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 interesting fact, as hydrochloric acid plays so important a part in the digestive process of animals.  Formic acid induces very slight inflection, and is not poisonous; 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 inducing 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 discussion, that most of the acids are poisonous, often highly so.  Diluted acids are known to induce negative 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 secretion 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. [page 198]

dency; in these two latter cases the surrounding fluid was not coloured pink, and the leaves were not poisoned.  On the other hand, propionic acid, which is poisonous, does not cause much mucus to be secreted, yet the surrounding fluid became slightly pink.  Lastly, as in the case of saline solutions, leaves, after being immersed in certain acids, were soon acted on by phosphate of ammonia; on the other hand, they were not thus affected after immersion in certain other acids.  To this subject, however, I shall have to recur. [page 199]

CHAPTER IX.

THE EFFECTS OF CERTAIN ALKALOID POISONS, OTHER SUBSTANCES AND VAPOURS.

Strychnine, salts of—­Quinine, sulphate of, does not soon arrest the movement of the protoplasm—­Other salts of quinine—­Digitaline—­Nicotine&m
dash;­Atropine—­Veratrine—­Colchicine—­ Theine—­Curare—­Morphia—­Hyoscyamus—­Poison of the cobra, apparently accelerates the movements of the protoplasm—­Camphor, a powerful stimulant, its vapour narcotic—­Certain essential oils excite movement—­Glycerine—­Water and certain solutions retard or prevent the subsequent action of phosphate of ammonia—­Alcohol innocuous, its vapour narcotic and poisonous—­Chloroform, sulphuric and nitric ether, their stimulant, poisonous, and narcotic power—­Carbonic acid narcotic, not quickly poisonous—­Concluding remarks.