全文获取类型
收费全文 | 10467篇 |
免费 | 2136篇 |
国内免费 | 2569篇 |
专业分类
测绘学 | 1156篇 |
大气科学 | 1977篇 |
地球物理 | 2517篇 |
地质学 | 6213篇 |
海洋学 | 1439篇 |
天文学 | 189篇 |
综合类 | 660篇 |
自然地理 | 1021篇 |
出版年
2024年 | 99篇 |
2023年 | 320篇 |
2022年 | 347篇 |
2021年 | 374篇 |
2020年 | 318篇 |
2019年 | 413篇 |
2018年 | 439篇 |
2017年 | 265篇 |
2016年 | 335篇 |
2015年 | 358篇 |
2014年 | 591篇 |
2013年 | 464篇 |
2012年 | 520篇 |
2011年 | 511篇 |
2010年 | 459篇 |
2009年 | 462篇 |
2008年 | 508篇 |
2007年 | 499篇 |
2006年 | 510篇 |
2005年 | 489篇 |
2004年 | 457篇 |
2003年 | 393篇 |
2002年 | 399篇 |
2001年 | 360篇 |
2000年 | 385篇 |
1999年 | 312篇 |
1998年 | 350篇 |
1997年 | 411篇 |
1996年 | 399篇 |
1995年 | 339篇 |
1994年 | 338篇 |
1993年 | 311篇 |
1992年 | 323篇 |
1991年 | 315篇 |
1990年 | 286篇 |
1989年 | 265篇 |
1988年 | 160篇 |
1987年 | 162篇 |
1986年 | 131篇 |
1985年 | 151篇 |
1984年 | 124篇 |
1983年 | 98篇 |
1982年 | 92篇 |
1981年 | 68篇 |
1980年 | 60篇 |
1979年 | 29篇 |
1978年 | 13篇 |
1959年 | 13篇 |
1958年 | 31篇 |
1957年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
21.
From elements formed in interstellar furnaces to humans peering back at the stars, the evolution of life has been a long, intricate and perhaps inevitable process. Life as we know it requires a planet orbiting a star at just the right distance so that water can exist in liquid form. It needs a rich supply of chemicals and energy sources. On Earth, the combination of chemistry and energy generated molecules that evolved ways of replicating themselves and of passing information from one generation to the next. Thus, the thread of life began. This chart traces the thread, maintained by DNA molecules for much of its history, as it weaves its way through the primitive oceans, gaining strength and diversity along the way. Organisms eventually moved onto the land, where advanced forms, including humans, ultimately arose. Finally, assisted by a technology of its own making, life has reached back out into space to understand its own origins, to expand into new realms, and to seek other living threads in the cosmos. 相似文献
22.
Chandna RC Sharma D 《Population geography : a journal of the Association of Population Geographers of India》1987,9(1-2):45-55
Using a sample of 885 females (1/2 of all married women of reproductive age), this study examines the role of education as a determinant of fertility among couples in Kullu town of Himachol Pradesh, India. Of the 885 respondents, only 149 were illiterate. The average family size was 2.88. Findings reveal that 1) the average family size was largest for illiterate respondents (3.57) and for illiterate males (3.76); 2) average family size declined consistently after the middle level of education to 1.29 for post graduate respondents and 2.33 for post graduate males; 3) a negative correlation exists between fertility and education of both husband and wife, with the wife's education having a stronger negative correlation with fertility; and 4) couples with an educational level of matriculation and above have a distinctly smaller family size than those less educated. 相似文献
23.
An observational test--the detection of a hydrogen coma around comets at large heliocentric distances--is proposed for determining whether comets were formed by the agglomeration of unaltered, ice-coated, interstellar grains. Laboratory experiments showed that amorphous water ice traps H2, D2, and Ne below 20 K and does not release them completely until the ice is heated to 150 K. Gas/ice ratios as high as 0.63 are obtainable. Thus, if the ice-coated interstellar grains were not heated above approximately 110 K, prior to their agglomeration into cometary nuclei, the inward propagating heat waves should release from the comets a continuous flux of molecular hydrogen. This flux would exceed that of water molecules at approximately 3 AU preperihelion and approximately 4 AU postperihelion. 相似文献
24.
25.
古代西域南道上的若干历史地理问题 总被引:1,自引:0,他引:1
本文探讨塔克拉玛干沙漠南沿、和田以东至若羌间的几个历史地理问题。1,历史时期,塔里木盆地南沿上是否有过一条与北方塔里木河相对应的“南河”?笔者认为是没有的。2,自古以来,和田以东的绿洲数目是在逐期增多或是逐期减少?笔者认为是逐期增多。3,历史时期,和田以东出现过几处绿洲城镇废弃和交通路线改线的现象,笔者就其中的安迪尔、且末、尼雅三地的历史演变作了初步探讨。 相似文献
26.
一万年来白洋淀的扩张与收缩 总被引:5,自引:0,他引:5
本文探讨了一万年来的全新世期间,白洋淀兴起、扩张和收缩的变迁过程,恢复了各个发展阶段的古湖淀范围,论述了相应时期的古环境特征,指出了白洋淀令后的演变趋势。 相似文献
27.
28.
天山乌鲁木齐河源1号冰川积累区气溶胶和表层雪中可溶性矿物粉尘的变化特征及相互关系——以Ca2+、Mg2+为例 总被引:1,自引:0,他引:1
对天山乌鲁木齐河源1号冰川(以下简称1号冰川)积累区海拔4 130 m处12个月、每周1次的气溶胶和表层雪样品进行了分析.结果表明,1号冰川气溶胶与表层雪中Ca2 、Mg2 的平均浓度均在夏季最高,冬季最低.气溶胶中Ca2 、Mg2 浓度变化规律极其相似,二者在一年内出现两次峰值,第1次出现在春季,第2次出现在夏末秋初.表层雪中这两种离子浓度变化趋势也相似,峰值均出现在夏季,且Ca2 浓度峰值的出现稍早于Mg2 ,二者浓度在冬季都有所升高,尤其Ca2 更为显著.气溶胶和表层雪中对应离子间在整体变化趋势上较为相似,且在日时间尺度上的对应关系有显著的特点.相应离子的对应关系,可能受粉尘微粒的溶解、离子的清除比率、离子的淋溶强弱等性质的影响. 相似文献
29.
An unusual feature has been observed in polychaetes regarding their capacity to accumulate high levels of relatively toxic forms of arsenic in specific tissues. Basal levels of arsenic and distribution of its compounds were investigated in tissues of the Mediterranean polychaete Sabella spallanzanii. Particularly high concentrations were measured in the branchial crown (1036+/-136 microg/g d.w.) and chemical speciation revealed that the predominant form was the relatively toxic dimethylarsinate (DMA). These data suggest a potential role of As as anti-predatory strategy in polychaetes. 相似文献
30.
能产生名贵珍珠的合浦珠母贝Pinctada martensi(Dunker)是一种栖息在近岸内湾的暖水性双壳类,是国内外珍珠养殖的主要母贝。 珍珠养殖包括母贝养成和植核育珠两大部分。从母贝育种至养成可供植核的大贝,不论从珍珠养殖的生物学考虑还是从企业的经济效 相似文献