MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 349 



replaced with hundredth-normal hydrochloric acid, traversed 1 1 mm. 

 of the scale in the third hour during which it was immersed in 

 hundredth-normal sodium hydrate, then shrank 5 mm. in an hour 

 in acid, then enlarged 9 mm. in three and a half hours in alkali, 

 after which it shrank 3 mm. between 8:30 P.^I. and 7 A.]\I. in 

 acid. A change to alkali gave an enlargement of 6 mm. in two hours 

 (Fig. 12). The auxograph was set to multiply so that the actual 



Fig. 12. Auxographic record of changes in section of agar 90 — tyrosin 

 10, .14 mm. in thickness. Immersed in water at A, alkali at B, acid at C. 

 alkali at Z), acid at £, alkali at F, and acid at G^. (Upper half of figure.) X lO- 



Auxographic record of changes in section of agar 90 — tyrosin 10, .14 mm. 

 in thickness. A in distilled wafer, B acid, C alkali, D acid, E alkali, F acid, 

 and G alkali. (Lower half of figure.) X 10. 



enlargement in the periods noted was one twentieth of the distance 

 traversed by the pen. The change from acidity to alkalinity is fol- 

 lowed by the most marked effects when the colloid has taken up a 

 fourth or a third of the possible total amount of water. Perhaps 

 the most striking feature is the response of the colloid to acidifica- 

 tion under the alternating conditions. Desiccated sections give a 

 greater total swelling in acid than in alkali, but when a certain 

 amount of swelling has already taken place under neutral or alkaline 

 conditions no further increase in acid solutions and actual shrink- 

 age ensues. A change to alkalinity is always followed by increased 

 imbibition. Sections of plates containing 90 parts agar and 10 

 parts of gelatine gave results similar to those of the tyrosin mix- 

 ture. No determinations of the minimum proportion of nitrog- 



