90 PHYSIOLOGICAL ACTIVITIES 



serum is more commonly said to be ''liquefied," as is the 

 case when gelatin is the substance changed. Most of these 

 bacteria have also the property of coagulating or curdling 

 milk in an alkaline medium and then digesting the curd. 

 A second class of bacteria has no effect on the complex 

 proteins just mentioned but readily attacks the products of 

 their first splitting, i. e., the proteoses, peptones, polypep- 

 tids and amino-acids. They are sometimes called sapro- 

 philic bacteria. 



Other bacteria derive their nitrogen from some of the 

 products of the first two groups and still further break dow^n 

 the complex protein molecule. Under normal conditions 

 these various kinds of bacteria all occur together and thus 

 mutually assist one another in what is equivalent to a sym- 

 biosis or rather a metabiosis, a "successive existence," one 

 set living on the products of the other. The result is the 

 complete splitting up of the protein molecule. A part of 

 the nitrogen is built up into the bodies of the bacteria which 

 are using it as food. A part is finally liberated as free nitrogen 

 or as ammonia after having undergone a series of transfor- 

 mations, many of which are still undetermined. 



One class of compounds formed received at one time 

 much attention because they were supposed to be respon- 

 sible for a great deal of illness. These are the "ptomaines," 

 basic nitrogen compounds of definite composition— amines 

 —some few of which are poisonous, most of them not. The 

 basic character of ptomaines may be understood if they be 

 regarded as made up of one or more molecules of ammonia 

 in which the hydrogen has been replaced by alkyl or other 

 radicals. Thus ammonia (NH)3 may be represented as 



/H /CH3 . . 



N— H. The simplest ptomaine is N— H, m which one H is 



\H "^ "^ \H 



replaced by methyl, methylamine, a gaseous ptomaine. 



/CHr 



With two hydrogens replaced by methyl, N— CH3, dimethyl- 



\H 

 amine, also a gas at ordinary temperature, is formed. Tri- 



methylamine, N— CH3, a liquid, results when three hydro- 



\CH3 



