Cuar. VII CONCLUDING REMARKS, 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 ‘ Elements of Chemistry,’ part i. 1867, p. 87. 
