Calculate the number of cubic centimeters of decinormal solution which would be required for 100 c.c. of stomach fluid. This gives, in degrees, _all the acidity except the combined hydrochloric acid_. The combined hydrochloric acid is then found by deducting this amount from the total acidity, which has been previously determined. _Example_.--Suppose that 5 c.c. of decinormal solution were required to produce the purple color in 10 c.c. gastric juice; then 5 X 10 = 50 = _all the acidity except combined hydrochloric acid_. Suppose, now, that the total acidity has already been found to be 70 degrees; then 70 - 50 = 20 _degrees_ of combined hydrochloric acid; and 20 X 0.00365 = 0.073 _per cent._ * * * * * (3) Organic Acids.--There is no simple direct quantitative method. After the total acidity has been {227} determined, organic acids may be removed from another portion of the gastric filtrate by shaking thoroughly with an equal volume of neutral ether, allowing the fluids to separate, and repeating this process until the gastric fluid has been extracted with eight or ten times its volume of ether. The total acidity is then determined, and the difference between the two determinations indicates the amount of organic acids. (4) Pepsin.--No direct method is available. The following is sufficient for clinical purposes: * * * * * Hammerschlag's Method.--To the white of an egg add twelve times its volume of 0.4 per cent. hydrochloric acid (dilute hydrochloric acid, U.S.P., 4 c.c.; water, 96 c.c.), mix well, and filter. This gives a 1 per cent. egg-albumen solution. Take 10 c.c. of this solution in each of three tubes or beakers. To _A_ add 5 c.c. gastric juice; to _B_, 5 c.c. water with 0.5 gm. pepsin; to _C_, 5 c.c. water only. Place in an incubator for an hour and then determine the amount of albumin in each mixture by Esbach's method. Tube _C_ shows the amount of albumin in the test-solution. The difference between _C_ and _B_ indicates the amount of albumin which would be digested by normal gastric juice. The difference between _C_ and _A_ gives the albumin which is digested by the fluid under examination. It has been shown that the amounts of pepsin in two fluids are proportionate to the squares of the products of digestion. Thus, if the amounts of albumin digested in tubes _A_ and _B_ are to each other as 2 is to 4, the amounts of pepsin are to each other as 4 is to 16. Certain sources of error can be eliminated by diluting the gastric juice several times before testing. * * * * * {228} D. MICROSCOPIC EXAMINATION A drop of unfiltered stomach-contents is placed upon a slide, covered with a cover-glass, and examined with the two-thirds and one-sixth objectives.