PHOSPHATE OF AMMONIA.
This salt is more powerful than the nitrate, even in a greater degree
than the nitrate is more powerful than the carbonate. This is shown by
weaker solutions of the phosphate acting when dropped on the discs, or
applied to the glands of the exterior tentacles, or when leaves are
immersed. The difference in the power of these three salts, as tried in
three different ways, supports the results presently to be [page 154]
given, which are so surprising that their credibility requires every
kind of support. In 1872 I experimented on twelve immersed leaves,
giving each only ten minims of a solution; but this was a bad method,
for so small a quantity hardly covered them. None of these experiments
will, therefore, be given, though they indicate that excessively minute
doses are efficient. When I read over my notes, in 1873, I entirely
disbelieved them, and determined to make another set of experiments
with scrupulous care, on the same plan as those made with the nitrate;
namely by placing leaves in watch-glasses, and pouring over each thirty
minims of the solution under trial, treating at the same time and in
the same manner other leaves with the distilled water used in making
the solutions. During 1873, seventy-one leaves were thus tried in
solutions of various strengths, and the same number in water.
Notwithstanding the care taken and the number of the trials made, when
in the following year I looked merely at the results, without reading
over my observations, I again thought that there must have been some
error, and thirty-five fresh trials were made with the weakest
solution; but the results were as plainly marked as before. Altogether,
106 carefully selected leaves were tried, both in water and in
solutions of the phosphate. Hence, after the most anxious
consideration, I can entertain no doubt of the substantial accuracy of
my results.
[Before giving my experiments, it may be well to premise that
crystallised phosphate of ammonia, such as I used, contains 35.33 per
cent. of water of crystallisation; so that in all the following trials
the efficient elements formed only 64.67 per cent. of the salt used.
Extremely minute particles of the dry phosphate were placed [page 155]
with the point of a needle on the secretion surrounding several glands.
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