Rennet could be much more easily kept sweet if put in an air-tight
vessel. The "American Dispensatory" says: "When gastric juice is
completely protected from the air it may be kept unchanged for a
longtime; but on exposure it speedily undergoes decomposition,
acquires a very offensive odor, and loses its characteristic digestive
property." We think that the Dispensatory is right. The composition of
pure gastric juice is as follows: Water, 97.00; salts, 1.75; pepsin,
1.25; total, 100.00. There is also a small amount of free acid. Both
rennet extract and pepsin are used as medicine.