3l8 L. H. HYMAN. 



RELATIVE TOXICITY OF DIFFERENT ACIDS. 



An attempt was made to discover the hydrogen ion concentra- 

 tion at which each of the acids used would cause the death of 

 Planaria within a relatively short period of time, say, two to 

 three hours. A sort of trial and error procedure is necessary to 

 determine this matter. The results are not very exact but the 

 relative toxicities of the different acids were plain enough after a 

 few trials. Butyric acid is by far the most toxic, as found also 

 by other investigators, and kills the" animals within two or three 

 hours at pH 5.0. Acetic acid ranks next in toxicity, killing in 

 the time specified at pH 4.4. Tartaric and citric acids come 

 next, killing within two or three hours at 3.6 and 3.4 respectively. 

 The three mineral acids are nearly equally toxic, sulphuric being 

 slightly more effective, killing in two or three hours at pH 3.2 to 

 3.4, while hydrochloric and nitric acids must be used at pH 3.0 

 to 3.2 to obtain the same result. The order of toxicity may then 

 be expressed as follows: butyric > acetic > tartaric > citric 

 > sulphuric > nitric = hydrochloric. 



Carbon dioxide at saturation (about pH 5.2) was not found to 

 kill the animals as long as the oxygen supply was adequate. 



The death of the animals took place with equal rapidity and 

 at the same hydrogen ion concentrations in both normal and 

 carbonate-free water. This proves that the death is not due to 

 the carbon dioxide set free by acids in normal water but is a 

 direct acid effect. This is further evidenced by differences in 

 the appearance and in the manner of disintegration of the dead 

 animals. Death in acids appears to be generally due to coagula- 

 tion. This was also noted by Mrs. Lewis, '23. 



In view of the fact that the death of the animals results from 

 a direct coagulating action of the acids while the depression of 

 the respiratory rate by acids is caused almost wholly by carbon 

 dioxide, no relation would be expected between the toxicity of 

 different acids and their depressing action. This is the case. 

 Butyric acid, the most toxic, is in non-lethal doses the poorest 

 depressant. The experiments do not of course serve to show the 

 amount of depression at or near the death point, except in the 

 cases of acetic and butyric acids, pH 5.0, Table VI. Since 

 different acids are equally lethal at different hydrogen ion 



