As all energy set free in the body, except in so far as energy of work is transferred outwards, leaves the body as heat, it is easy, by computing the amount of heat lost, to furnish a statement of the calories of food required to make good the loss. This can be effected by means of the respiration calorimeter, a chamber in which a man may live from one day or less to two weeks, and by which the output of carbon dioxide, water and energy, and also the intake of oxygen, can be accurately studied. The total heat output of individuals of different size, age, sex and degree of bodily activity is thus easily measured, and the amount of work performed can be controlled with precision by means of a stationary bicycle with variable friction. Such instruments are now in existence in various countries, but perhaps the most satisfactory examples are to be found in the Nutrition Laboratory, Boston, Massachusetts, a branch of the Carnegie Institute of Washington. During a recent visit I examined two splendid specimens which had been constructed under the personal supervision of the Director, Dr. Francis G. Benedict, whose great experience in the science of calorimetry is so well-known, and who hopes by building three more shortly to complete the equipment of a laboratory to be entirely devoted to the study of nutrition. I make special mention of this institution because for years to come its work is to be confined to the study of the production of energy in man and the lower animals, and because, so far as my experience goes, its apparatus is the most complete and comprehensive in existence. Estimations of all the three modes of calorimetry which have been practised in the past, viz., the Pettenkofer-Voit method, the Zuntz and the Jaquet methods, can be accomplished by its use. Whatever the future may hold in store, however, in this direction, it will never suffice to efface the memory of the brilliant calorimetric researches of Rubner, to whom we are indebted for the mean "physiological combustion values" just detailed, and who established the fact that the law of the conservation of energy obtains also in the human organism.