18 MACDOUGAL— ACTION OF BASES AND SALTS 



making of the plates which furnished the sections used in the swell- 

 ing tests. It is seen therefore that the swelling of agar shows a 

 maximum in a hydrogen ion concentration greater than its own as 

 well as in the hydroxide solution at Ph ii. 



The above results are to be applied in correction of statements 

 made in many previous publications as to the retarding action of 

 the hydrogen ion on the swelling of pentosans. This mistaken con- 

 clusion has been most recently made in the Report of the Depart- 

 ment of Botanical Research of the Carnegie Institution of Wash- 

 ington for 1920, pp. 54 and 55, in which it is said that " The 

 pentosans are weak acids and in general their hydration capacity is 

 lessened by hydrogen ions. Hydroxyl ions and compounds contain- 

 ing the amino-groups, such as may be in solutions of phenylalanin, 

 alanin, asparagin and glycocoll, may exert an effect by which hydra- 

 tion capacity is increased above that in pure water. Tvlucilages de- 

 rived from various sources show some differences in reactions to 

 the solutions named while conforming to the generalizations given. 

 Their hydration is but little affected by the presence of the common 

 sugars in the water of suspension or dispersion." 



Some dift'erences in the reactions of the different plant muci- 

 lages to the action of hydrogen and hydroxyl ions might reasonably 

 be attributed to the varying acidity of these substances. Thus com- 

 mercial acacia gum (gum arabic) and purified cherry gum in a 1 

 per cent, solution have a Ph value of 5.1, which is not far from that 

 of a sample of " Bacto " gelatine in an 8 per cent, solution. The 

 mucilage of Opuniia, which is taken to form an actual part of the 

 plasmatic mass in the cells of this cactus has a Ph value of 5.8 as 

 determined by the indicator method. 



The data presented in the foregoing table show that the chlo- 

 rides of the four metals at o.oooi M w'ith a Ph value ranging from 

 5.6 to 6, cause an excessive hydration, which in terms of water as 

 100, would be CaClo, no; MgCL, 116; KCl, 113; NaCl, 150. At 

 o.ooi M KCl which has a Ph value of 5.8 as compared with a value 

 of 5.7 at 0.0001 M also causes an excessive hydration. Marked dif- 

 ferences are shown by the sections from the two plates in solutions 

 in this concentration, the swelling of one in the sodium being ex- 

 cessive and that of the other lessened. It is to be noted that in the 



