Following Rubner, Atwater, at one time a pupil of Voit, with the aid of Rosa, the physicist, constructed a large calorimeter capable of measuring to a nicety the amount of heat given off by a man living in it. This apparatus confirmed Rubner's experiments and has shown that the energy expended by a man in doing any work, such as bicycle-riding, is exactly equal to the energy set free by metabolism in the body. Ex nihilo nihil fit.

This apparatus was the product of many years of labor and it's cost was borne by the United States Government. Armsby has completed a similar one for use with cattle for the Agricultural Station of the State of Pennsylvania. Benedict with great success has extended Atwater's work in the notable Nutrition Laboratory of the Carnegie Institution in Boston. This is housed in a new building splendidly equipped with apparatus for the simultaneous determination of metabolism and heat production. The work has been still further extended by the construction for the Physiological Laboratory of the Cornell University Medical College in New York City of a small respiration calorimeter1 suitable for use with babies, dwarfs, and dogs, and of the Sage respiration calorimeter2 constructed in Bellevue Hospital by the Russell Sage Institute of Pathology for the determination of metabolism in diseased conditions. The two machines have been under the general management of the writer. These elaborate and costly devices prove and confirm the general laws of metabolism in the body enunciated above, through a knowledge of which alone proper systems of nutrition for people under various conditions may be devised (see p. 56). The American Indian when first shown a watch thought it was alive. We, on the other hand, have come to look upon the living organism as a machine. Like the moving locomotive, we burn more if we are to attain a faster speed, or if we are to keep all parts warm in the winter's cold. In both cases the motion and the heat are derived from the power in the fuel. The casual observer sees the moving train, but the expert engineer alone knows how and why the wheels go round. The physiologist busies himself answering the similar how and why regarding the mechanism of living things.

1 Williams: "Journal of Biological Chemistry," 1912, xii, 317. 2 Riche and Soderstrom: "Archives of Internal Medicine," 1915, xv, 805; Lusk, same "Archives," p. 793.

Before taking up the details of the work we may copy the last general pronouncement of Voit1 upon the subject of metabolism. It reads:

"The unknown causes of metabolism are found in the cells of the organism. The mass of these cells and their power to decompose materials determine the metabolism. It is absolutely proved that protein fed to the cells is the easiest of all the food-stuffs to be destroyed, next carbohydrates, and lastly fat. The metabolism continues in the cells until their power to metabolize is exhausted. All kinds of influences may act upon the cells to modify their ability to metabolize, some increasing it or others decreasing it. To the former category belong muscular work, cold of the environment (in warmblooded animals), abundant food, and warming the cells. To the latter, cooling the cells, certain poisons, etc.

"In speaking of the power of the cells to metabolize, I have not meant thereby, as may be seen from all my writings, that the cells must always use energy in order to metabolize, but rather I have understood thereby the sum of the unknown causes of the metabolic ability of the cells - as one speaks of the fermentative 'power' of yeast cells.

"The metabolism of the different food-stuffs varies with the quality and quantity of the food. Protein alone may burn, or little protein and much carbohydrate and fat. I have determined the amount of the metabolism of the various food-stuffs under the most varied conditions. All the phases of metabolism originate from processes in the cells. In a given condition of the cells available protein may be used exclusively if enough be furnished them. If the power of the cells to metabolize is not exhausted by the protein furnished, then carbohydrates and fats are destroyed up to the limit of the ability of the cells to do so.

1 Voit: "Munchener medizinische Wochenschrift," 1902, xlix, 233.

"From this use of materials arise physical results, such as work, heat, and electricity, which we can express in heat units. This is the power derived from metabolism.

"It is possible to approach the subject in the reverse order, that is, to study the energy production (Kraftwechsel) and to draw conclusions regarding the metabolism (Stoffwechsel). It is perfectly possible to say that the requirement of energy in the body or the production of the heat necessary to cover heat loss, or for energy to do work, are controlling factors of the metabolism; since on cooling the body or on working correspondingly more matter is destroyed. But one must not conclude that the loss of body heat and muscular work are the immediate causes of this increased metabolism. The causes lie in the peculiar conditions of the organism, and muscle work and loss of heat are merely factors acting favorably upon those causes, raising the power of the cells to metabolize. In virtue of this more is destroyed, and secondarily the power to work and increased heat production are determined.

"The requirement for energy cannot possibly be the cause of metabolism, any more than the requirement for gold will put it into one's pocket. Hence the production of energy has a very definite upper limit, which is afforded by the ability of the cells to metabolize. If the cells will metabolize no more, then further increase of work ceases even in the presence of direst necessity; and this is also the case with the heat production, even though it were very necessary, and we were likely to freeze.

"I therefore maintain my 'older' point of view, that of pure metabolism, in order to explain the phenomena of nutrition. I am convinced that it is the right way, and that the clearest and most unifying development will be possible as one investigates what substances are destroyed under different circumstances, such as the performance of work, and loss of heat, and how much of the different materials must be fed to maintain the body in condition".