I made other experiments in which harvesting ants were imprisoned along with various seeds in small, cylindrical, closed vessels containing a little damp sand. Here the vessels were frequently rolled from side to side or shaken, during the twenty-two hours for which the experiment lasted, so as to excite the ants and make them give off such odours as they possessed, but no trace of injurious influence was produced upon the seeds, which germinated and grew normally afterwards. At Mr. Darwin's suggestion I made a long series of experiments with formic acid, in which measured quantities, pure or diluted, were placed in a watch-glass on damp sand and surrounded by seeds, the whole being enclosed in a covered tumbler, so that the effects produced on the seeds by the vapour rising from the acid might be noted. Similar seeds were sown at the same time and in the same way, but without the acid, so as to permit of comparison. These experiments have afforded some interesting results,[116] but do not supply any positive data which might help us to discover the secret of the ants. They narrow, indeed, the area in which search can profitably be made, indicating as they do that the vapour of formic acid is incapable of rendering the seeds dormant after the manner of the ants, and showing, on the contrary, that its influence is always injurious to the seeds, even when present only in excessively minute quantities. [Footnote 116: I hope shortly to offer these observations, together with another series of a similar nature in which my friend Mr. J. B. Andrews has taken part, to the Linnean Society.] It appears to me now that the most promising field for experiments made with a view to clearing up this difficulty, is that afforded by the closer investigation of the phenomena of normal germination, and by a study of the conditions under which seeds remain dormant, as they are occasionally known to do, in situations which our general experience would have selected as favourable to germination. I have good hopes, also, that when we come to know more of the habits of harvesting ants in tropical countries, and when naturalists have excavated and described their subterranean stores--a thing which has not yet been done as far as I know--we may gather fresh indications to guide us in our search.