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   212篇
  免费   0篇
  国内免费   1篇
系统科学   3篇
教育与普及   2篇
理论与方法论   1篇
现状及发展   49篇
研究方法   35篇
综合类   118篇
自然研究   5篇
  2022年   1篇
  2020年   1篇
  2019年   2篇
  2018年   2篇
  2017年   1篇
  2016年   1篇
  2015年   2篇
  2014年   1篇
  2013年   4篇
  2012年   16篇
  2011年   24篇
  2010年   10篇
  2009年   4篇
  2008年   7篇
  2007年   17篇
  2006年   14篇
  2005年   14篇
  2004年   10篇
  2003年   10篇
  2002年   14篇
  2001年   1篇
  2000年   7篇
  1994年   1篇
  1992年   2篇
  1990年   1篇
  1989年   1篇
  1988年   4篇
  1987年   1篇
  1984年   2篇
  1981年   1篇
  1979年   3篇
  1978年   2篇
  1977年   5篇
  1976年   3篇
  1975年   2篇
  1974年   2篇
  1973年   4篇
  1971年   4篇
  1970年   2篇
  1967年   1篇
  1966年   3篇
  1965年   1篇
  1964年   1篇
  1957年   2篇
  1955年   1篇
  1954年   1篇
排序方式: 共有213条查询结果,搜索用时 125 毫秒
51.
At some point during the 1950s, mainstream American philosophy of science began increasingly to avoid questions about the role of non-cognitive values in science and, accordingly, increasingly to avoid active engagement with social, political and moral concerns. Such questions and engagement eventually ceased to be part of the mainstream. Here we show that the eventual dominance of ‘value-free’ philosophy of science can be attributed, at least in part, to the policies of the U.S. National Science Foundation's “History and Philosophy of Science” sub-program. In turn, the sub-program's policies were set by logical empiricists who espoused value-free philosophy of science; these philosophers' actions, we also point out, fit a broad pattern, one in which analytic philosophers used institutional control to marginalize rival approaches to philosophy. We go on to draw on existing knowledge of this pattern to suggest two further, similar, contributors to the withdrawal from value-laden philosophy of science, namely decisions by the editors of Philosophy of Science and by the editors of The Journal of Philosophy. Political climate was, we argue, at most an indirect contributor to the withdrawal and was neither a factor that decided whether it occurred nor one that was sufficient to bring it about. Moreover, we argue that the actions at the National Science Foundation went beyond what was required by its senior administrators and are better viewed as part of what drove, rather than as what was being driven by, the adoption of logical empiricism by the philosophy of science community.  相似文献   
52.
Two federally listed fishes, the Foskett speckled dace and Warner sucker, are endemic to Warner Basin in south central Oregon. The Foskett speckled dace is native only to a single spring in Coleman Valley. A nearby spring was stocked with dace in 1979 and 1980, and now provides a second population. The present numbers of dace probably are at their highest levels since settlement of the region. The Warner sucker historically occurred throughout much of the Warner Valley, but its distribution and abundance have been reduced by construction of reservoirs and irrigation dams and the introduction of predatory game fishes. Lentic habitats have become dominated by introduced fishes, particularly white crappie, black crappie, and brown bullhead. The largest remaining population of Warner suckers occurs in Hart Lake, where successful reproduction was documented but there is no evidence of recruitment to the adult population.  相似文献   
53.
54.
Heit JJ  Apelqvist AA  Gu X  Winslow MM  Neilson JR  Crabtree GR  Kim SK 《Nature》2006,443(7109):345-349
The growth and function of organs such as pancreatic islets adapt to meet physiological challenges and maintain metabolic balance, but the mechanisms controlling these facultative responses are unclear. Diabetes in patients treated with calcineurin inhibitors such as cyclosporin A indicates that calcineurin/nuclear factor of activated T-cells (NFAT) signalling might control adaptive islet responses, but the roles of this pathway in beta-cells in vivo are not understood. Here we show that mice with a beta-cell-specific deletion of the calcineurin phosphatase regulatory subunit, calcineurin b1 (Cnb1), develop age-dependent diabetes characterized by decreased beta-cell proliferation and mass, reduced pancreatic insulin content and hypoinsulinaemia. Moreover, beta-cells lacking Cnb1 have a reduced expression of established regulators of beta-cell proliferation. Conditional expression of active NFATc1 in Cnb1-deficient beta-cells rescues these defects and prevents diabetes. In normal adult beta-cells, conditional NFAT activation promotes the expression of cell-cycle regulators and increases beta-cell proliferation and mass, resulting in hyperinsulinaemia. Conditional NFAT activation also induces the expression of genes critical for beta-cell endocrine function, including all six genes mutated in hereditary forms of monogenic type 2 diabetes. Thus, calcineurin/NFAT signalling regulates multiple factors that control growth and hallmark beta-cell functions, revealing unique models for the pathogenesis and therapy of diabetes.  相似文献   
55.
Pluto's first known satellite, Charon, was discovered in 1978. It has a diameter (approximately 1,200 km) about half that of Pluto, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful, but they were not sensitive to objects less, similar150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1's diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of approximately 38 days (P1) and approximately 25 days (P2).  相似文献   
56.
The low temperature EPR spectrum of a quickly reacted mixture of oxyhemoglobin and phenylhydrazine was studied. With the use of a computer, the spectral contribution of methemoglobin in the region of g = 2 was subtracted. The remaining spectrum was that of an axial free radical (g perpendicular = 2.00, g parallel = 2.06) having the magnetic parameters of superoxide anion. In the presence of superoxide dismutase, this axial radical was not seen, confirming that superoxide anion is indeed generated by the reaction.  相似文献   
57.
58.
Bizzarro M  Baker JA  Haack H  Ulfbeck D  Rosing M 《Nature》2003,421(6926):931-933
The 176Lu to 176Hf decay series has been widely used to understand the nature of Earth's early crust-mantle system. The interpretation, however, of Lu-Hf isotope data requires accurate knowledge of the radioactive decay constant of 176Lu (lambda176Lu), as well as bulk-Earth reference parameters. A recent calibration of the lambda176Lu value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3.9 Gyr, implying widespread continental crust extraction from an isotopically enriched mantle source more than 4.3 Gyr ago, but does not provide evidence for a complementary depleted mantle reservoir. Here we report Lu-Hf isotope measurements of different Solar System objects including chondrites and basaltic eucrites. The chondrites define a Lu-Hf isochron with an initial 176Hf/177Hf ratio of 0.279628 +/- 0.000047, corresponding to lambda176Lu = 1.983 +/- 0.033 x 10-11 yr-1 using an age of 4.56 Gyr for the chondrite-forming event. This lambda176Lu value indicates that Earth's oldest minerals were derived from melts of a mantle source with a time-integrated history of depletion rather than enrichment. The depletion event must have occurred no later than 320 Myr after planetary accretion, consistent with timing inferred from extinct radionuclides.  相似文献   
59.
Summary 2-m latex particles accumulated in macrophages in intestinal Peyer's patches of mice given latex suspensions as drinking fluid for 2 months. The number of particles accumulating was a direct (but nonlinear) function of the number ingested. Some of the latex particles were still present in Peyer's patches 6 weeks after the cessation of latex feeding.Acknowledgments. Research supported by the U. S. Energy Research and Development Administration. We thank Mr W. Marin, Division of Photography and Graphic Arts, Brookhaven National Laboratory, for figure 2.The research described in this report involved animals maintained in animal care facilities fully accredited by the American Association for Accreditation of Laboratory Animal Care.  相似文献   
60.
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