Another notable case of efficiency in nature has already been remarked, namely, the conversion by the animal frame of fuel-values into mechanical work. This is of a piece with the chief task of the engineer as he puts his engines in motion by burning coal or wood, oil or gas. It is a remarkably good steam engine which yields as much as one tenth as a working dividend. Gas engines have sprung into wide popularity because they yield larger results, in extremely favorable cases reaching thirty per cent. A heat engine, of any type, has its effectiveness measured by comparing in absolute units the heat which enters it with the heat which remains after its work is done. The zero of the absolute scale is 460° below the zero of Fahrenheit. So that if an engine begins work at 920° Fahr. (1380° absolute), and the working substance is lowered in temperature by its action in the machine until it falls to 460° Fahrenheit (920° absolute), the engine has a gross efficiency of one third. Economy depends upon employing a working substance at the highest feasible temperature in such a mode that it leaves the engine at the lowest temperature possible. Hence we see engineers devising superheaters for their steam, and producing metal surfaces which either need no lubrication at all, or employ such a lubricant as graphite, which bears high temperatures without injury. Now let us glance at the mechanism of our own frames, which, according to Professor W. O. Atwater, converts about twenty per cent. of the energy value of our food into mechanical work. This is a remarkable performance, especially when we remember that in health the bodily warmth does not rise above 98° Fahrenheit. What explains this amazing effectiveness at a temperature so far below that of either a steam engine or a gas engine? A simple experiment may be illuminating. We take a plate of zinc and a plate of copper; although they seem to be at rest we know them to be in active molecular motion, which motion is set free when they combine with oxygen or other elements. This combination may take place in two quite different ways, which we will now compare. In a glass jar, nearly filled with a solution of sulphuric acid and water, we immerse the plates of zinc and copper without their touching each other; both rise in temperature as they corrode, as they unite with oxygen from the surrounding liquid. We may, if we wish, employ this heat in driving an air engine; but we can do better than that, for an air engine wastes most of the heat supplied to it. We stop the heating process by joining the two plates with a wire through which now passes an electric current, our simple apparatus now forming a common voltaic cell. This current we apply to lift weights, propel a fan, or execute any other task we please, all with scarcely any waste of energy whatever. The instructive point is that now chemical union is taking place without heat, in a mode