All the juices poured out by these glands, indeed nearly all the fluids or juices in our bodies, are either _acid_ or _alkaline_. By acid we mean sour, or sharp, like vinegar, lemon juice, vitriol (_sulphuric acid_), and _carbonic acid_ (which forms the bubbles in and gives the sharp taste to plain soda-water). By alkaline we mean "soap-like" or flat, like soda, lye, lime, and soaps of all sorts. If you pour an acid and an alkali together--like vinegar and soda--they will "fizz" or effervesce, and at the same time _neutralize_ or "kill" each other. The Use of the Saliva. As the chief purpose of digestion is to prepare the food so that it will dissolve in water, and then be taken up by the cells lining the food-tube, the saliva, like the rest of the body juices, consists chiefly of water. Nothing is more disagreeable than to try to chew some dry food--like a large, crisp soda cracker, for instance--which takes more moisture than the salivary glands are able to pour out on such short notice. You soon begin to feel as if you would choke unless you could get a drink of water. But it is not altogether advisable to take this short cut to relief, because the salivary juice contains what the drink of water does not--a _ferment_, or digestive substance (_ptyalin_), which possesses the power of turning the _starch_ in our food into _sugar_. As starch is only very slowly soluble, or "meltable," in water, while sugar is very readily so, the saliva is of great assistance in the process of melting, known as _digestion_. The changing of the starch to sugar is the reason why bread or cracker, after it has been well chewed, begins to taste sweetish. This change in the mouth, however, is not of such great importance as we at one time thought, because even with careful mastication, a certain amount of starch will be swallowed unchanged. Nature has provided for this by causing another gland farther down the canal, just beyond the stomach, called the _pancreas_, to pour into the food tube a juice which is far stronger in sugar-making power than the saliva, and this will readily deal with any starch which may have escaped this change in the mouth. Moreover, this "sugaring" of starch goes on in the stomach for twenty to forty minutes after the food has been swallowed. Starchy foods, like bread, biscuit, crackers, cake, and pastry, are really the only ones which require such thorough and elaborate chewing as we sometimes hear urged. Other kinds of food, like meat and eggs--which contain no _starch_ and consequently are not acted upon by the saliva--need be chewed only sufficiently long and thoroughly to break them up and reduce them to a coarse pulp, so that they can be readily acted upon by the acid juice of the stomach.