486 DIGESTION. 



form a jelly sooner than dilute hydrochloric acid alone. Soak 

 gelatin in cold water till it swells up completely, and then add 

 sufficient boiling water to it to form a concentrated solution. 

 Put some of it into two test-tubes, and add to each its own 

 bulk of 0.2 per cent, hydrochloric acid. Put into one test- 

 tube a little glycerine solution of pepsin, and into the other 

 the same amount of glycerin, and place them in the water- 

 1 t:ith at 40 C. Take them out after an hour or so, and let 

 tlu'in cool. If both gelatinize, replace them for a while, and 

 then cool them again, repeating the experiment if necessary. 

 In this way the gelatin in the gastric juice will be found to 

 lose its power of gelatinizing somewhat sooner than the other. 



* 121. Effect of Temperature on Digestion. A low 

 temperature arrests the action of pepsin temporarily, but does 

 not destroy its activity. It acts more and more rapidly as the 

 temperature increases, until it attains its maximum between 

 30 C. and 50 C. Above this the action becomes slower. It 

 is completely annulled by boiling. The activity of a dilute 

 solution of pepsin is destroyed by exposure to a temperature 

 of 70 C. for two minutes, and by a still lower temperature 

 when exposed for a longer time. The activity of a concen- 

 trated solution is not so readily "destroyed, and that of an 

 undiluted glycerin solution is retained after being exposed to 

 80 C. for two minutes. 



To show the action of temperature, take four tubes, and put 

 into each equal quantities of 0.1 per cent, hydrochloric acid, 

 to which a little glycerin solution of pepsin has been added. 

 Put one in pounded ice, the second in a test-tube rack on the 

 table, the third in the water-bath at 40 C., and boil the fourth, 

 and then put it also in the water-bath. Put into each a bit of 

 fibrin, and let them stand. The fibrin in the third tube will 

 dissolve quickly, that in the second much more slowly, that in 

 the first and fourth not at all. After a while say half an 

 hour take the tube out of the ice and put it in the water- 

 bath. The fibrin will then dissolve quickly, showing that the 

 activity of the pepsin has been only suspended. That in the 

 fourth will not dissolve at all, showing that the pepsin has 

 been destroyed. 



* 122. Strength of Acid required for Digestion. 

 The strength of acid with which albuminous bodies are most 

 quickly digested by pepsin varies with the nature of the body, 

 and al.io with the amount of pepsin present. Very dilute 

 solutions of pepsin digest best with very dilute acids, while 

 more concentrated pepsin solutions act more quickly with a 

 somewhat stronger acid. There seems, indeed, to be a definite 

 relation between the amount of pepsin and the strength of the 

 acid, though what this is has not yet been determined. The 

 proper strength of acid for any albuminous body may be 



