Lead poisoning in children: detection by ion-selective electrode.

curve appears to drop off sharply (which is expected behavior for lambdatype transitions) and then resumes with 'normal" heat-loss characteristics. Thus, even if the thermal arrests are not as prolonged as observed (1, 4) or due entirely to magnetic ordering, detailed temperature-time curves are likely to yield new information on mineralogical and compositional variations of the lunar surface. The temperatures of the heat capacity maxima related to magnetic transitions in lunar materials can be expected to vary widely because of the known variations in the bulk compositions of minerals (6) and from the effect of particle size on the ordering temperatures of very finely divided substances (7). It is not reasonable at this time, therefore, to attempt a correlation of Allen and Ney's (1) data with a specific mineral or mineral assemblage, inasmuch as detailed lowtemperature heat capacity data are not yet available for basalt, ferrosilite (Fe2Si206) or for any Fe-Mg-Mn-silicate solutions. Some theoretical consideration has been given to the effects of simple variations in Cp with temperature (8), but the present lack of empirical data on the above substances and on samples of lunar surface material precludes further theoretical analysis. Large discontinuities in thermal expansion are also associated with some ordering transitions, as for example, in magnetite (9) and quartz. If such is the case for magnetic ordering in lunar materials, repeated cycling through one or more phase transition of this type would be an effective mechanism for fracture and erosion of the lunar surface. D. R. WALDBAUM Department of Geological Sciences, Harvard University, Cambridge, Massachuisetts 02138