全文获取类型
收费全文 | 42739篇 |
免费 | 78篇 |
国内免费 | 134篇 |
专业分类
系统科学 | 197篇 |
丛书文集 | 848篇 |
教育与普及 | 129篇 |
理论与方法论 | 239篇 |
现状及发展 | 19197篇 |
研究方法 | 1612篇 |
综合类 | 20221篇 |
自然研究 | 508篇 |
出版年
2013年 | 242篇 |
2012年 | 516篇 |
2011年 | 1045篇 |
2008年 | 710篇 |
2007年 | 709篇 |
2006年 | 741篇 |
2005年 | 760篇 |
2004年 | 741篇 |
2003年 | 734篇 |
2002年 | 739篇 |
2001年 | 1224篇 |
2000年 | 1140篇 |
1999年 | 785篇 |
1994年 | 369篇 |
1992年 | 718篇 |
1991年 | 602篇 |
1990年 | 661篇 |
1989年 | 627篇 |
1988年 | 640篇 |
1987年 | 678篇 |
1986年 | 694篇 |
1985年 | 822篇 |
1984年 | 639篇 |
1983年 | 571篇 |
1982年 | 494篇 |
1981年 | 480篇 |
1980年 | 608篇 |
1979年 | 1315篇 |
1978年 | 1147篇 |
1977年 | 1102篇 |
1976年 | 800篇 |
1975年 | 913篇 |
1974年 | 1260篇 |
1973年 | 1083篇 |
1972年 | 1122篇 |
1971年 | 1426篇 |
1970年 | 1775篇 |
1969年 | 1393篇 |
1968年 | 1314篇 |
1967年 | 1307篇 |
1966年 | 1130篇 |
1965年 | 823篇 |
1964年 | 250篇 |
1959年 | 479篇 |
1958年 | 785篇 |
1957年 | 620篇 |
1956年 | 524篇 |
1955年 | 481篇 |
1954年 | 510篇 |
1948年 | 328篇 |
排序方式: 共有10000条查询结果,搜索用时 500 毫秒
981.
In 1793 Lord Macartney arrived in China as ambassador of King George III. The aims of his embassy were largely directed towards the enlargement of British trade with the far east, and especially with China. The embassy also had a diplomatic and cultural mission, to impress the Chinese with British achievements. They were to do so largely by distributing presents of British manufactures, chief among them being scientific instruments. The Chinese refused the embassy's requests, and clearly regarded the gifts of instruments as merely ingenious toys. This paper describes the role of instruments in the embassy, and contrasts British expectations with Chinese attitudes to scientific instruments. The embassy's failure is shown to reveal fundamental differences in British and Chinese eighteenth-century responses to science, and has wide cultural implications. 相似文献
982.
L.J.M. Coleby M.A. M.Sc. Ph.D. 《Annals of science》2013,70(2):101-123
The development of the circular dividing engine in England is traced from Henry Hindley and Jesse Ramsden through the improvements introduced by Ramsden's successors to the self-acting engine of William Simms. Particular emphasis is given to the invention, evolution and transmission of the methods used to achieve accuracy in: dividing the wheel; ratching the teeth and matching them to the endless screw; and mounting the cutter. The procedures adopted by Ramsden and Troughton for correcting initial dividing marks are also described. 相似文献
983.
Two populations of Helianthus niveus (Benth.) Brandegee ssp. canescens (A. Gray) Heiser were discovered in northern Arizona extending the range 300 miles north for this species. Helianthus deserticola Heiser range was extended 100 miles east in both Arizona and Utah. 相似文献
984.
Angela M. Ortega-León Maricela Villagrán-Santa Cruz J. Jamie Zú?iga-Vega Raúl Cueva-del Castillo Fausto R. Méndez-de la Cruz 《西北部美国博物学家》2011,69(1)
Asynchronous reproduction is a common phenomenon in high-elevation populations of lizards from Central México. Sperm storage in the reproductive tract of females is the mechanism for making oocyte fertilization possible. Our study addresses questions related to functional oviductal sperm storage of females mating on different dates throughout the reproductive season. A population of Sceloporus mucronatus with copulation in the summer and ovulation in the fall was chosen for this experiment. Eleven females that copulated in the field during June and 13 females that copulated in captivity during August were maintained in the laboratory until parturition. The number of pregnant females and the litter sizes produced in each experimental group were indicative of the viability of the stored sperm. Sperm stored in the reproductive tract of females were able to fertilize eggs after 4 months. No significant differences were found in the number of pregnant females between the 2 experimental groups nor in the litter sizes that they produced. We found that the amount of time sperm were held in the female reproductive tract (ca. 3 months) had no effect on the capacity of sperm to fertilize eggs. Histological examination of 8 oviducts collected before the mating season eliminated the possibility of sperm storage from one year to the next. In this system, sperm retention could have evolved as a response mechanism to deal with the asynchrony between sexes in the reproductive cycles. However, we cannot rule out alternative hypotheses. 相似文献
985.
Five additional species of wood rotting basidiomycetes on Arizona junipers are described and illustrated. All cause a white rot of dead, fallen trees. Leptosporomyces juniperinus is proposed as a new species. Vararia fibra is reported from the United States for the first time. 相似文献
986.
987.
P.A. Bretscher N. Ismail J.N. Menon C.A. Power J. Uzonna G. Wei 《Cellular and molecular life sciences : CMLS》2001,58(12-13):1879-1896
The occurrence of infectious disease represents a failure of the immune system, a failure that must be prevented by effective vaccination or remedied by treatment. Vaccination against acute diseases such as smallpox and polio are very effective, due to the rapid and increased immune response of vaccinated individuals upon natural infection. In contrast, effective vaccination against intracellular pathogens that cause chronic diseases, such as the leishmaniases, tuberculosis and AIDS, has not been achieved. Clinical observations suggest cell-mediated, Th1 responses, exclusive of antibody production and the generation of Th2 cells, are optimally protective against these intracellular pathogens. Effective vaccination must ensure the generation of such a protective response. We explore here whether understanding very broad features of the regulation of the immune response can accommodate modern findings on the immunological features of these diseases, and provide a perspective within which strategies for effective vaccination and treatment can be developed. 相似文献
988.
989.
The effect of Si on the austenite stabilization, martensite morphology, and magnetic properties in Fe-26%Ni-x%Si (x=3.5, 5, and 6) alloys have been studied by means of transmission electron microscopy (TEM) and M?ssbauer spectroscopy techniques. TEM observations reveal that the martensite morphology is closely dependent on the Si content. The volume fraction changes of martensite and austenite phases, the hyperfine magnetic field, and isomer shift values have been determined by Mssbauer spectroscopy. The M?ssbauer study reveals that the hyperfine magnetic field, the isomer shift values and the volume fraction of martensite decrease with increasing Si content. 相似文献
990.
Karen Schrader Jisen Huai Lars Jöckel Carolin Oberle Christoph Borner 《Cellular and molecular life sciences : CMLS》2010,67(10):1607-1618
Caspases are the most important effectors of apoptosis, the major form of programmed cell death (PCD) in multicellular organisms. This is best reflected by the appearance of serious development defects in mice deficient for caspase-8, -9, and -3. Meanwhile, caspase-independent PCD, mediated by other proteases or signaling components has been described in numerous publications. Although we do not doubt that such cell death exists, we propose that it has evolved later during evolution and is most likely not designed to execute, but to amplify and speed-up caspase-dependent cell death. This review shall provide evidence for such a concept. 相似文献