It may be calculated by measuring the amount of carbon dioxide gas exhaled during a unit of time, and the number of calories of heat radiated by the skin simultaneously.

It may be noted that Mr. Lowell objects to the carbon-dioxide theory of the formation of the snow-caps, that this gas at low pressures does not liquefy, but passes at once from the solid to the gaseous state, and that only water remains liquid sufficiently long to produce the blue colour' which plays so large a part in his argument for the mild climate essential for an inhabited planet.