238 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



Letts and McKay have also shown that carbon dioxide, 

 itself produced by the decomposition of organic matter, can 

 decompose sulphides, such as ferrous sulphide, FeS, yielding 

 H 2 S. Such sulphides can also be decomposed by fatty acids 

 produced by other fermentations. The two sources, a and b, 

 of sulphuretted hydrogen may now be separately considered : 

 (a) The decomposition of albumin. The formation of 

 hydrogen sulphide by the decomposition of albumin, through 

 the action of bacteria, can be readily demonstrated. If a few 

 drops of lead acetate solution are added to a small bottle full 

 of sewage, the bottle closed, and placed in an incubator for a 

 day or two, the solution turns black from the presence of lead 

 sulphide. The actual organisms capable of decomposing 

 albumin, with formation of hydrogen sulphide, can be recog- 

 nised by an elegant method suggested by Beyerinck : 



To ordinary nutrient gelatine, sufficient white lead is added 

 to obtain a perfectly white plate ; when the medium is poured 

 into the Petri dish, a little sewage diluted with distilled water 

 is poured over the plate. After it is set, and as the colonies 

 develop, black dots of lead sulphide will indicate the presence of 

 these organisms, which are capable of breaking down albumin 

 with production of hydrogen sulphide. 



A very serious case of nuisance has for a long time existed 

 on the shores of Belfast Lough. Here great quantities of a 

 seaweed, Ulva latissima, flourish. Professor Letts has shown 

 that this seaweed contains an abnormally high albumin con- 

 tent ; when deprived of its natural conditions of growth, the 

 Ulva is capable of fermentation, apparently in two distinct 

 and successive stages. The first stage results in the produc- 

 tion of fatty acids, mainly propionic, together with carbon 

 dioxide and hydrogen ; in the second stage of fermentation, 

 in which a different species of micro-organism is concerned, 

 sulphuretted hydrogen is produced. 



It is not at present certain what are the exact sources, in 

 the first place of the fatty acids, and in the second place of the 



