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   295966篇
  免费   22520篇
  国内免费   318篇
  2018年   8282篇
  2017年   8132篇
  2016年   7154篇
  2015年   3878篇
  2014年   4615篇
  2013年   6585篇
  2012年   9301篇
  2011年   14430篇
  2010年   10671篇
  2009年   9979篇
  2008年   12233篇
  2007年   13936篇
  2006年   6798篇
  2005年   7290篇
  2004年   6967篇
  2003年   7046篇
  2002年   6840篇
  2001年   12128篇
  2000年   12145篇
  1999年   9149篇
  1998年   2676篇
  1997年   2758篇
  1996年   2714篇
  1995年   2479篇
  1994年   2429篇
  1993年   2345篇
  1992年   7195篇
  1991年   7005篇
  1990年   7061篇
  1989年   6858篇
  1988年   6358篇
  1987年   6018篇
  1986年   5356篇
  1985年   5661篇
  1984年   4474篇
  1983年   3876篇
  1982年   2682篇
  1981年   2493篇
  1980年   2303篇
  1979年   4106篇
  1978年   3133篇
  1977年   2879篇
  1976年   2827篇
  1975年   3276篇
  1974年   3496篇
  1973年   3522篇
  1972年   3627篇
  1971年   3345篇
  1970年   2534篇
  1969年   2299篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
951.
952.
Alkaliphilic anaerobic community at pH 10   总被引:7,自引:0,他引:7  
Relict or ancient microbial communities in extreme environment might be analogous to the centers of origin of bacterial diversity. A bacterial community of an alkaline lake was investigated, and the diversity of bacteria found there indicates that both conditions of autonomy and phylogenetic variety are fulfilled for anaerobic bacteria developing at pH 10±0.2. Major functional groups in the trophic network were present. Representatives of proteolytic, bacteriolytic, cellulolytic, saccharolytic, dissipotrophic, acetogenic, sulfate-reducing, methanogenic bacteria were isolated.  相似文献   
953.
Enumeration of potentially pathogenic bacteria from sewage sludges.   总被引:9,自引:8,他引:1       下载免费PDF全文
To ascertain the health risks that may be posed by the land application of sewage sludges, a scheme was devised to determine the types and numbers of pathogenic and potentially pathogenic bacteria present in sludges. A processing treatment was adapted to sludge to give a homogenate which yielded the greatest numbers of viable bacteria. Conventional methods were successful in enumerating Klebsiella, Staphylococcus, gram-negative enteric bacteria, and commonly used indicator organisms. Modifications of conventional methods improved the enumeration of Salmonella, Mycobacterium sp., fluorescent Pseudomonas sp., and Clostridium perfringens. However, Shigella methodology yielded only one isolate. Utilizing the proposed scheme, the population densities of these organisms were estimated in three domestic wastewater sludges. In light of these results, the potential impact of land application of sewage sludges is discussed.  相似文献   
954.
Understanding the nature of organic matter is a necessary first step in assessing contaminant bioavailability and allowing water supply managers to optimise the treatment train in the aim of providing safe and inexpensive drinking water. This study provides further insight into the composition, structure and functional groups of dissolved organic matter (DOM) (both hydrophobic and hydrophilic) from urban aquatic systems by means of various analytical techniques (DAX-8/XAD-4 fractionation, elemental analysis, UV and FTIR spectroscopies, 13C and 15N isotopic analysis, size exclusion chromatography and Pyrolysis-GC-MS). The analytical range chosen for this study constitutes a powerful tool in the characterisation of DOM in urban water. The inclusion of information from one technique to the next might not only serve as a support to each one, but also as a complement. The DOM fraction from treated effluent and, more generally, DOM from urban water (i.e. receiving treated effluent) display a strong hydrophilic characteristic [i.e. low humic substance (HS) content, low SUVA], along with a high distribution in molecular weights observed by SEC and low average molecular weight. Due to the origin of this DOM, proteinaceous structures constitute the main compounds, as observed by FTIR and Py-GC-MS. Such characteristics (i.e. heterogeneity, low average molecular weight and diverse functional groups, which make up a total of N) could explain that DOM from treated effluent displayed a strong reactive potential metals pollutants as previously demonstrated.  相似文献   
955.
Six polymorphic dinucleotide microsatellite loci were isolated and characterized from the White‐chinned Petrel Procellaria aequinoctialis, using a degenerate primer and PCR‐based technique to construct and screen an enriched genomic library. Preliminary data on three populations show heterozygosity levels ranging from 0.22 to 0.67 and allele numbers from three to nine. Preliminary data also suggest genetic distance between these three populations (FST 0.088). Cross‐species amplification of these six microsatellite loci and one further locus were tested in six other procellariiform species of the genus Procellaria, Macronectes, Thalassarche and Diomedea.  相似文献   
956.
957.
958.
Social justice issues remain some of the most pressing problems in the United States. One aspect of social justice involves the differential treatment of demographic groups in the criminal justice system. While data consistently show that Blacks and Hispanics are often treated differently than Whites, one understudied aspect of these disparities is how police officers'' assessments of suspects'' size affects their decisions. Using over 3 million cases from the New York Police Department (NYPD) Stop, Question, and Frisk (SQF) Database, 2006–2013, this study is the first to explore suspects'' race, perceived size, and police treatment. Results indicate that tall and heavy black and Hispanic men are at the greatest risk for frisk or search. Tall and heavy suspects are at increased risk for experiencing police force, with black and Hispanic men being more likely to experience force than white men across size categories.  相似文献   
959.
960.
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