There are many differences of opinion among physiological writers respecting the proper time for the use of water. There is one rule, however, which when properly followed leads to the best result, namely, that water should be drunk when one is thirsty. The quantities of water, for instance, which are required in winter when evaporation from the surface of the body is restricted, is very much less than that which would be used by a man in summer, especially if engaged in hard labor. A good rule is to drink frequently, rather than to drink much at a time.

There is much difference of opinion respecting the use of water during meals. Formerly physiologists were almost unanimous in their belief that the quantity of water used with meals should be restricted, if used at all. The later investigations by Professor P. B. Hawk and his assistants favor the use of a reasonable quantity of water with meals. The secretion of saliva containing the enzyme ptyalin, which converts starch into sugar, does not appear to be restricted by the use of water while eating, but rather promoted. There should, of course, be a sufficient quantity of liquid in the masticated food to favor the action of the digestive enzymes in the stomach. When the food is too dry the action of these enzymes must be to that extent restricted. Theoretically too great concentration of the food in the stomach would interfere more seriously with digestive action than too great dilution. The present status of the problem, therefore, favors a somewhat generous use of liquids, such as milk and water, at meals. This liquid should be taken when no food remains in the mouth, in other words "between bites."