All non-plastic substances--and of each kind there are numerous varieties--are capable of conversion, in the animal mechanism, into fat and oil. The non-plastic food substances do not contain nitrogen, hence they are commonly termed non-nitrogenous elements. The oily and fatty matters contain a large proportion of carbon, their next most abundant component is hydrogen, and they contain but little oxygen. Unlike the plastic elements, they are--except the fats of the brain and nervous tissue--altogether destitute of sulphur and phosphorus. The starchy, saccharine, and gummy substances are composed of the same elements as the fatty bodies, but they contain a higher proportion of oxygen. According to Liebig, fat is used in the animal economy as a source of internal heat. We all know that it is a most combustible body, and that during its inflammation the most intense heat is developed. It is less evident, but not less true, that heat is evolved during its slow oxidation, or decay. The more rapidly a body burns, the greater is the amount of heat evolved by it in a _given time_; but the total amount of heat developed by a specific weight of the body is the same, whether the combustion takes place rapidly or slowly. An experiment performed with phosphorus illustrates the case perfectly. If we burned two pieces of equal weight, the one in oxygen, the other in atmospheric air, we should find that the former would emit a light five times as brilliant as that evolved by the latter, for the simple reason that its combustion would be five times as rapid. The white, vapor-like matter into which phosphorus is converted by its combustion, is termed _phosphoric acid_. It is composed of phosphorus and oxygen. In forming an ounce of this compound, by the direct oxidation, or combustion of phosphorus, the amount of force, either as heat, or as heat and light, evolved is precisely the same, whether the time expended in the process be a minute or a month.[5] If, in the experiment I have described, we were to substitute two pieces of fat for the fragments of phosphorus, the results would be precisely similar. The fat burned in oxygen gas would emit intense light and heat; but the total amount of these forces evolved would be neither greater nor less than that developed during the slower and therefore less brilliant combustion of the fat in ordinary atmospheric air. Now, as we can demonstrate that an ounce of fat will emit a certain amount of heat, if burned within a minute of time, and that neither a larger nor a smaller amount will be developed if the combustion of the fat extend over a period of five minutes, I think we may fairly assume that the amount of heat evolved by the complete oxidation of a specific quantity of fat is constant under all conditions, except, as I have already explained, at high temperatures, when a portion of the heat is converted into light.