1900-1901.] Prof. Letts and Mr Hawthorne on Ulva latissima. 287 
The farmers on the shores of Belfast Lough have discovered the 
value of the ulva as a manure, and large quantities are carted away 
by them and used on their land. It no doubt fails in phosphates, 
but contains the necessary potassium salts. It is probable that it 
would be greatly improved for most crops by the addition of 
calcium phosphate or basic slag. 
2. Assimilation experiments . — Our first idea was to contrast the 
extent of growth of the ulva in pure sea water and in polluted sea 
water respectively ; and accordingly, as far as possible, similar 
tufts of the growing weed, adhering to stones, etc., and freshly 
removed from the sea-shore, were placed in two glass aquaria, one 
of which was filled with the pure sea water of the Irish Channel 
and the other with the same water to which 2 J per cent, of Belfast 
sewage had been added. Photographs were then taken of the 
two tanks, with the object of contrasting them with photographs 
on the same scale to be taken later. 
The result of this experiment was, however, curious, as the weed 
in both tanks soon became unhealthy and died in a month or two. 
"We believe that the explanation was that in both cases the ulva 
was killed by the strong sunlight to which at times it was 
exposed, as the two aquaria containing it were placed in a 
window facing south-west, and the experiment was made in the 
spring-time. 
Since then we have had a specimen of the seaweed growing in 
a glass dish placed near a window with a northern aspect ; for 
months, and as we write, it is still in a perfectly healthy condition. 
It is a mere frond of the ulva , and was picked up on the shore, 
unattached to any support, and indeed, when we commenced our 
experiments with it, we were afraid that it would be of no use to 
us. But this frond has remained in perfect health for seven 
months, during which time it has been treated with several dif- 
ferent specimens of foul sea water, and in the intervals has not 
been supplied with any pure sea water ; for after the failure of our 
first experiment it occurred to us that a far better method of 
investigation would be to examine the water in which the weed 
was growing, and not the weed itself. 
The methods of water analysis are delicate, and by contrasting 
the composition of samples of sea water both before and after the 
