Since the days of Albrecht von Hailer, the emphasis has been on devising experiments and methods that allow for first-hand observation of the processes at work--the chymification and chylification of aliments, to use the language of eighteenth- and nineteenth-century physiology.
He cites the latter both to refute the chemical as well as the mechanical explanation of chymification.
It is true, that by the agency of gastric juice on food out of the body, a change very similar to chymification can be effected on it; but when we remember that chyme, or the result of real digestion, is essentially the same in its elementary or component principles, whatever be the kind of food from which it is formed, and that as yet we are acquainted with no purely chymical agent which, applied to different substances, gives rise to the same uniform product, we shall be more willing to believe that chymification is neither a purely mechanical nor a purely chymical operation; but the result of a vital process, to which both mechanical and chymical forces contribute, and which no action or combination of inanimate matter can either exactly imitate or supersede.
As Combe states: "If physiologists experience much difficulty in satisfactorily explaining all the phenomena of chymification, the reflecting reader will not be surprised to learn that they are still more puzzled to account for those of chylification.
If chymification had already transformed the diversity of aliments into a homogenous mass with barely detectible traces of its original components, chylification completed the process, so much so that its chemical composition stood in no relation whatsoever to the material from which it was formed.