says, Take a narrow strip of paper 83 ft. 4 in. in length, and stretch
it along the wall of a large hall; then mark off at one end the tenth
of an inch. This tenth will represent a hundred, and the entire strip a
million. [page 170]
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]
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