Cuar, VII. SUMMARY, SALTS OF AMMONIA. 171 
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. T'wo 
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 hape 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 muck 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 
