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tainly a most surprising fact that the 1/19760000 of a grain, or in round numbers the one-twenty-millionth of a grain (.0000033 mg.), of the phosphate should affect any plant, or indeed any animal; and as this salt contains 35.33 per cent. of water of crystallisation, the efficient elements are reduced to 1/30555126 of a grain, or in round numbers to one-thirty-millionth of a grain (.00000216 mg.). The solution, moreover, in these experiments was diluted in the proportion of one part of the salt to 2,187,500 of water, or one grain to 5000 oz. The reader will perhaps best realise this degree of dilution by remembering that 5000 oz. would more than fill a 31-gallon cask; and that to this large body of water one grain of the salt was added; only half a drachm, or thirty minims, of the solution being poured over a leaf. Yet this amount sufficed to cause the inflection of almost every tentacle, and often of the blade of the leaf.
I am well aware that this statement will at first appear incredible to almost everyone. Drosera is far from rivalling the power of the spectroscope, but it can detect, as shown by the movements of its leaves, a very much smaller quantity of the phosphate of ammonia than the most skilful chemist can of any substance.* My results were for a long time incredible
* When my first observations were made on the nitrate of ammonia, fourteen years ago, the powers of the spectroscope had not been discovered; and I felt all the greater interest in the then unrivalled powers of Drosera. Now the spectroscope has altogether beaten Drosera; for according to Bunsen and Kirchhoff probably less than one 1/200000000 of a grain of sodium can be thus detected (see Balfour Stewart, 'Treatise on Heat,' 2nd edit. 1871, p. 228). With respect to ordinary chemical tests, I gather from Dr. Alfred Taylor's work on 'Poisons' that about 1/4000 of a grain of arsenic, 1/4400 of a grain of prussic acid, 1/1400 of iodine, and 1/2000 of tartarised antimony, can be detected; but the power of detection depends much on the solutions under trial not being extremely weak. [page 171]
even to myself, and I anxiously sought for every source of error. The salt was in some cases weighed for me by a chemist in an excellent balance; and fresh water was measured many times with care. The observations were repeated during several years. Two of my sons, who were as incredulous as myself, compared several lots of leaves simultaneously immersed in the weaker solutions and in water, and declared that there could be no doubt about the difference in their appearance. I hope that some one may hereafter be induced to repeat my experiments; in this case he should select young and vigorous leaves, with the glands surrounded by abundant secretion. The leaves should be carefully cut off and laid gently in watch-glasses, and a measured quantity of the solution and of water poured over each. The water used must be as absolutely pure as it can be made. It is to be especially observed that the experiments with the weaker solutions ought to be tried after several days of very warm weather. Those with the weakest solutions should be made on plants which have been kept for a considerable time in a warm greenhouse, or cool hothouse; but this is by no means necessary for trials with solutions of moderate strength.
I beg the reader to observe that the sensitiveness or irritability of the tentacles was ascertained by three different methods--indirectly by drops placed on the disc, directly by drops applied to the glands of the outer tentacles, and by the immersion of whole leaves; and it was found by these three methods that the nitrate was more powerful than the carbonate, and the phosphate much more powerful than the nitrate; this result being intelligible from the difference in the amount of nitrogen in the first two salts, and from the presence of phosphorus in the third. It may aid the [page 172] reader's faith to turn to the experiments with a solution of one grain of the phosphate to 1000 oz. of water, and he will there find decisive evidence that the one-four-millionth of a grain is sufficient to cause the inflection of a single tentacle.