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1.
Zraket CA 《Science (New York, N.Y.)》1984,224(4655):1306-1311
Command, control, communications, and intelligence (C(3)l) for nuclear forces are essential elements in the deterrence of nuclear war. The present C(3)l) system has vulnerabilities associated with its reliability, survivability, and endurance under attack, thereby weakening deterrence by increasing the ambiguity in our capabilities. Development of a reliable and enduring C(3)l) system would reduce this ambiguity. Its reliable, positive control of nuclear forces would give the national leadership more time to assess situations, ensure discriminate retaliation, and improve our ability to manage crises in general. These capabilities could help to stop a war rapidly should one start. A reliable and enduring C(3)1) system will be needed for a long time to come, even if a freeze on strategic nuclear forces is accomplished or other arms control successes achieved. Indeed, C(3)l) may be the best source today of confidence-building measures to reduce tensions and the threat of nuclear catastrophe.  相似文献   

2.
The roles of photoperiodism and related light-dependent and hormonal processes in plants are not well understood. Rapid growth, aquatic habit, and adaptability to axenic culture make the Lemnaceae, or duckweeds, excellent material for investigating these topics and others in which highly defined conditions or the presence of organic substances are crucial. As a major example among several that are described, recent work with one species suggests a relation between some features of carbon dioxide flux and the photoperiodic timing mechanism, thus providing a system in which the biochemical basis of the latter may be explored.  相似文献   

3.
Kroto H 《Science (New York, N.Y.)》1988,242(4882):1139-1145
Although carbon has been subjected to far more study than all other elements put together, the buckminsterfullerene hollow-cage structure, recently proposed to account for the exceptional stability of the C(60) cluster, has shed a totally new and revealing light on several important aspects of carbon's chemical and physical properties that were quite unsuspected and others that were not previously well understood. Most significant is the discovery that C(60) appears to form spontaneously, and this has particularly important implications for particle formation in combustion and in space as well as for the chemistry of polyaromatic compounds. The intriguing revelation that 12 pentagonal "defects" convert a planar hexagonal array of any size into a quasi-icosahedral cage explains why some intrinsically planar materials form quasi-crystalline particles, as appears to occur in the case of soot. Although the novel structural proposal has still to be unequivocally confirmed, this article pays particular attention to the way in which it provides convincing explanations of puzzling observations in several fields, so lending credence to the structure proposed for C(60).  相似文献   

4.
The primary rocks are a sequence of titanium-rich basic volcanics, composed of clinopyroxene, plagioclase, and ilmenite with minor olivine, troilite, and native iron. The soil and microbreccias are respectively loose and compacted mixtures of fragments and aggregates of similar rocks, minerals, and glassy fragments and spheres. Impact events are reflected by the presence of shock metamorphosed rock fragments, breccias, and glasses and their resulting compaction to form complex breccias, glass-spattered surfaces, and numerous glass-lined craters. Chemistry of the glasses formed by the impact events is highly variable, and the high iron and nickel content of a few moundlike features suggests that at least some of the projectiles are iron and nickel-rich meteorites.  相似文献   

5.
重金属稀土元素污染在水生物体内的生物富集   总被引:2,自引:0,他引:2  
对铅、镉、汞以及镧、铈在鲤鱼体内的生物富集作用进行了室内试验和讨论。结果表明,重金属及稀土元素在鱼体各部位均有较强的富集作用,其中肌肉部分富集能力较弱,内脏富集能力最强;鱼体各部位对重金属及稀土元素的富集作用随时间的增加而增加,在一定的时间后趋于平衡;并且,鱼体中各部位对重金属及稀土元素的生物富集作用存在较大差异。  相似文献   

6.
7.
Macklis RM 《Science (New York, N.Y.)》2002,295(5560):1647-1648
Although Marie Curie is known primarily for her discovery of radium, her true gift to science was her realization that radioactivity is an intrinsic atomic property of matter rather than the result of chemical processes. She was one of the few Nobel laureates to win the prize twice (physics and chemistry). During her career and as one of the first prominent women scientists, she became increasingly aware of the need for funding for research and of the scientific freedom that money can bring. By nature shy and reserved, Marie's fame, as both a scientist and as an exemplar of a liberated professional woman of the roaring twenties, grew to superstar proportions.  相似文献   

8.
Changes in the US food system and an interest in changing dietary habits among youth have impelled numerous schools and communities to develop programs such as community gardens. Youth community gardens have the potential to positively influence dietary behaviors and enhance environmental awareness and appreciation. However, actual data supporting youth gardening and its influence are limited. The purpose of this study was to explore the effects of community gardens on youth dietary behaviors, values and beliefs, and cooking and gardening behaviors. Focus groups were conducted with inner-city youth living in Minneapolis/St. Paul, Minnesota and compared those involved in a youth garden program with those uninvolved in order to investigate whether the gardening program influenced their habits, beliefs, and values. Findings indicate that youth garden program participants were more willing to eat nutritious food and try ethnic and unfamiliar food than those not in the program. Additionally, it was apparent that garden participants had a stronger appreciation for other individuals and cultures and were more likely to cook and garden on their own than youth not involved in a garden program. The findings suggest that garden programs positively impact youth garden habits, food choice, social skills, nutrition knowledge, and cooking skills. Lauren Lautenschlager is currently a graduate student at the University of Minnesota and working toward obtaining her MS in nutrition and becoming a registered dietitian. She is interested in the education of youth on the food system. Chery Smith is an associate professor in the Department of Food Science and Nutrition at the University of Minnesota. Her research interests include the dietary behavior and nutritional status of low-income and homeless people, community and international nutrition, and food systems.  相似文献   

9.
Quantifying the impact of density, extrinsic climatic fluctuations, and demography on population fluctuations is a persistent challenge in ecology. We analyzed the effect of these processes on the irregular pattern of population crashes of Soay sheep on the St. Kilda archipelago, United Kingdom. Because the age and sex structure of the population fluctuates independently of population size, and because animals of different age and sex respond in different ways to density and weather, identical weather conditions can result in different dynamics in populations of equal size. In addition, the strength of density-dependent processes is a function of the distribution of weather events. Incorporating demographic heterogeneities into population models can influence dynamics and their response to climate change.  相似文献   

10.
The water content of the breccia is 150 to 455 ppm, with a deltaD from-580 to -870 per mil. Hydrogen gas content is 40 to 53 ppm with a deltaD of -830 to -970 per mil. The CO(2) is 290 to 418 ppm with delta (13)C = + 2.3 to + 5.1 per mil and delta(18)O = 14.2 to 19.1 per mil. Non-CO(2) carbon is 22 to 100 ppm, delta(13)C = -6.4 to -23.2 per mil. Lunar dust is 810 ppm H(2)O (D = 80 ppm) and 188 ppm total carbon(delta(13)C = -17.6 per mil). The (18)O analyses of whole rocks range from 5.8 to 6.2 per mil. The temperature of crystallization of type B rocks is 1100 degrees to 1300 degrees C, based on the oxygen isotope fractionation between coexisting plagioclase and ilmenite.  相似文献   

11.
Time, structure, and fluctuations   总被引:1,自引:0,他引:1  
Fundamental conceptual problems that arise from the macroscopic and microscopic aspects of the second law of thermodynamics are considered. It is shown that nonequilibrium may become a source of order and that irreversible processes may lead to a new type of dynamic states of matter called "dissipative structures." The thermodynamic theory of such structures is outlined. A microscopic definition of irreversible processes is given, and a transformation theory is developed that allows one to introduce nonunitary equations of motion that explicitly display irreversibility and approach to thermodynamic equilibrium. The work of the group at the University of Brussels in these fields is briefly reviewed. In this new development of theoretical chemistry and physics, it is likely that thermodynamic concepts will play an ever-increasing role.  相似文献   

12.
棉花丰产株型,株高,茎粗与单株成铃的关系   总被引:3,自引:1,他引:2  
对棉花株高、茎粗与单株成铃性进行了调查研究,所调查群体17.05万株.hm-2;株高平均81.9厘米,茎粗平均0.86厘米.主茎节间长度均匀,单株成铃10.5个。性状分析结果表明,单株成铃与珠高、茎粗相关极显著。由此推断:在保证茎杆粗壮和节间长度均匀的前提下,适当增加棉株高度,对进一步提高产量有利。  相似文献   

13.
The finding of appreciable quantities of p,p'-DDT after feeding o,p'-DDT to rats led to the proposal of a theory, that an isomeric metabolic conversion occurs. The presence of p,p'-DDT as an impurity in supposedly pure samples of o,p'-DDT is the correct explanation for the appearance of p,p'-DDT. Purified o,p'-DDT and (14)C-labeled o,p'-DDT yielded no data to support the idea that o,p'-DDT is converted to the p,p'-DDT isomer.  相似文献   

14.
In the last decade the systems approach to agricultural research has begun to subsume the older reductionist approaches. However, proponents of the systems approach often accept without critical examination a number of features that were inherited from previously accepted approaches. In particular, supporters of the systems approach frequently ignore the ironies and tragedies that are a part of all human endeavors. They may also fail to consider that all actual systems are temporally and spatially bounded. By incorporating such features into a systems perspective, it becomes possible to consider them as involving the manipulation of things, the reconstruction of institutions, and the reformulation of policies in accordance with democratic goals and objectives as part of a single web of interrelationships.  相似文献   

15.
Fiber, food, fuel, and fungal symbionts   总被引:2,自引:0,他引:2  
Virtually all plants of economic importance form mycorrhizae. These absorbing organs of higher plants result from a symbiotic union of beneficial soil fungi and feeder roots. In forestry, the manipulation of fungal symbionts ecologically adapted to the planting site can increase survival and growth of forest trees, particularly on adverse sites. Vesicular-arbuscular mycorrhizae, which occur not only on many trees but also on most cultivated crops, are undoubtedly more important to world food crops. Imperatives for mycorrhizal research in forestry and agriculture are (i) the development of mass inoculum of mycorrhizal fungi, (ii) the interdisciplinary coordination with soil management, plant breeding, cultivation practices, and pest control to ensure maximum survival and development of fungal symbionts in the soil, and (iii) the institution of nursery and field tests to determine the circumstances in which mycorrhizae benefit plant growth in forestry and agri-ecosystems.  相似文献   

16.
浅谈自然生态节约型园林景观   总被引:2,自引:0,他引:2  
刘娜 《现代农业科学》2009,(4):131-131,158
随着社会经济的发展,人们对园林景观的要求也不断提高,然而,这也使部分园林景观设计者会走入一个误区,那就是生搬硬套,盲目地追逐潮流走。其实最好的景观有时就在眼前,而强硬地将现有的景观资源改为另一种景观,这不仅不能做到师法自然,反而浪费了大量的人力、财力和自然资源。作为园林景观设计师应该真正地利用自然资源,因地制宜,设计出真正的自然、生态和节约型的园林景观。  相似文献   

17.
Individual carbon nanotubes are like minute bits of string, and many trillions of these invisible strings must be assembled to make useful macroscopic articles. We demonstrated such assembly at rates above 7 meters per minute by cooperatively rotating carbon nanotubes in vertically oriented nanotube arrays (forests) and made 5-centimeter-wide, meter-long transparent sheets. These self-supporting nanotube sheets are initially formed as a highly anisotropic electronically conducting aerogel that can be densified into strong sheets that are as thin as 50 nanometers. The measured gravimetric strength of orthogonally oriented sheet arrays exceeds that of sheets of high-strength steel. These nanotube sheets have been used in laboratory demonstrations for the microwave bonding of plastics and for making transparent, highly elastomeric electrodes; planar sources of polarized broad-band radiation; conducting appliqués; and flexible organic light-emitting diodes.  相似文献   

18.
为选育适宜苏中地区种植的水稻新品种,选用淮稻14号为母本、徐23121为父本杂交配组,经多年系统选育而成迟熟中粳稻新品种"淮稻18号"。该品种具有丰产性好、品质优、综合抗性强等特点,其全生育期155.1d,株高98.7cm,每穗实粒数121.9粒,结实率92.4%,千粒重28.0g。2012-2014年参加江苏省水稻新品种区域试验,淮稻18号每667m2平均产量分别达到709.7、660.1和658.0kg,较对照淮稻9号分别增产8.64%、7.19%和13.47%。2015年通过江苏省农作物品种审定委员会审定,适宜在江苏省苏中及南京、镇江、扬州丘陵地区种植。  相似文献   

19.
The global distribution of fluorocarbon-12 and fluorocarbon-11 is used to establish a relatively fast interhemispheric exchange rate of 1 to 1.2 years. Atmospheric residence times of 65 to 70 years for fluorocarbon-12 and 40 to 45 years for fluorocarbon-l1 best fit the observational data. These residence times rule out the possibility of any significant missing sinks that may prevent these fluorocarbons from entering the stratosphere. Atmospheric measurements of methyl chloroform support an 8-to 10-year residence time and suggest global average hydroxyl radical (HO) concentrations of 3 x 10(5) to 4 x 10(5) molecules per cubic centimeter. These are a factor of 5 lower than predicted by models. Additionally, methyl chloroform global distribution supports Southern Hemispheric HO levels that are a factor of 1.5 or more larger than the Northern Hemispheric values. The long residence time and the rapid growth of methyl chloroform cause it to be a potentially significant depleter of stratospheric ozone. The oceanic sink for atmospheric carbon tetrachloride is about half as important as the stratospheric sink. A major source of methyl chloride (3 x 10(12)grams per year), sufficient to account for nearly all the atmospheric methyl chloride, has been identified in the ocean.  相似文献   

20.
Before the launch of the Compton Gamma Ray Observatory (CGRO), the only source of >100-megaelectron volt (MeV) gamma radiation known outside our galaxy was the quasar 3C 273. After less than a year of observing, 13 other extragalactic sources have been discovered with the Energetic Gamma Ray Experiment Telescope (EGRET) on CGRO, and it is expected that many more will be found before the full sky survey is complete. All 14 sources show evidence of blazar properties at other wavelengths; these properties include high optical polarization, extreme optical variability, flat-spectrum radio emission associated with a compact core, and apparent superluminal motion. Such properties are thought to be produced by those few, rare extragalactic radio galaxies and quasars that are favorably aligned to permit us to look almost directly down a relativistically outflowing jet of matter expelled from a supermassive black hole. Although the origin of the gamma rays from radio jets is a subject of much controversy, the gamma-ray window probed by CGRO is providing a wealth of knowledge about the central engines of active galactic nuclei and the most energetic processes occurring in nature.  相似文献   

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