170 DROSERA ROTUNDIFOLIA. Cuar. VIL 
tainly a most surprising fact that the y57ss5000 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 s553s5705 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 3l-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 ten- 
tacle, and often of the blade of the leaf. 
I am well aware that this statement will at first 
appear incredible to almost every one. 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 am- 
monia than the most skilful chemist can of any 
substance.* My results were for a long time incredible 
* When my first observations ‘Treatise on Heat,’ 2nd edit. 
were made on the nitrate of am- 
monia, 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 al- 
together beaten Drosera; for ac- 
eee to Bunsen and Kirchhoff 
probably less than one of 
a grain of sodium can ee thus 
detected \sce Balfour Biovark 
1871, p. 228). With respect to 
ordinary chemical tests, I -gather 
from Dr. Alfred Taylor's work 
on ‘Poisons’ that about yh, of a 
grain of arsenic, ¥ib5 of a grain 
of prussic acid, rm of iodine, 
and x5 of tartarised antimony, 
can be detected; but the power 
of detection depends much on the 
solutions under trial not being 
extremely weak. 
