galaxies form in the first 5 Gyr (refs 4-7), but uncertainties remain owing to the lack of spectra to confirm the redshifts (which are estimated from the colours) and the role of obscuration by dust. Here we report the results of a spectroscopic redshift survey that probes the most massive and quiescent galaxies back to an era only 3 Gyr after the Big Bang. We find that at least two-thirds of massive galaxies have appeared since this era, but also that a significant fraction of them are already in place in the early Universe.