484 MINNA E. JEWELL 



The results for H3PO4 were not very conclusive, as several of 

 the tadpoles died or appeared abnormal. The total amount 

 regenerated was, however, greater in the three controls. The 

 neutral-phosphate control lay about midway between the two 

 distilled-water controls, showing that the small amounts of Na, 

 K, and PO4 ions employed were neither beneficial nor detrimental. 

 The results of the NaOH series were definite, showing a progres- 

 sive decrease in both the rate of regeneration and the amount 

 regenerated as the concentration of base increased. While the 

 regeneration of the tadpoles in 5 cc. of NaOH averaged above the 

 controls, the hydrogen ion determinations show that this solution 

 was not only neutralized, but became slightly acid within twenty- 

 four hours, and it has already been shown (table 4) that this 

 same concentration of base is distinctly harmful if the volume 

 is sufficient to prevent it from being neutralized. 



The next series of experiments, involving 124 tadpoles, was 

 made in order to compare the effects upon regeneration of the 

 various acids. The experiments were carried on in finger bowls 

 containing 200 cc. of the solution. Four tadpoles were placed 

 in each dish, and the solutions were changed alternate days. 

 The acids used were H2SO4, HBr, HNO3, and H3PO4. The 

 tadpoles in the HNO3 series were slightly smaller than those in 

 H2SO4 and H3PO4; those in HBr were somewhat larger. The 

 H2SO4 series, which is regarded as typical, is given in detail 

 (table 5) in order to show the extent of variation among indi- 

 viduals of a set. About the same extent of individual variation 

 is found in the experiments with bases and other acids. 



A comparison of figures 13, 15, and 17 shows that in HNO3, 

 HBr, and H2SO4 the retardation is at first comparatively slight, 

 and increases gradually in concentrations up to 12.5 cc. (0.00065 

 N). As the concentration rises above this, the retardation 

 increases veiy rapidly, the order of toxicity of the acids being 

 HNO3, HBr, H2SO4. This might be explained on the ground 

 that in the lower concentrations the acids are so nearly completely 

 ionized as to have about the same effects, but as the concentra- 

 tion is increased the concentration of hydrogen ions, and conse- 

 quently the toxicity, increases more rapidly in HNO3 than in 



