92 PHYSIOLOGICAL ACTIVITIES 



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 saprophilic 

 bacteria. 



Other bacteria derive their nitrogen from some of the 

 products of the first two groups, and still further break down 

 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 complex 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 transformations 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 in which the hydrogen has been replaced by alkyl or other 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 (NH3) may be represented as 



N^H; the simplest ptomaine is n^h , methylamine, in 



which one H is replaced by methyl, a gaseous ptomaine; 



/CHs 



with two hydrogens replaced by methyl, N^CHs, dimethyl- 

 amine, a gas at ordinary temperature also; and trimethyl- 

 amine, N^CHs, a liquid. All three of these occur in her- 



