THE PHYSIOLOGY OF DIGESTION
Young's experiments on frogs had already taught him, in confirmation of Spallanzani, that the gastric juice resists decay and it "even restored putrid substances to their original sweetness." Here is an additional experiment upon himself:
On an empty stomach I made a light dinner, on chicken pye, and drank simple water: in half an hour, by irritating my fauces, it was thrown up; at this time it was plentifully supplied with gastric fluid, as well as saliva, as the quantity of food was but small. It was then exposed in a tumbler to a heat equal to the human temperature. For the space of nine hours there was not the least intestine motion nor any disengagement of air. As digestion is performed sooner than this period, it was not attended to any longer.
Young convinced himself of the acid character of the gastric fluid and attempted to identify the acid present.
A piece of fresh veal was introduced into the empty stomach of one of the large frogs: in two hours it was examined; the surface was a little tender; upon being touched with litmus paper it was turned red. Here digestion was progressing quite regular, yet an acid was present. It appeared impossible at the same time to conceive the meat could become sour in so very short a time, and in so very low a temperature; it was therefore conjectured, the acid was to be referred not to the meat, but to the gastric juice, which the following experiments confirmed us in. A frog was kept starving for two days; a piece of litmus paper was then forced into its empty stomach by means of a pair of forceps; upon being drawn out, it was covered with gastric juice, and the litmus turned red. The naked gastric juice was afterwards often examined, by bringing it out of their stomachs with a teaspoon, and constantly found to be slightly acid. Being thus fully persuaded the acid, in the digested food of frogs, did not arise from a fermentation, but was to be referred to their gastric juice, we were led by analogy to suppose the acid of our own stomachs was to be attributed to the same origin: but this analogical reasoning might be called mere probability; the following experiment was therefore performed. Early in the morning, my stomach being empty, I irritated my fauces
with a view of throwing up some gastric juice: though many efforts were made, none could be vomited. The following day I took some meat on an empty stomach: in half an hour afterwards, by irritating my fauces, the meat was thrown up, and with it some gastric fluid: upon being tested, an acid was very evidently present. Here no one can suppose the acid was to be referred to the meat. We have little hesitation, therefore, in saying that the acid so constantly found in the stomach of man, and almost, probably, all animals, is to be referred to their gastric fluid.
Young's friend, Mr. Mitchell, "being in good health and having the power to ruminate," collected gastric fluid for him. The analysis of the filtered fluid was performed by precipitating with acetate of lead. The precipitate was treated with muriatic acid "which decomposed it, a very white powder remaining at the bottom, and a fluid above." From analogy with the behavior of urine similarly treated the author concluded:
Though great accuracy and many varied experiments are required to ascertain certainly the presence of an unknown acid, yet we are disposed to believe any person who had witnessed the great similarity in the comparative precipita-