CHAP. VIII. CONCLUDING EEMARKS, ACIDS. 197 



criterion of the power of an acid on Drosera, as citric 

 and tartaric acids are very sour, yet do not excite 

 inflection. It is remarkable how acids differ in 

 their power. Thus, hydrochloric acid acts far less 

 powerfully than hydriodic and many other acids of the 

 same strength, and is not poisonous. This is an in- 

 teresting fact, as hydrochloric acid plays so important 

 a part in the digestive process of animals. Formic 

 acid induces very slight inflection, and is not poison- 

 ous ; whereas its ally, acetic acid, acts rapidly and 

 powerfully, and is poisonous. Malic acid acts slightly, 

 whereas citric and tartaric acids produce no effect. 

 Lactic acid is poisonous, and is remarkable from in- 

 ducing inflection only after a considerable interval of 

 time. Nothing surprised me more than that a solution 

 of benzoic acid, so weak as to be hardly acidulous to 

 the taste, should act with great rapidity and be highly 

 poisonous ; for I am informed that it produces no 

 marked effect on the animal economy. It may be 

 seen, by looking down the list at the head of this dis- 

 cussion, that most of the acids are poisonous, often 

 highly so. Diluted acids are known to induce nega- 

 tive osmose,* and the poisonous action of so many 

 acids on Drosera is, perhaps, connected with this 

 power, for we have seen that the fluids in which they 

 were immersed often became pink, and the glands 

 pale-coloured or white. Many of the poisonous acids, 

 such as hydriodic, benzoic, hippuric, and carbolic (but 

 I neglected to record all the cases), caused the secre- 

 tion of an extraordinary amount of mucus, so that 

 long ropes of this matter hung from the leaves when 

 they were lifted out of the solutions. Other acids, 

 such as hydrochloric and malic, have no such ten- 



* Miller's c Elements of Chemistry,' part i. 1867, p. 87. 



