Pavy says (‘Treatise on Food and Dietetics,’ page 6): ‘In the liberation of actual force, a complete analogy may be traced between the animal system and a steam-engine. Both are media for the conversion of latent into actual force. In the animal system, combustible material is supplied under the form of the various kinds of food, and oxygen is taken in for the process of respiration. From the chemical energy due to the combination of these, force is liberated in an active state; and, besides manifesting itself as heat, and in other ways peculiar to the animal system, is capable of performing mechanical work.’ In another place (page 59 of same work), after describing Liebig’s view, Dr. Pavy says: ‘The facts which have been already adduced’ (those above described on the nitrogen eliminated by the kidneys), ‘suffice to refute this doctrine. Indeed, it may be considered as abundantly proved that food does not require to become organised tissue before it can be rendered available for force-production.’ On page 81 he says: ‘While nitrogenous matter may be regarded as forming the essential basis of structures possessing active or living properties, _the non-nitrogenous principles may be looked upon as supplying the source of power_. The one may be spoken of as holding the position of the instrument of action, while the other supplies the motive power. Nitrogenous alimentary matter may, it is true, by oxidation contribute to the generation of the moving force, but, as has been explained, in fulfilling this office there is evidence before us to show that it is split up into two distinct portions, one containing _the nitrogen, which is eliminated as useless, and a residuary non-nitrogenous portion which is retained and utilised in force-production_.’ The italics are mine, for reasons presently to be explained. Pavy’s work contains repetitions and further illustrations of this attribution of the origin of force to the non-nitrogenous elements of food. Then we have a statement of the experiments of Joule on the mechanical equivalent of heat, connected with experiments of Frankland with the apparatus that is used for determining the calorific value of coal, &c.—viz. a little tubular furnace charged with a mixture of the combustible to be tested, and chlorate of potash. This being placed in a tube, open below, and thrust under water, is fired, and gives out all its heat to the surrounding liquid, the rise of temperature of which measures the calorific value of the substance (see fig. 7, page 21, ‘Simple Treatise on Heat’). From this result is calculated the mechanical work obtainable from a given quantity of different food materials. That from a gramme is given as follows: Beef fat 27,778 } Units of work, Starch (arrowroot) 11,983 } or number of Lump sugar 10,254 } pounds lifted Grape sugar 10,038 } one foot.