292 Proceedings of Royal Society of Edinburgh. [sess. 
water, its actual weight was 0*0000404 x 16 = 0*0006464 grm., 
and this, multiplied by the factor gives 0*0104 grm., or about 
1 centigram, as the actual weight of seaweed formed. 
A series of determinations showed that 1 square inch of the dried 
ulva weighs on an average 0*009 grm., so that in this experiment 
say, 1*1 square inches, of the ulva were formed, which is 
equivalent to nearly 0*8 per cent, of the original frond. 
We hope to make further experiments in order to ascertain 
whether the rate of nitrogen assimilation is constant, or varies with 
the concentration, and also to what extent the rate is affected by 
differences in illumination. 
3. The localities in which Ulva latissima occurs in quantity 
contrasted with those from which it is virtually absent . — We may 
first of all draw attention to two particular localities which have 
come more immediately under our observation where this seaweed 
is abundant, and one from which it is almost entirely absent, 
because an examination of the conditions obtaining in these, offers 
some very striking evidence in favour of the view mentioned above, 
viz., that the occurrence of the ulva in quantity is an indication of 
sewage pollution. 
The first two localities we refer to are Belfast Lough and a 
part of Dublin Bay, and the second is Strangford Lough. 
Belfast Lough . — According to the statements of some of the 
older inhabitants of the neighbourhood, Ulva latissima was not 
present in former times in the very large quantities in which it 
now occurs in the upper reaches of the Lough, hut the Zostra 
marina , or sea grass, now found only in small quantities, was 
abundant. 
Up to the year 1889 the bulk of the sewage of the city of 
Belfast was allowed to flow directly into the Lagan river. But in 
that year a new main drainage system was inauguratedhy which the 
greater part of the sewage is collected in two main channels, and 
from them pumped into a tank, the contents of which are dis- 
charged (on the ebb-tide only) through a submarine culvert 
opening some distance seawards. Belfast, as every one knows, 
has grown with remarkable rapidity, and there can therefore he no 
question that for that reason alone very much more sewage makes 
its way into the Lough now than formerly, and this amount has 
