Chap. VII.] SUMMARY, SALTS OF AMMONIA. 139 



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 incredi- 

 ble 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 meas- 

 ured many times with care. The observations were repeated 

 during several years. Two of my sons, who were as incredu- 

 lous 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 

 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 car- 

 bonate, and the phosphate much more powerful than the 

 nitrate; this result being intelligible from the difference in 



* When my first observations Stewart, ' Treatise on Heat,* 2nd 



were made on the nitrate of am- edit. 1871, p. 228). With respect 



monin, fourteen years ago, the to ordinary chemical tests, I 



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



not been discovered; and I felt work on ' Poisons ' that about 



all the greater interest in the ^Ja <>f fi K^aln ot nmonic, jjm nt a 



then unrivalled powers of Dro- gruinof prussicacid, ,^'oo of iodine, 



sera. Now the spectroscope has and ko* of tartarised antimony, 



altogether beaten Drosera: for, can be detected: but the power 



according to Bunsen and Kirch- of detection depends much on 



hofr, probably less than one the solutions under trial not be- 



in'Of a grain of sodium can ing extremely weak, 

 be thus detected (ee Balfour 



