In our review we survey the observed properties of metal-insulator-metal devices involving such insulating layers, and discuss the mechanisms which have been proposed for their operation. Further, since the unusual properties may be technologically important, we outline some of the possible uses of these devices and the relevance of the phenomena to the mechanism of corrosion of certain metals.

Materials displaying the remarkable combination of high electrical conductivity and optical transparency already from the basis of many important technological applications, including flat panel displays, solar energy capture and other opto-electronic devices. Here we present the basic materials physics of these important materials centred on the nature of the doping process to generate n-type conductivity in transparent conducting oxides, the associated transition to the metallic (conducting) state and the detailed properties of the degenerate itinerant electron gas. The aim is to fully understand the origins of the basic performance limits of known materials and to set the scene for new or improved materials which will breach those limits for new-generation transparent conducting materials, either oxides, or beyond oxides.

A B S T R A C T An isotope dilution method, using 'Plabeled pyrophosphate, has been developed for the measurement of inorganic pyrophosphate (PP.) in human plasma. The specificity of the method was better than 90% as assessed by elution patterns during ion-exchange chromatography, by paper chromatography, and by incubation with inorganic pyrophosphatase. The 99% confidence limits for a single estimation of plasma PP. was -+-13%. There were no differences in plasma PPi between men and women, but the values in young people (0-15 yr) were slightly higher than in older people.The mean concentration (±+sE) of PP1 in the plasma of 73 men and women was 3.50 ±0.11 Amoles/liter (0.217 ±0.007 Ag P/ml) and the normal range (99%

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1. The effects of two diphosphonates, disodium ethane-I-hydroxy-l,1-diphosphonate (EHDP) and disodium dichloromethylene diphosphonate (C1,MDP) on several variables of calcium metabolism have been measured in the intact rat using the model of .2. The animals were bred from weaning on a diet containing 1 -3% calcium. At the age of 54 days they were switched to a diet with 0.5% calcium. Some of the animals were also given daily injections of one of the diphosphonates from the day the intake was decreased. The doses of the compounds were 0-01-10 mg of P kg body wt.-' day-'.3. C12MDP caused a decrease in the rates of bone formation (Vo+) and bone resorption (Vo-), and these effects were proportional to the logarithm of the dose of the compound. There was a small increase in the retention of calcium due to a decrease in the endogenous faecal calcium.4. EHDP in doses of up to 1 mg of P kg-' day-' decreased bone resorption but had little effect on bone formation. The retention of calcium increased due to an increased absorption of dietary calcium and a decreased endogenous faecal calcium. EHDP at a dose of 10 mg of P kg-' day-' caused a further fall in Vo-but also a great decrease in Vo + which was larger than the fall in Vo -. The net absorption of calcium was diminished, the urinary excretion of calcium was increased, the retention of calcium was greatly diminished and there was a greatly diminished ash content in the femur. Thus, for equal doses of up to 1 mg of P kg-' day-', C1,MDP was more potent in decreasing bone turnover than EHDP, but the two compounds caused a similar increase in the retention of calcium. At high doses EHDP inhibited the mineralization of epiphyseal cartilage.5. These results are discussed in relation to the possible clinical applications of the diphosphonates.

SUMMARY Serum total calcium was measured in 1693 patients during a four-month period. We examined the effects of adjustment for albumin concentration on the interpretation of single measurements of serum total calcium and on the variation of series of measurements in individual patients.Markedly abnormal total calcium concentrations-2 75 mmol/l (11 0 mg/100 ml) or more, or 2-00 mmol/1 (8-0 mg/100 ml) or less-were found in 115 patients, but only 24 (21 %) remained markedly abnormal after adjustment for albumin. Three patients, two with malignant disease and one with primary hyperparathyroidism, had significant hypercalcaemia which was masked by hypoalbuminaemia. The serum total calcium measured on a subsequent occasion had changed 0S15 mmol/l (0-6 mg/100 ml) or more in 60 patients, but after adjustment for albumin this number was reduced to 27 (45 %). The within-person standard deviation for serum total calcium was calculated in 26 patients with normal mean adjusted calcium concentrations who had had six or more sequential measurements. The mean standard deviation was 0-148 mmol/l (0 59 mg/100 ml) and, after adjustment for albumin, this was reduced to 0 100 mmol/l (0 40 mg/100 ml). We conclude that adjustment of serum total calcium concentration for albumin is essential to detect abnormal values and to assess changes in a value.

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