Measurement of the heat output from animals had its beginnings just over two hundred years ago. Adair Crawford in Glasgow measured the rise in temperature of a water jacket surrounding an animal chamber and, at about the same time, Lavoisier in Paris measured the heat output from a guinea-pig by recording the quantity of ice melted inside an insulated chamber which contained the ice and the animal. There is some doubt as to whether Crawford or Lavoisier made the first of these measurements (Mendelsohn, 1964; Blaxter, 1978). From the earlier work by Boyle, Hooke, Mayow and Priestley it was known that ‘pure air’ was necessary to support both animal life and the flame of a candle, and it was believed that both the animal and the candle ‘phlogisticated’ the air, thus gradually rendering it unfit for the continuing support of either. Both Crawford and Lavoisier compared the amount of heat given off by their animals with that given off by a lighted candle. Crawford believed that both animal heat and the heat produced by ‘combustible bodies’ was evolved from the air as phlogiston was formed but it was Lavoisier who made the breakthrough by recognising that the vital process involved was not the production of phlogiston, but the consumption of a new element which he named ‘oxygene’. The heat from living animals, like that from the burning candle, was evolved during the oxidation of carbon and the formation of carbon dioxide gas.