1900-1901.] Prof. Letts and Mr Hawthorne on Ulva latissima. 291 
Sufficient ammonium chloride was added to this sea water to 
bring up the nitrogen as free ammonia to 0*042 parts per 100,000, 
and this strength was verified by a determination made as before. 
The frond of ulva was now drained from the sea water in which 
it had been immersed for some months, rinsed with the new sample 
prepared as just described, and the dish then filled with the latter. 
After twenty-four hours, 200 c.c. of the water were removed 
from the dish, distilled, and the distillate Nesslerised, when no 
free ammonia was found, proving that the seaweed was still in a 
perfectly vigorous condition. This was also shown by the copious 
evolution of oxygen which had occurred from it, the gas remaining 
entangled in the folds of the frond. 
The water in the dish was next poured off, and sufficient of a 
standard solution of potassium nitrate added to it to bring up the 
nitric nitrogen to 0*05 parts per 100,000, when it was emptied 
back again. The frond of ulva was now in contact with ammonia- 
free sea water containing nitrates, and was allowed to remain thus 
for 70 hours, when a portion of the water was removed from the 
dish, and the nitrates determined by the same process as before. 
The water was found to contain 0*005 parts of nitric nitrogen, 
showing that the ulva had absorbed 90 per cent, of the amount 
originally present. 
The results of the preceding experiment leave no doubt as to the 
energetic power which Ulva latissima possesses of absorbing nitrogen 
from polluted sea water, both in the form of ammonia and of nitrates. 
They also clearly demonstrate that this seaweed can flourish in 
highly polluted water ; and in addition, they lend a good deal of 
support to the theory which we had gradually been led to form, 
that the occurrence of the ulva in quantity in a given locality may 
be regarded as a sign of sewage pollution. 
From the results of these experiments it is possible to calculate 
the rate of growth of the ulva under the existing conditions ; for, as 
its tissues contain 6*18 per cent, of nitrogen, it is obvious that the 
nitrogen lost by the water in which it was placed, multiplied by 
the factor -Fj, gives the weight of the seaweed formed. 
Thus, in experiment 2, the water lost 0*049 per 100,000 of free 
or saline ammonia in 17 hours. This is equivalent to 0*0404 parts 
of nitrogen per 100,000; and as it was removed from 1600 c.c. of 
