Iron spin transition in Earth's mantle.

High-pressure Mössbauer spectroscopy on several compositions across the (Mg,Fe)O magnesiowüstite solid solution confirms that ferrous iron (Fe(2+)) undergoes a high-spin to low-spin transition at pressures and for compositions relevant to the bulk of the Earth's mantle. High-resolution x-ray diffraction measurements document a volume change of 4-5% across the pressure-induced spin transition, which is thus expected to cause seismological anomalies in the lower mantle. The spin transition can lead to dissociation of Fe-bearing phases such as magnesiowüstite, and it reveals an unexpected richness in mineral properties and phase equilibria for the Earth's deep interior.

Submandibular duct repositioning after excision of floor of mouth cancer.

PURPOSE: Floor-of-mouth cancers frequently involve the submandibular duct. When neck dissection, which removes the submandibular gland, is not planned, the duct requires repositioning and cannulation to avoid obstructive sialoadenitis caused by scarring and stricture after excision of the primary lesion. A swollen submandibular salivary gland may be difficult to differentiate from a metastatic node and complicate subsequent oncologic management. This study reviews the clinical course and complications after repositioning and cannulation of the submandibular duct. PATIENTS AND METHODS: Thirteen patients who underwent 21 duct cannulations over a 25-month period were reviewed for assessment of submandibular gland function and complications of the procedure. RESULTS: Only two patients (15%) had episodes of postoperative neck swelling during follow-up. The commonest complication was early loss of the cannula in 7 of 21 patients (33%). There were no cases of local recurrence of the tumor, and only 1 patient of 10 with carcinomas had later cervical metastasis. CONCLUSIONS: Repositioning and cannulation of the submandibular duct is a sound procedure that does not compromise the cancer resection. The problem of later chronic swelling of the salivary gland is avoided. Fixation of the catheter has been inadequate in one third of patients.

Contextual effects of prekindergarten classrooms for disadvantaged children on cognitive development: the case of Chapter 1.

This study explores the effects of the social context of Chapter 1 prekindergarten classrooms on children's learning. Chapter 1 (also called Title I) is a federal government preschool program directed at children in low-income schools who are at risk of later school failure. Using hierarchical linear modeling (HLM) and a sample of 677 4-year-olds in 55 1990-91 Chapter 1 prekindergarten classes in 5 states, the study explores factors that influence gains on the Preschool Inventory (PSI) over the preschool year. Social context is defined here mainly in terms of the cognitive and social composition of the classroom. Contextual factors defined in terms of demographics are shown to be related to learning, but the average cognitive level of the class is not. On average, children learn less in classrooms with high concentrations of minorities, children with special needs, recent immigrants, and children whose mothers have little education. The study explores differential effects of racial concentration on race differences in learning. Policy implications are discussed.

Are Head Start effects sustained? A longitudinal follow-up comparison of disadvantaged children attending Head Start, no preschool, and other preschool programs.

This study investigates the sustained effects into kindergarten and grade 1 of Project Head Start for disadvantaged black children. Participation in generic Head Start programs was compared to both no preschool and other preschool experience for disadvantaged children in two American cities in 1969-1970. Incorporating both pretest/posttest and comparison group information, the study has advantages over other Head Start impact studies. Both preprogram background and cognitive differences were controlled in a covariance analysis design, using dependent measures in the cognitive, verbal, and social domains. Children who attended Head Start maintained educationally substantive gains in general cognitive/analytic ability, especially when compared to children without preschool experience. These effects were not as large as those found immediately following the Head Start intervention. Findings suggest an effect of preschool rather than of Head Start per se. Initial findings of greater effectiveness of Head Start for children of below average initial ability were reduced but not reversed. The diminution of effects over time, especially for low-ability children, may reflect differences in quality of subsequent schooling or home environment.