CHAP. VII. SUMMARY, SALTS OF AMMONIA. 17i 



even to myself, and I anxiously sought for every 

 source of error. The salt was in some cases weighed 

 fo>* 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 incredulous as myself, compared 

 several lots of leaves simultaneously immersed in the 

 weaker solutions and in water, and declared that there 

 could be no doubt about the difference in their ap- 

 pearance. 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 ab- 

 solutely pure as it can be made. It is to be especially 

 observed that the experiments with the weaker solu- 

 tions 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 

 irritability 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 carbonate, and the 

 phosphate much more powerful than the nitrate ; this 

 result being intelligible from the difference in the 

 amount of nitrogen in the first two salts, and from the 

 presence of phosphorus in the third. It may aid the 



