首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   25228篇
  免费   58篇
  国内免费   72篇
系统科学   108篇
丛书文集   295篇
教育与普及   66篇
理论与方法论   104篇
现状及发展   10291篇
研究方法   1062篇
综合类   13003篇
自然研究   429篇
  2013年   233篇
  2012年   353篇
  2011年   780篇
  2010年   158篇
  2008年   401篇
  2007年   490篇
  2006年   479篇
  2005年   482篇
  2004年   448篇
  2003年   435篇
  2002年   446篇
  2001年   882篇
  2000年   862篇
  1999年   494篇
  1992年   467篇
  1991年   376篇
  1990年   404篇
  1989年   436篇
  1988年   403篇
  1987年   383篇
  1986年   419篇
  1985年   493篇
  1984年   373篇
  1983年   349篇
  1982年   277篇
  1981年   280篇
  1980年   335篇
  1979年   820篇
  1978年   628篇
  1977年   616篇
  1976年   484篇
  1975年   527篇
  1974年   777篇
  1973年   618篇
  1972年   647篇
  1971年   767篇
  1970年   962篇
  1969年   732篇
  1968年   764篇
  1967年   670篇
  1966年   652篇
  1965年   453篇
  1964年   111篇
  1959年   245篇
  1958年   411篇
  1957年   312篇
  1956年   257篇
  1955年   235篇
  1954年   230篇
  1948年   186篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
231.
In 1670, the Bolognese mathematician Pietro Mengoli published his Speculationi di musica, a highly original work attempting to found the mathematical study of music on the anatomy of the ear. His anatomy was idiosyncratic and his mathematics extraordinarily complex, and he proposed a unique double mechanism of hearing. He analysed in detail the supposed behaviour of the subtle part of the air inside the ear, and the patterns of strokes made on the eardrum by simultaneous sounds. Most strikingly, he divided the musical octave into a continuous set of regions which he colour-coded to show their effects on a listener. His work did not find its way into the mainstream of seventeenth-century mathematical studies of music, but when examined in its context it has the potential to shed light on that discipline, as well as being of considerable interest in its own right. Here, I focus on the anatomical and mathematical basis of Mengoli's work.  相似文献   
232.
233.
In 1639–1640 Benedetto Castelli (1577–1643) wrote a treatise on the loadstone which is quite unlike any of its contemporaries. In it are the origins of the notion of elementary magnets sharing a common alignment, the idea that all materials are magnetic in different ways, and the first intimation of the conception of magnetic domains. Castelli did not publish his treatise. Nevertheless his work was noted during his life-time, and may have exerted an influence on the development of magnetic theory in the 17th century. The treatise was published in 1883. Since then, however, it has either been neglected or not appreciated. It deserves being rescued from the neglect of more than three centuries.  相似文献   
234.
235.
We present an analysis, and first full English translation, of a paper by Kant entitled ‘Über die Vulcane im Monde’ (1785). Kant became interested in the question of whether the mountains of the Moon were extinct volcanoes. Stimulated by the work of Herschel, Aepinus, and others, he considered the appearance of the Moon's surface and the possibility of lunar vulcanism. From this, he was led to consider the structures of mountain ranges on the Earth, which he decided were non-volcanic in origin, being produced by eruptions of vapours from the interior of the Earth soon after it formed from an original ‘chaos’. Kant developed his ideas in such a way as to yield a characteristic eighteenth-century ‘theory of the Earth’. We argue that the empirical base of his theory was provided by knowledge of the mountain ranges of Bohemia and Moravia. Analogies based on observations of the Moon further assisted in the construction of the theory. But the reasoning ran in two directions: what was seen on the Moon was construed in terms of what Kant knew of the Earth's topography; and the Earth's topography was presumed to be analogous to that of the Moon, for both the Earth and the Moon (and indeed all heavenly bodies) supposedly had essentially similar origins. Kant's ideas of 1785 are related to his earlier writings of 1754, 1755, and 1756, and also to the lectures on physical geography that he presented at Königsberg.  相似文献   
236.
As the most famous woman scientist of the twentieth century, there has been no shortage of books and articles on the life and career of Marie Curie (1867–1934). Her role as a director of a laboratory-based research school in the new scientific field of radioactivity, a field which embraced both chemistry and physics, however, has never been examined. In recent years, there has been a growing interest in the question of research schools, and Morrell, Ravetz, Geison, and Klosterman, amongst others, have written on this subject. Using, in part, the methodology of Morrell, this paper investigates the role of Marie Curie as a school director in the Paris Faculty in the years 1907–14, examining the work and characteristics of her school and assessing her effectiveness as a director.  相似文献   
237.
Pest insect species are a burden to humans as they destroy crops and serve as vectors for a wide range of diseases including malaria and dengue. Chemical insecticides are currently the dominant approach for combating these pests. However, the de-registration of key classes of chemical insecticides due to their perceived ecological and human health risks in combination with the development of insecticide resistance in many pest insect populations has created an urgent need for improved methods of insect pest control. The venoms of arthropod predators such as spiders and scorpions are a promising source of novel insecticidal peptides that often have different modes of action to extant chemical insecticides. These peptides have been optimized via a prey–predator arms race spanning hundreds of millions of years to target specific types of insect ion channels and receptors. Here we review the current literature on insecticidal venom peptides, with a particular focus on their structural and pharmacological diversity, and discuss their potential for deployment as insecticides.  相似文献   
238.
Two new species of tarantulas from Mexico are described and illustrated: Bonnetina tenuiverpis and Bonnetina juxtantricola, from the states of Mexico and Guerrero, respectively. Male palpal bulbs, tibial apophyses and spermatheca are among the most taxonomically informative characters. Male bulb microstructure is revealed from scanning electron microscopy; both new and homologous structures to other Theraphosinae genera are identified and described. Nomenclatural changes for male tibial apophyses are also proposed. The holotype male and allotype female of one of the species are molecularly characterized and matched from CO1 partial sequence.

http://zoobank.org/urn:lsid:zoobank.org:pub:BBDBBEDE-6983-4F5C-B34C-4622DF494C84  相似文献   
239.
Parasites of the grey mullet Mugil cephalus collected from Lower K?z?l?rmak Delta in Turkey were investigated. Trichodina puytoraci, Trichodina lepsii, Ligophorus mediterraneus, Ligophorus cephali, Gyrodactylus sp., Microcotyle mugilis, Ascocotyle (Phagicola) longa, Haplosplanchnus pachysomus, Diplostomum spathaceum, Tylodelphys clavata, Posthodiplostomum sp., Neoechinorhyncus agilis and Ergasilus lizae were identified. Overall infection prevalence (%), mean intensity and mean abundance values were 99.61%, 458.10 ± 75.72 and 456.30 ± 75.44, respectively. Infection indices of parasite species in relation to season, lakes/lagoons and some physicochemical parameters of water such as temperature, salinity, dissolved oxygen and nitrate are determined and discussed. Posthodiplostomum sp. is a new parasite record for mugilids and Diplostomum spathaceum is the first record in M. cephalus, in the present study. Moreover, Ligophorus mediterraneus, Ligophorus cephali and Ergasilus lizae are new records for Turkish parasite fauna of fish and Mugil cephalus is a new host record for Neoechinorhyncus agilis in Turkey.  相似文献   
240.
Membrane proteins are key elements in cell physiology and drug targeting, but getting a high-resolution structure by crystallographic means is still enormously challenging. Novel strategies are in big demand to facilitate the structure determination process that will ultimately hasten the day when sequence information alone can provide a three-dimensional model. Cell-free or in vitro expression enables rapid access to large quantities of high-quality membrane proteins suitable for an array of applications. Despite its impressive efficiency, to date only two membrane proteins produced by the in vitro approach have yielded crystal structures. Here, we have analysed synergies of cell-free expression and crystallisation in lipid mesophases for generating an X-ray structure of the integral membrane enzyme diacylglycerol kinase to 2.28-Å resolution. The quality of cellular and cell-free-expressed kinase samples has been evaluated systematically by comparing (1) spectroscopic properties, (2) purity and oligomer formation, (3) lipid content and (4) functionality. DgkA is the first membrane enzyme crystallised based on cell-free expression. The study provides a basic standard for the crystallisation of cell-free-expressed membrane proteins and the methods detailed here should prove generally useful and contribute to accelerating the pace at which membrane protein structures are solved.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号