1900-1901.] Prof. Letts and Mr Hawthorne on Ulva latissima. 285 
We have not had time to pursue the bacteriological investigation 
further, which very possibly in more experienced hands might have 
given more definite results, but we believe that the following con- 
clusions are warranted from our experiments : — 
(1) When the Ulva latissima ferments in water, it is attacked 
by a species of micro-organism, which is a spore-forming bacillus, 
and which probably infests the weed itself. The products of this 
fermentation consist mainly of propionic acid, but other fatty 
acids are formed in smaller quantities, together with carbonic an- 
hydride and hydrogen. This micro-organism 'probably attacks the 
albuminoids of the seaweed. 
(2) The fermenting ulva is attacked later by a second species of 
micro-organism, with the production eventually of ferrous sulphide 
and sulphuretted hydrogen. It seems probable that these sulphur 
compounds are produced from the sulphates of the sea water (or 
those contained in the tissues of the ulva), and not from the 
albuminoids of the seaweed, and that the micro-organisms are 
derived from the mud of the foreshore where the ulva grows. 
Our experiments so far do not enable us to decide definitely 
whether the sulphuretted hydrogen is produced directly from the 
sulphates (or possibly the albuminoids), or indirectly from the 
ferrous sulphide, by the action of the organic acids. We are, how- 
ever, of the opinion that some of the gas at least owes its origin to 
the second of these two causes. 
Ulva latissima in relation to Sewage Pollution. 
The evidence which we have collected tending to prove that the 
occurrence of Ulva latissima in quantity in any locality is asso- 
ciated with the pollution of the sea water by sewage is of three 
kinds. 
First, that afforded by the composition of the weed itself, or 
rather by the proportion of nitrogen it contains. Second, from 
experiments made on the assimilation of nitrogenous compounds 
by the growing ulva from sea water purposely polluted; and 
third, from an examination of the localities in which the weed 
occurs in abundance, and of those from which it is virtually 
absent. 
