58 



CARNEGIE INSTITUTION OF WASHINGTON. 



Agar is a carbohydrate and as such exhibits some of the properties of 

 an exceedingly weak acid. The amino-acids are amphoteric electro- 

 lytes and therefore behave like acids toward bases and like bases 

 toward acids. A comparative study of the swelling of agar in aqueous 

 solutions of the simpler organic acids and their corresponding a-amino 

 compounds showed that in the latter class of substances agar swells 

 more than in the fatty-acid solutions. In solutions more dilute than 

 0.01 normal the swelling in the a-amino acids exceeds that attained in 

 distilled water. Gelatine, on the other hand, attains its greatest 

 swelling in solutions of the fatty acids, then in water, and less than the 

 latter in solutions of the a-amino acids. 



Further insight as to whether the increased swelling of agar in 

 amino-acids is due to salt formation between the weak acid, agar, and 

 the amphoteric amino-acid was sought in a study of the behavior of 

 agar toward ammonium hydroxide and other bases. From these 

 experiments it would appear that the stronger the base, as indicated 

 by its position in the electromotive series, the less is the effect on the 

 swelling of agar. Thus we have in effect : KOH < NaOH < LiOH < 

 C2H5NH2<NH40H. In solutions of greater concentration than 0.01 

 normal the conditions in case of the ethyl amine and ammonium 

 hydroxide are somewhat complicated, owing to the complexities of 

 equilibria between dissolved NH3 and C2H5NH2 and their respective 

 hydroxides. 



Of great importance in experiments with these bases is the frequent 

 renewal of the solutions in which the agar plates are swelling. Owing 

 to the absorption of C02from the air and, in case of NH3 and C2H5NH2, 

 the volatilization, the strength of the solutions decreases considerably 

 in time. If the solutions are not renewed the final swellings in all 

 solutions are very nearly equal to that attained in water. If, however, 

 the solutions are renewed every 12 hours very marked differences are 

 obtained. Especially noteworthy is the fact that ammonium hy- 

 droxide in 0.001 normal concentration produces a swelling considerably 

 in excess of water. 



The following shows the relative swelling of dried agar plates at 

 15° C. in alkaline-hydroxide solutions renewed every 12 hoiu"s; the 

 total swelling of the plates in water is 3,950 per cent: 



When the agar plates are made so as to incorporate in them the 

 kations, K or NH4, by originally making up the 2.5 per cent agar with 

 0.01 normal KOH and NH4OH instead of with water and then drying, 



