Isochore selection

Statistical thermodynamics gives us the recipes to perform this average. The most appropriate Gibbsian ensemble for our problem is the canonical one (namely the isochoric-isothermal ensemble N,V,T). We remark, in passing, that other ensembles such as the grand canonical one have to be selected for other solvation problems). To determine the partition function necessary to compute the thermodynamic properties of the system, and in particular the solvation energy of M which we are now interested in, of a computer simulation is necessary [1],... 

Abstract A synthetic pure water fluid inclusion showing a wide temperature range of metastability (Th - Tn 50°C temperature of homogenization Th = 144°C and nucleation temperature of Tn = 89°C) was selected to make a kinetic study of the lifetime of an isolated microvolume of superheated water. The occluded liquid was placed in the metastable field by isochoric cooling and the duration of the metastable state was measured repetitively for 7 fixed temperatures above Tn. Statistically, metastability lifetimes for the 7 data sets follow the exponential reliability distribution, i.e., the probability of non nucleation within time t equals. This enabled us to calculate the half-life periods of metastability r for each of the selected temperature, and then to predict i at any temperature T > Tn for the considered inclusion, according to the equation i(s) = 22.1x j Hence we conclude that... 

An isochoric equation has been developed for computing thermodynamic functions of pure fluids. It has its origin on a given liquid-vapor coexistence boundary, and it is structured to be consistent with the known behavior of specific heats, especially about the critical point. The number of adjustable, least-squares coefficients has been minimized to avoid irregularities in the calculated P(p,T) surface by using selected, temperature-dependent functions which are qualitatively consistent with isochores and specific heats over the entire surface. Several nonlinear parameters appear in these functions. Approximately fourteen additional constants appear in auxiliary equations, namely the vapor-pressure and orthobaric-densities equations, which provide the boundary for the P(p,T) equation-of-state surface. 

Figure 12,2. A. Scheme of negative selection in the conservative mode of evolution. Isochores (solid lines) that drift beyond the GC thresholds indicated by the broken lines are counterselected. B. Scheme of negative and positive selection in the transitional or shifting mode of genomes evolution. Isochores (solid lines) with decreasing GC levels are counterselected, whereas those with increasing GC levels are selected for. (From...

This proposal reverses a trend that has dominated the literature for the past 40 years. The reasoning was that since the different base compositions of prokaryotic genomes were due to biases in replication enzymes (Freese, 1962 Sueoka, 1962), the same process should also explain the formation and maintenance of isochores in vertebrate genomes. The results presented in this book in favour of natural selection being responsible for the isochores of vertebrates and the data of Fig. 12.20 lead, in contrast, to explaining the different compositions of prokaryotic genomes according to the natural selection schemes proposed for the isochorcs of vertebrates. 

Eyre-Walker A. (1999) Evidence of selection on silent site base composition in mammals potential implications for the evolution of isochores and junk DNA. Genetics 152 675-683. 

Francino M.P. and Ochman H. (1999) Isochores result from mutation not selection. Nature 400 30-31. 

Note the qualifier not necessarily. Sometimes the most direct pathway is the most economical. More to the point, the special pathways of Chapter 4 offer the maximum economy in one or more state variables. For isobaric, isochoric, adiabatic, and isothermal cases, Ix p, Ix<, and 0, respectively for every path of a closed system, Ix = 0. Note that special pathways afford straight-line representations in select planes, for example, isobaric and isochoric in pV, and adiabatic in the TS plane. A Carnot cycle appears as a square or rectangle when drawn in the TS plane. 

Historically, the first experimental determinations of the vapor densities and pressures approaching the critical region of a metal were made for mercury. Bender (1915, 1918) carried out pioneering measurements of vapor densities up to about 1400 °C. The samples in these studies were enclosed in strong fused quartz capillaries. In 1932, Birch made the first measurements of the vapor pressure of mercury and obtained realistic values for the critical temperature and pressure. Birch found values = 1460 °C and = 1610 bar, results that are remarkably close to the most accurate values available today (Table 1.1). A number of groups in various countries have contributed subsequently to the pool of pVT data currently available (Hensel and Franck, 1966, 1968 Kikoin and Senchenkov, 1967 Postill et al., 1968 Schonherr et al., 1979 Yao and Endo, 1982 Hubbard and Ross, 1983 Gotzlaff, 1988). The result is that the density data for mercury are now the most extensive and detailed available for any liquid metal. Data have been obtained by means of isothermal, isobaric, or isochoric measurements, but as we have noted in Sec. 3.5, those obtained under constant volume (isochoric conditions) tend to be preferable. In Fig. 4.10 we present a selection of equation-of-state data that we believe to be the most reliable now available for fluid... 

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