3. On introducing into the stomach alimentary substances inclosed in metallic balls perforated with holes, or in pieces of porous cloth, it is found, on removing these bodies from the stomach, after a certain time, that the alimentary substances contained in them are as completely digested as if they had been in actual contact with the surface of the stomach; the metallic ball and the cloth remaining wholly unchanged. This experiment, which has been often performed with the same uniform result, was the first that led to the discovery of the true nature of the digestive process. 4. Though it be impossible to imitate out of the stomach all the circumstances under which the food is placed within it, yet, on procuring gastric juice from the stomachs of various animals, and mixing it with different alimentary substances, it is found not only to dissolve them, but to convert them into a pultaceous mass, closely resembling chyme. Gastric juice thus procured was put into a glass tube with boiled beef, which had been masticated; the tube was then hermetically sealed, and exposed near the fire to a uniform heat: by the side of this tube was placed another, containing the same quantity of flesh immersed in water. In twelve hours, the flesh in the tube containing the gastric juice began to lose its fibrous structure; in thirty-five hours it had nearly lost its consistence, being reduced to a soft homogeneous pultaceous mass. It experienced no further change during the two following days. On the other hand, the flesh that had been immersed in water was putrid in sixteen hours. 638. Since alimentary substances under the action of the stomach present precisely the appearance exhibited by bodies exposed to the influence of chemical agents, it appears that the gastric juice not only dissolves the food, but dissolves it by a chemical agency. Its action bears no proportion to the mechanical texture of bodies, nor to any of their physical properties. It acts upon the densest membrane, dissolves even bone itself; and yet produces no effect on other substances of the most tender and delicate texture. On the skin of fruit, on the finest fibre of flax and cotton, it is incapable of making the slightest impression. In this selection of substances it perfectly resembles a chemical agent acting by chemical affinity. On certain substances its action is unquestionably of a chemical nature. It occasions the coagulation of albuminous fluids; it prevents the accession of putrefaction; it stops the process after it has commenced. From the whole, it follows that the food in the stomach is converted into chyme by the agency of a fluid secreted by the inner surface of the stomach, and that this change is effected by a proper chemical action.