170 DROSERA EOTUNDIFOLIA. CiiAr. VII. 



taiuly a most surprising fact that the TT7TTr'oirrr-o- 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 ai'e reduced to 3-TT^^Vmr 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 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. 



wore inado on the nitrate of am- 1871, p. 228). With respect to 



monia, fourteen years ago, the ordinary chemical tests, I gather 



powers of the spectroscope had from Dr. Alfred Taylor's work 



not Leen discovered; and I felt on 'Poisons' that about 5^ of a 



all the greater interest in the grain of arsenic, ^^ of a grain 



then unrivalled powers of Drosera. of prussic acid, -^ of iodine, 



Now the spectroscope has al- and ^^ of tartarised antimony, 



together beaten Drosera; for ac- can be detected; but the power 



cording to Bunsen and Kirchhoff of detection depends much on the 



probaljly less than one sijTmnniWo of solutions under trial not being 



a grain of sodium can be thus extremely weak, 

 detected (see Balfour Stewart, 



