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The structure of pyridine-2,6-dicarboxylic acid, C(7)H(5)NO(4), has been determined at 0.71 A resolution. The molecule is located on a site with mirror symmetry. A one-dimensional supramolecular structure is stabilized in the solid state through a strong symmetric double hydrogen bond, with H* * *O distances of 1.86 (3) A and O-H* * *O angles of 167 (3) and 171 (5) degrees. This arrangement is similar but not identical to that reported for the isoelectronic isophthalic acid (benzene-1,3-dicarboxylic acid).

A new dicationic ring [(water) 6 -(ammonium) 2 ] was found as a building block for a 3D supramolecular assembly of the compound [(CH 3 ) 2 NC 5 H 4 NH] 4 (NH 4 ) 2 [V 10 O 28 ]Á8H 2 O, 1, where (CH 3 ) 2 NC 5 H 4 NH = 4-(N,N-(dimethylamino)pyridinium) (DMAP ? ). The title compound was synthesized and characterized by single crystal X-ray diffraction at 100 K, infrared spectroscopy and thermogravimetric analysis. Compound 1 crystallizes in space group P-1 in the triclinic system with cell parameters: a = 11.1296(4) Å , b = 11.4790(4) Å , c = 11.8691(4) Å , a = 109.631(1)°, b = 100.601(1)°, c = 92.909(1)°and R(F 2 ) = 0.021. The threedimensional supramolecular structure is driven by intermolecular hydrogen bonds OÁÁÁH-N and OÁÁÁH-O from decavanadate anion with NH 4 ? and [(CH 3 ) 2-NC 5 H 4 NH] ? cations, as well as those formed between water molecules and decavanadate ions. The embedded rings consist of two sets of water-water-waterammonium chains joint together in a twisted chair conformation, in which water-ammonium pairs are at the top and bottom sites. The ring is hydrogen bond stabilized and represents to our knowledge, the first example of building block with this composition for a supramolecular structure to be reported.

Small groups of fish of a schooling species were trained in successive discrimination reversals with an avoidance paddle. In this method, fish are transitorily swept out of the water by a moving paddle passing through their tank if they fail to swim through an opening, which is shifted rightleft in successive reversals. Errors declined significantly through reversals. This is an unusual result for fish, which generally fail in reversal learning problems.

This article describes the results of a qualitative study, based on the perspective of critical discourse analysis, which explores the discourse on childhood of 10-and 11-year-old boys and girls from a middle-income socioeconomic sector in Santiago, Chile. Among the findings, a complex and relational notion of childhood is highlighted. The children perceive themselves as overwhelmed and subjected to excessive demands by grown-ups, and conceive of adulthood as a state without real freedom due to the excessive demands of work and family.