Cuar. VII.] SUMMARY, SALTS OF AMMONIA. 139 
sufficed to cause the inflection of almost every tentacle, and 
often of the blade of the leaf. 
Iam 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 ammonia than the most skilful chemist can 
of any substance.* My results were for a long time incredible 
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 incre- 
dulous as myself, compared several lots of leaves simultane- 
ously 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 in- 
duced 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 irri- 
tability of the tentacles was ascertained by three different 
methods—indirectly by drops placed on the disc, directly by 
* When my first observations were (see Balfour Stewart, ‘Treatise on 
made on the nitrate of ammonia, Heat, 2nd edit. 1871, p. 228). With 
fourteen years ago, the powers of respect to ordinary chemical tests, I 
the spectroscope had not been dis- gather from Dr. Alfred Tay lor’s 
covered ; and I felt all the greater work on ‘ Poisons’ that about 1055 Of 
interest in the then unrivalled powers a grain of arsenic, 3/55 of a grain of 
of Drosera. Now the spectroscope  prussic acid, phy of iodine, and B05 
has altogether beaten Drosera; for, of tartarised antimony, can be de- 
according to Bunsen and Kirchhoff, tected; but the power of detection 
probably less than one skg of a depends much on the solutions under 
grain of sodium can be thus detected trial not being extremely weak. 
