All reliable sources of data for the static dielectric constant or relative pennittivity of water and steam, many of them unpublished or inaccessible, have been collected, evaluated, corrected when required, and converted to the ITS-90 temperature scale. The data extend over a temperature range from 238 to 873 K and over a pressure range from 0.1 MPa up to 1189 MPa. The evaluative part of this work includes a review of the different types of measurement techniques, and the corrections for frequency dependence due to the impedance of circuit components, and to electrode polarization. It also includes a detailed assessment of the uncertainty of each particular data source, as compared to other sources in the same range of pressure and temperature. Both the raw and the corrected data have been tabulated, and are also available on diskette. A comprehensive list of references to the literature is included.

Note on the Small Cloud.-It has been suggested that the Small Magellanic Cloud is also a quasi-barred spiral like the Large Cloud and that its elongation is an indication of its tilt. The variable stars provide no supporting evidence. For 211 cepheid variables in five areas outside the central body of the Small Cloud, we have assembled the mean systematic deviations of the observed median magnitudes from the adopted period-magnitude curve, as shown in the accompanying tabulation. No. of Systematic Area Position Variables Deviation

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The reversible, concentration-dependent dissociation of the alpha beta dimer of bovine brain tubulin (purified by phosphocellulose chromatography) has been demonstrated by equilibrium ultracentrifugation. The dissociation constant is approximately 8 X 10(-7) M at 4.6 degrees C in PM buffer (0.1 M piperazine-N, N'-bis(2-ethanesulfonic acid), 2 mM ethylene glycol bis (beta-aminoethyl ether)-N, N'-tetraacetic acid, 1 mM MgSO4, 0.1 MM guanosine triphosphate, 2mM dithioerythritol, at pH 6.9). This result was confirmed by observation of an appropriate dependence of the sedimentation coefficient of very dilute (is less than 0.5 mg/mL) tubulin on its concentration. Small zone gel filtration experiments on Bio-Gel P-150 also demonstrated an increase in peak elution volume with decreasing column load concentration. Reversibility of the dissociation was demonstrated directly by sedimentation velocity and gel filtration ion experiments on tubulin reconcentrated from dilute solution by pressure ultrafiltration. Control experiments accompanying the sedimentation equilibrium experiments showed that this tubulin retained, under the conditions of the experiments, both its ability to form microtubules and more than 70% of its initial colchicine-binding activity.

Electrophoresis of microtubule preparations purified from calf brain by repeated cycles of assembly and disassembly shows that they contain many proteins in addition to alpha- and beta-tubulin. These additional proteins constitute about 17% of the total material present after five cycles of assembly and disassembly. Both one-dimensional and two-dimensional (P.H. O'Farrell (1975), J. Biol. Chem. 250, 4007) electrophoretic techniques have been used to characterize them. They can be divided into two groups: one that contains proteins which remain in constant quantitative ratio to tubulin during the purification cycles, and one composed of proteins which are removed during purification, although inefficiently. Gel-filtration chromatography of cold-depolymerized microtubule preparations yields a polydisperse fraction of high molecular weight containing most of the non-tubulin proteins. This fraction contains flexible filaments about 100 A in diameter similar to those reported by R.A.B. Keats and R.H. Hall ((1975), Nature (London) 247, 418). It is suggested that these fibers are neurofilaments, and that they may be the major source of the group of inefficiently removed proteins.