Lavoisier1 was the first to recognize that animal heat was derived from the oxidation of the body's substance and to compare animal heat to that produced by a candle. To prove this he burned a known quantity of carbon in an ice-chamber and noted the amount of ice melted. He then calculated the amount of heat produced from a unit of carbon. He and Laplace put a guinea-pig in an ice-chamber and noted the amount of ice which melted during ten hours and calculated the heat given off from the animal. They then determined how much carbon dioxid the guinea-pig gave off. The animal yielded 31.82 calories to the ice-chamber, while a calculation from the respiratory analysis showed that 25.408 calories could have been derived by the burning of enough carbon to yield the same amount of carbon dioxid as was eliminated by the animal.

Lavoisier realized several of the errors in his work. For example, the calorimetric determination on the animal was made at a different temperature from that of the respiratory experiment, and Lavoisier knew that cold would raise the carbon dioxid output. Also cold reduced the heat in the animal itself, and, further, the water of respiration was added to that of the melting ice. But Lavoisier concluded that the source of the heat lay in the oxidation of the body.

Crawford, in England in 1777, found after burning wax and carbon, or on leaving a live guinea-pig in his water calorimeter, that for every 100 ounces of oxygen used the water was raised the following number of degrees Fahrenheit:

1 Lavoisier and Laplace: Academie des Sciences, 1780, p. 379.

Crawford concluded that the heat above produced was due to the transformation of pure air into fixed air (carbon dioxid) and water.

The methods of Crawford, though primitive, were based on fundamental principles, for according to the modern computation of Zuntz the values of heat production where 1 liter of oxygen is used to burn the different food-stuffs in the body are very nearly identical (see p. 62).

In 1823 the French Academy awarded a prize for the best essay on the subject of animal heat. Depretz and Dulong competed for the prize and it was awarded to the former.

Depretz1 calculated the amount of heat which would have been liberated in burning the carbon and hydrogen of the metabolism to carbon dioxid and water, and compared this with the amount of heat given off by the animal. The heat as calculated was only 74 to 90 per cent. of what was found, a discrepancy due to faults in the method employed (see p. 43). So Depretz concluded that although the respiration was the principal source of animal heat, food, the motion of the blood, and friction yielded the remainder. Interpretation along the lines of the law of the conservation of energy was obviously beyond the ideas of the time.

Dulong's2 experiments also led to the same conclusion, that oxidation was insufficient to explain the cause of animal heat, and that there must be other sources of it.

Regnault and Reiset, writing in 1849 regarding the computation of heat production from the oxygen absorbed by an animal, remark, "The phenomena are evidently so complex that it is scarcely probable that one will ever be able to submit them to calculation".

About 1842 James P. Joule supplied the chief experimental data which established the mechanical equivalent of heat. In 1845 J. R. Mayer laid down the law of the conservation of energy, and in 1847 Helmholtz independently made the same discovery. Both contributions were rejected by the leading German scientific journal of the day.1 This should encourage all workers to rest assured of the ultimate recognition of work that is worth while.

1 Depretz: "Journal de Physiologie," 1824, iv, 143. 2Dulong: Ibid., 1823, iii, 45.

Energy cannot arise from nothing, nor can energy disappear into nothing. Where energy is active it must have been elsewhere potential. The sum total of energy remains constant in the universe, but energy may vary in kind. The kinds include mechanical energy, heat, electricity, magnetism, and potential energy. The source of energy on the earth is the sun, excepting the energy of the tides, which is due principally to the moon. The sun unevenly warms the atmosphere, producing winds which drive ships and windmills. The sun's heat lifts the vapor of water into the atmosphere, producing rain, in consequence of which rivers are made to turn machinery. The sunlight acts upon a mixture of hydrogen and chlorin gas, causing them to unite with a loud explosion, and the sun acts upon the green leaf of the plant, causing it to unite carbon dioxid and water, with the production of formic aldehyd, which is built up into sugar, oxygen being given off in the process. The sun's energy required to build up the compound becomes latent or potential in it. Whenever and wherever this sugar is again converted into carbon dioxid and water by oxidation, exactly the same quantity of energy taken from the sun and made potential in the sugar is set free. This sugar in the plant may be further converted into starch, cellulose, fat, and possibly into protein. Plants furnish wood and coal as fuel for the steam-engine. They also furnish the basis of animal food, yielding substances which can build up animal tissues, and which can furnish the energy necessary to maintain those motions in the cells whose aggregate is called life. These motions appear in the body as heat, mechanical work, and electric currents, all of which may be measured as heat. Is this energy completely derived from the metabolism? This question is but the continuation of the old one of Lavoisier in the light of newer science.

1 "Wiener klin. Wochenschr.," S. Exner, 1914, xxvii, 1529.

Bischoff and Voit1 in 1860 still calculated the heat value of the metabolism from the heat developed in burning the carbon and hydrogen elements of the metabolism. They recognized, as had Bidder and Schmidt2 before them, that this was a false method, and stated that they should employ the calorific value of fat, starch, and protein, less the urea, since they recognized that urea was capable of undergoing combustion with liberation of heat.