Even in man, the gastric juice undergoes considerable modifications, not merely according to the kind of aliment habitually used, but also according to the time of life, the wants of the system, the season of the year, and the state of the health; so that while sudden and great changes from one kind of diet to another are positively hurtful on the one hand, absolute uniformity is not less objectionable on the other, because it leads to so great a uniformity in the quality of the gastric juice habitually secreted as to render it incapable of acting with due effect on any accidental variety of food, to which a change of circumstances may compel us. Many attempts have been made to ascertain to which of the elements of the gastric juice its power is chiefly to be ascribed, and experiments have been instituted on them individually to discover which of them is most nearly analogous to it in effect. From the general results, it appears that acetic acid (vinegar) and muriatic acid have a wider range of influence, and produce solvent effects more closely resembling those of gastric juice, than any other known substances. Both of these acids, it will be recollected, are constituent elements of the gastric fluid; and it has, in consequence, been argued, that to them it is indebted for all its energy. And, indeed, without laying too much stress on this real or supposed analogy, it is impossible to overlook the well-known fact, that scurvy, and a highly alkaline state of the system, are generally induced by a diet restricted for a long time to animal food alone, and are prevented or cured most easily by a free use of lemon juice or of vegetable matter, either fresh or fermented. In these circumstances, the vegetable acid is probably efficacious both by directly improving digestion, and by combining with the excess of the alkaline salts already existing in the system. It is worthy of remark, too, that in weak stomachs _acidity_ is almost invariably induced by the use of vegetable food, possibly to some extent for the very purpose of effecting its digestion; for it has been ascertained beyond a doubt, that in herbivorous animals the gastric juice always contains some free or uncombined acid--and in man also, after living much on vegetables for some time. The necessity of acid for the chymification of vegetable food, affords an explanation of the fondness which the Germans and Dutch display for _saur-kraut_--or cabbage in a state of acetous fermentation--and of its alleged easy digestibility. It explains, also, the general use of vinegar along with salads, cucumbers, oysters, salmon, and other substances of difficult digestion, and shews that its utility is not imaginary, but loudly proclaimed by Nature’s own acts.