_Lability._--Neutral secretin is but feebly attacked by a temperature of 100 C. If heated in an autoclave (so as to prevent oxidation), this temperature can be continued for thirty minutes without any change in its activity. Increasing the temperature increases the speed of destruction, so that at 140 C. the destructive action is marked.[61] Autoclaving at 15 pounds for fifteen minutes, as an ordinary sterilization of culture mediums, produces, we found, a distinct though not serious decrease in activity. Secretin acidified to fifth-normal with hydrochloric acid loses 60 per cent. of its activity on fifteen minutes boiling. Secretin, alkalinized to fifth-normal with sodium hydroxid loses 95 per cent. of its activity in five minutes’ boiling; decreases to a trace in thirty minutes, and disappears entirely in sixty minutes. At room temperature, with fifth-normal alkalinity, 80 per cent. of secretin is destroyed in eight hours.[61] The destruction probably means a secondary cleavage of the secretin molecule itself. [61] Lalou (Note 21). May: Jour. Physiol. =30=:400, 1904. Secretin is oxidized readily. If left standing uncovered for a summer’s day, the preparation will be inactive.[51] Even if kept in the ice-chest (no other precaution being taken), its activity is lost in a very few days. Sunlight undoubtedly hastens the oxidative process. If care is taken as to sterility, however, and the secretin is kept in the ice-chest, well stoppered and in a dark flask, it will retain its activity for several weeks. Dixon and Hamill[51] claimed that secretin disappears quantitatively on passage through a Berkefeld filter at 5 mm. pressure. Lalou,[62] using higher pressure, was unable to confirm the finding, but obtained a marked decrease in activity. Our results are in accord with those of Lalou. [62] Launoy: Arch. internat. de Physiol. =3=:62, 1906. Morel and Terroine: Compt. rend. Soc. de biol. =67=:36, 1909. Zunz: Arch. internat. de physiol. =8=:181, 1909. Lalou: Jour. de physiol. =14=:465, 1912. _Analogy to Epinephrin._--The analogy of secretin to epinephrin does not generally receive enough emphasis. Both substances are nonspecific in distribution, but specific chemically, and especially physiologically, epinephrin acting on the myoneural junctions, secretin on intestinal digestion. They are both relatively simple substances of low molecular weight, and subject to rapid oxidation whereby their properties disappear. The action in both cases is very transient. They are the two examples of what Starling calls the “acute hormones,” in which it is essential that reaction take place immediately, and shall disappear as soon as the exciting cause is removed.[63] [63] Starling: Proc. Roy. Soc. Med., 8, No. 4, 1914, Therap. and Pharm. Section, p. 29. CLINICAL USE OF SECRETIN