CH. Xl] BACTERIA IN THE SEA 263 



of the air), marsh gas, nitrogen and possibly free hydrogen. The 

 sulphuretted hydrogen becomes oxidised in various ways, and then 

 all that remains of the proteid are a few innocuous and inoffensive 

 compounds, which rapidly diffuse into the water of the soil, into 

 rivers, &c. 



Fermentation bacteria. In the organic body there are three 

 great classes of compounds, the nitrogenous proteids, the non-nitro- 

 genous carbohydrates, and the fats. The proteids are the " flesh " 

 of the body, and many other tissues not popularly termed " flesh," 

 such as horn, chitin, &c. The carbohydrates are starch, sugar, 

 cellulose, &c. The fats are compounds of a fatty acid with glycerine. 

 Putrefactive decomposition concerns itself with the proteids. But 

 the carbohydrates and the fats are also broken down by the agency 

 of micro-organisms. The fats are split into their fatty acids and 

 glycerine, and then each of these compounds is broken down by a 

 process which we distinguish from putrefaction and call ferment- 

 ation. Fermentative bacteria doubtless exist in the sea though 

 we have little knowledge of their distribution. 



Fermentation processes are among the most common things of 

 everyday life. The formation of alcohol in the manufacture of 

 wines, spirits and beer, is a process of fermentation. The souring 

 of wine or beer, or of milk, is also a fermentation. So is the pro- 

 duction of vinegar, the rising of bread in baking, the ripening of 

 cheese, and many other processes used in the domestic arts. In all 

 fermentation processes some carbohydrate substance is decomposed 

 by the vital action of bacteria and a simpler substance or many 

 simpler substances are formed. Always, or nearly always, carbon 

 dioxide is also formed. But whereas the end products of a putre- 

 factive process are ammonia and sulphuretted hydrogen and some 

 other bodies, those of fermentation are carbon dioxide and water. 

 The characteristic feature of a putrefactive or fermentative 

 process is that a large amount of the food material of the micro- 

 organism (bacterium, mould or yeast) is broken down in order to 

 obtain a comparatively small amount of energy. In the assimila- 

 tion of sugar, as a food, by a higher organism this substance is 

 completely oxidised and the products of the combustion appear as 

 carbon dioxide and water. All the available energy of the food 



