24 
methane is formed by the fermentation of cellulose; but it is quite possible, that 
methane may be formed by the fermentation of other substances, e. g. pectoses. 
But besides this formation of methane there is a formation of ferrous sul- 
phide in the mud on the bottom of Roskilde Fjord. The black colour of this 
mud is mainly due to ferrous sulphide. Adding acid to the mud, it assumes a 
somewhat lighter colour. The same happens, when the mud remains in the air, 
the ferrous sulphide being oxidized. In an undried bottom-sample from Roskilde 
Fjord the quantity of ferrous sulphide was determined in the following manner. 
The bottom-sample was boiled with addition of water on the water-bath to drive 
off the free hydrogen sulphide. Then an excess of sulphuric acid was added to 
free the hydrogen sulphide, which was driven out by heating up the water-bath 
and carrying through a stream of air. The sulphuretted hydrogen was absorbed 
in ammoniated silver nitrate. The silver sulphide precipitated was filtered off, 
dissolved in nitric acid and precipitated with hydrochloric acid. From the silver 
chloride precipitated, the combined sulphuretted hydrogen was calculated to amount 
to 0.38 %/, of the dried material. The method is not altogether exact, and the 
result is probably somewhat too low; but it shows that the quantities of ferrous 
sulphide formed are fairly considerable. 
A detailed study of the formation of ferrous sulphide in freshwater has 
been made by Beyerinck: »Ueber Spirillum desulfuricans als Ursache von Sul- 
fatreduktion« (Centrbl. f. Bact. 2 Bbth., 1, 1895 p. 1, 49 and 104). He remarks 
first of all, that the sulphide formation by organisms may proceed in the following 
4 ways: (1) by splitting up the sulphur-containing albuminous materials; (2) from 
sulphur (3) from sulphites and thiosulphates, where the last must be split up first 
of all into sulphur and sulphide; (4) through reduction of sulphates. It is the last 
mode of formation which is most important in nature; in the canals of several 
Dutch towns this formation of sulphide of iron is extremely troublesome. The 
bacteria which causes the reduction of sulphates is anaerob. In order that this 
may be able to develop, the water must first be deprived of its oxygen. This 
desoxidation of the water is due to other hbacteria, the growth and development of 
which are conditioned by the presence of organic materials in the water. Simul- 
taneous with the production of the ferrous sulphide in the water, the quantity of 
the sulphates is reduced. 
The formation of sulphide of iron in saltwater seems to proceed under 
the same conditions as in freshwater, though it is probably other bacteria which 
effect the reduction. The parallel which, according to the above, must exist be- 
tween the quantity of organic matter and the sulphate reduction, can also be 
readily shown in our fjords. The blackest and most stinking mud, which certainly 
also contains the greatest quantity of ferrous sulphide, is found im Roskilde Bred- 
ning, where we also have the greatest quantity of organic matter; but it is 
certain, that Holbæk Fjord and Bramsnæsvig also contain large quantities of fer- 
rous sulphide. In the Limfjord again, it is the places where most organic matter 
is found, that we meet with the ferrous sulphide, in Louns Bredning for example; 
on the other hand, not in Thisted, Kaas and Livø Bredninger. In Vilsund we 
have a sandy bottom with remains of Zostera buried in the sand. It was of 
interest to note, that the Zostera remains and the sand lying around these were 
