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31.
Two central nervous system stimulants, magnesium pemoline and dextroamphetamine, were tested to see if they facilitate learning in human subjects. Subjects under placebo learned faster than the subjects under any of the several doses of magnesium pemoline; however, none of these differences reached statistical significance. Subjects who received dextroamphetamine learned significantly more slowly than those who received placebo. 相似文献
32.
Desmosterol as the major sterol in L-cell mouse fibroblasts grown in sterol-free culture medium 总被引:5,自引:0,他引:5
The principal sterol synthesized by L-cell mouse fibroblasts is desmosterol. Cholesterol was not detected in these cells when they were grown in a sterol-free culture medium. These findings indicate that, in cells, cholesterol can be replaced by desmosterol. Sterol analyses of six other tissue culture cell lines revealed cholesterol synthesis. 相似文献
33.
By treatment of muscovite with molten lithium nitrate at 300 degrees C we have been able to remove a large portion of the interlayer potassium and observe a decrease in the apparent b-axis length as a function of potassium content over the range of 8.8 to 0.7 percent potassium. The b-axis length of Silversheen mica (a 2M(1) muscovite) decreases from 9.024 to 8.988 A as the potassium content decreases from 8.79 to 3.31 percent. The contraction of the b-dimension results in an effective decrease in the size of the ditrigonal opening in the planar surfaces with a consequent increase in the d(001) upon re-saturation with potassium. 相似文献
34.
Plastic particles in surface waters of the northwestern atlantic 总被引:1,自引:0,他引:1
35.
Current biogeographic models hypothesize that brown bears migrated from Asia to the New World ~100 to 50 thousand years ago but did not reach areas south of Beringia until ~13 to 12 thousand years ago, after the opening of a mid-continental ice-free corridor. We report a 26-thousand-year-old brown bear fossil from central Alberta, well south of Beringia. Mitochondrial DNA recovered from the specimen shows that it belongs to the same clade of bears inhabiting southern Canada and the northern United States today and that modern brown bears in this region are probably descended from populations that persisted south of the southern glacial margin during the Last Glacial Maximum. 相似文献
36.
Jupiter's main ring shows vertical corrugations reminiscent of those recently detected in the rings of Saturn. The Galileo spacecraft imaged a pair of superimposed ripple patterns in 1996 and again in 2000. These patterns behave as two independent spirals, each winding up at a rate defined by Jupiter's gravity field. The dominant pattern originated between July and October 1994, when the entire ring was tilted by about 2 kilometers. We associate this with the Shoemaker-Levy 9 impacts of July 1994. New Horizons images still show this pattern 13 years later and suggest that subsequent events may also have tilted the ring. Impacts by comets or their dust streams are regular occurrences in planetary rings, altering them in ways that remain detectable decades later. 相似文献
37.
Burns CJ 《Science (New York, N.Y.)》2005,309(5742):1823-1824
38.
39.
Lin EK Soles CL Goldfarb DL Trinque BC Burns SD Jones RL Lenhart JL Angelopoulos M Willson CG Satija SK Wu WL 《Science (New York, N.Y.)》2002,297(5580):372-375
The continuing drive by the semiconductor industry to fabricate smaller structures using photolithography will soon require dimensional control at length scales comparable to the size of the polymeric molecules in the materials used to pattern them. The current technology, chemically amplified photoresists, uses a complex reaction-diffusion process to delineate patterned areas with high spatial resolution. However, nanometer-level control of this critical process is limited by the lack of direct measurements of the reaction front. We demonstrate the use of x-ray and neutron reflectometry as a general method to measure the spatial evolution of the reaction-diffusion process with nanometer resolution. Measuring compositional profiles, provided by deuterium-labeled reactant groups for neutron scattering contrast, we show that the reaction front within the material is broad rather than sharply defined and the compositional profile is altered during development. Measuring the density profile, we directly correlate the developed film structure with that of the reaction front. 相似文献
40.
The effect of surface growth of blue-green algae and bryophytes on some microbiological,biochemical, and physical soil properties 总被引:3,自引:0,他引:3
Summary The influence of surface growth of inoculated cyanobacteria (blue-green algae) on subsurface properties of a brown earth, silt loam soil was studied in reconstituted flooded soil columns. One blue-green algae species, Nostoc muscorum, become dominant within the first 7 days of inoculation. In light control columns (not inoculated) a bryophyte, Barbula recurvirostra, was dominant although significant growth of indigenous blue-green algae occurred. The blue-green algae counts were in the range of 1×106 g-1 dry soil in the surface layer (0–0.7 cm) in both columns. Any effect of surface phototrophic growth on soil properties was restricted to the surface layer. In inoculated columns there was a twofold increase in microbial biomass and an eightfold increase in bacterial numbers by week 13. However, bacterial numbers declined so that there was only a 2.8-fold increase by week 21. Dehydrogenase (x2.1), urease (x2.8) and phosphatase (x3.1) activities and polysaccharides (+69%) increased by week 21 as a result of the blue-green algae inoculation along with a significant improvement in soil aggregation. However, similar increases occurred in the light control columns, indicating that given appropriate conditions of light and moisture indigenous species may be ultimately as effective as introduced species in bringing about biochemical and microbiological changes to soil. 相似文献