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The silica-supported transition metal hydrides (=Si-O-Si=)(=Si-O-)2Ta-H and (=Si-O-)xM-H (M, chromium or tungsten) catalyze the metathesis reaction of linear or branched alkanes into the next higher and lower alkanes at moderate temperature (25degrees to 200degreesC). With (=Si-O-Si=)(=Si-O-)2Ta-H, ethane was transformed at room temperature into an equimolar mixture of propane and methane. Higher and lower homologs were obtained from propane, butane, and pentane as well as from branched alkanes such as isobutane and isopentane. The mechanism of the step leading to carbon-carbon bond cleavage and formation likely involves a four-centered transition state between a tantalum-alkyl intermediate and a carbon-carbon final sigma-bond of a second molecule of alkane. 相似文献
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Spatial Variability of Turbulent Mixing in the Abyssal Ocean 总被引:3,自引:0,他引:3
Ocean microstructure data show that turbulent mixing in the deep Brazil Basin of the South Atlantic Ocean is weak at all depths above smooth abyssal plains and the South American Continental Rise. The diapycnal diffusivity there was estimated to be less than or approximately equal to 0.1 x 10(-4) meters squared per second. In contrast, mixing rates are large throughout the water column above the rough Mid-Atlantic Ridge, and the diffusivity deduced for the bottom-most 150 meters exceeds 5 x 10(-4) meters squared per second. Such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry. 相似文献
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Six horses, which had a foreign body obstruction of the small colon showed abdominal pain of progressing severity and intestinal tympany. On rectal examination the caecum and large colon were distended with ingesta and gas but the obstructing mass could be palpated in only 3 cases. All horses had elevated indirect blood pressure and in 3 there was also fluid distension of the stomach. Only one horse had known access to foreign material in the diet, but a further 3 were related to an exceptionally dry climate period. Five of the 6 horses recovered following surgery. 相似文献
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B.W. Parry BVSc PhD B.A. Christie BVSc MACVSc C.C. Gay DVM MVSc FACVSc R.N. Haywood BVSc 《Journal of Equine Veterinary Science》1983,3(1):24-27
A case of acute circulatory failure in a horse is described. The response to intravenous fluid therapy was monitored by measurement of heart and respiratory rates, and arterial and central venous pressures. Heart rate was an unreliable guide to improved circulatory status. Central venous pressure and indirect arterial blood pressure were valuable guides to the efficacy of fluid therapy. 相似文献
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Evaluation of the agar gel immunodiffusion test for diagnosis of subclinical paratuberculosis in cattle 总被引:2,自引:0,他引:2
Concurrent bacteriologic culture of feces and agar gel immunodiffusion (AGID) testing was performed on all cows and bred heifers over 14 months old in 10 dairy herds during a 32-month period to determine the effectiveness of the AGID test for the detection of subclinical paratuberculosis. Herds were sampled 5 times and, when possible, culled animals were tested again at slaughter. During 5 herd-wide samplings, Mycobacterium paratuberculosis was isolated from 139 fecal specimens obtained from 109 cattle. Results of the AGID test were simultaneously positive 40 of 139 times (28.8%). Thirty-six of the 109 cattle (33.0%) determined to be infected had a positive AGID test result at some point during the 5 herd-wide samplings. When results of tests performed at time of slaughter were included, 117 cattle were identified as infected by culture methods; 55 of these (47.0%) were AGID test-positive at some point during the study. The upper limit of the maximal false-positive rate for the AGID test was 2.1%. On the basis of colony counts from cultures, subclinically infected cows shedding higher numbers of M paratuberculosis in their feces were more likely to have positive AGID test results (P less than 0.0001). In known infected cattle, neither the culture nor AGID test results were consistently positive on repeated testing. Of 48 official calfhood paratuberculosis vaccinates tested as adults, 3 had positive AGID test results and in 1 of these, M paratuberculosis was also isolated from the feces, indicating that the rate of false-positive AGID test results in calfhood vaccinates is low. 相似文献