This molecular agitation accompanies the falls from unstable to stable molecular combinations; whether they be those from the most complex to the less complex compounds, or whether they be those ultimate falls which end in fully oxidized and relatively simple compounds; and whether they be those of the nitrogenous matters composing the tissues or those of the non-nitrogenous matters diffused through them. In the one case as in the other, the heat must be regarded as a concomitant. Whether the distinction, originally made by Liebig, between nitrogenous substances as tissue-food and non-nitrogenous substances as heat-food, be true or not in a narrower sense, it cannot be accepted in the sense that tissue-food is not also heat-food. Indeed he does not himself assert it in this sense. The ability of carnivorous animals to live and generate heat while consuming matter that is almost exclusively nitrogenous, suffices to prove that the nitrogenous compounds forming the tissues are heat-producers, as well as the non-nitrogenous compounds circulating among and through the tissues: a conclusion which is indeed justified by the fact that nitrogenous substances out of the body yield heat, though not a large amount, during combustion. But most likely this antithesis is not true even in the more restricted sense. The probability is that the hydrocarbons and carbo-hydrates which, in traversing the system, are transformed by communicated chemical action, evolve, during their transformation, not heat alone but also other kinds of force. It may be that as the nitrogenous matter, while falling into more stable molecular arrangements, generates both that molecular agitation called heat and such other molecular movements as are resolved into forces expended by the organism; so, too, does the non-nitrogenous matter. Or perhaps the concomitants of this metamorphosis of non-nitrogenous matter vary with the conditions. Heat alone may result when it is transformed while in the circulating fluids, but partly heat and partly another force when it is transformed in some active tissue that has absorbed it; just as coal, though producing little else but heat as ordinarily burnt, has its heat partially transformed into mechanical motion if burnt in a steam-engine furnace. In such case the antithesis of Liebig would be reduced to this--that whereas nitrogenous substance is tissue-food _both_ as material for building-up tissue and as material for its function; non-nitrogenous substance is tissue-food _only_ as material for function.