There are marked differences among bacteria in their ability to attack this class of compounds. Some can break up the most complex natural proteins such as albumins, globulins, glyco-, chromo-, and nucleoproteins, nucleins and albuminoid derivatives like gelatin. The term _saprogenic_ (σαπρος = rotten) is sometimes applied to bacteria which have this power. These proteins are large-moleculed and not diffusible, so that the first splitting up that they undergo must occur outside the bacterial cell. The products of this first splitting may diffuse into the cell and be utilized there. The bacteria of this class attack not only these proteins in the natural state or in solution, but also in the coagulated state. The coagulum becomes softened and finally changed into a liquid condition. The process when applied to the casein of milk is usually called “digestion,” also when coagulated blood serum is acted on. In the latter case the 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, polypeptids 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 symbiosis 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 complete 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 responsible 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 /H / N--H \ \H. The simplest ptomaine is /CH₃ / N--H \ \H, in which one H is replaced by methyl, methylamine, a gaseous ptomaine. With two hydrogens replaced by methyl, /CH₃ / N--CH₃ \ \H, dimethylamine, also a gas at ordinary temperature, is formed. Trimethylamine,