H. BOTANY AND HORTICULTURE. 349 



tion have not been determined as thoroughly as those of fer- 

 mentation, yet we know that all putrefaction -is accompanied 

 by the development of Bacteria, and is entirely prevented by 

 their exclusion ; it progresses in the same ratio as these small- 

 est of all organisms increase, and ceases as soon as this in- 

 crease ceases. The Bacteria are then precipitated as a pow- 

 dery deposit, or as gelatinous lumps (Zooglcea), just as the 

 fungi are in sugar solutions when fermentation is finished. 

 The question as to how Bacteria enter into nitrogenous sub- 

 stances has usually been answered by the assertion that they 

 float in the air like the spores of fungi. This has been suc- 

 cessfully refuted by Sanderson, who maintains that the infec- 

 tion is only caused by contact with unclean surfaces (of the 

 skin, of tools, or vessels), or by the water, which, when not 

 recently distilled, always contains germs of Bacteria. Even 

 saliva, urine, blood, milk, and albumen of eggs become only 

 mouldy, without putrefying, protected against the contact 

 with water or other bodies containing Bacteria. Mr. Cohn's 

 researches, however, do not absolutely confirm Sanderson's 

 observations, as he is quite certain that germs of Bacteria 

 may be evaporated to a slight extent. 



Mr. Cohn also demonstrated that sugar or other fermenta- 

 ble matter is not necessary for the development of Bacteria ; 

 they propagate quite normally in any liquid which contains 

 carbon in addition to ammonia or nitric acid. 



Since Bacteria only assimilate nitrogen in the form of am- 

 monia or nitric acid, their action in putrefaction may be con- 

 sidered as causing the division of albuminous substances into 

 ammonia, which is assimilated, and into other bodies which 

 give rise to the collateral products of putrefaction, so that 

 the process is similar to fermentation, where the sugar is di- 

 vided into alcohol and carbonic acid. 



Sometimes the products of albumen, decomposed by putre- 

 faction, are colored (as in boiled potatoes, bread, etc.), and in 

 such cases another form of Bacteria the globular is always 

 found. These are imbedded in slime, and without proper 

 motion. 



In several contagious diseases Bacteria have been found in 

 the blood and the secretions, and may be considered as the 

 carriers of infection. They disturb the normal functions of 

 the organism by decomposing the blood, and, as they always 



