“Most of these substances are formed also by the chemical decomposition of proteids, but there is a sixth group which may be termed specific putrefactive products. These are the so-called ptomaines or putrefactive alkaloids.” Some of these bodies are either not poisonous or only poisonous in large doses, whilst others, derived from putrid foods of various kinds (sausages, cheese), are highly toxic (ptomatropine, tyrotoxine). [Sidenote: Definition of some terms.] Concerning the work done by micro-organisms, it may not be out of place here to define the meaning of certain terms, and to direct attention at the same time to the modifications in the results brought about by the presence or absence of air during the various stages of the process. The terms mineralisation, disintegration, oxidation, hydrolysis, bacteriolysis, nitrification, decomposition, eremacausis, putrefaction, fermentation, etc., have by many been used somewhat promiscuously, and this has led to a good deal of confusion, bewilderment and misconception. The cause of this has been undoubtedly our small amount of knowledge concerning this process and the changes brought about therein, but this would appear to be no reason why complication should be made worse. For the purposes of these remarks the undermentioned terms shall have the following meaning. [Sidenote: Mineralisation.] The term “mineralisation” is used for describing the whole process of the disintegration and conversion of organic into mineral matter, and no distinction shall be made between organic matter containing nitrogenous and organic matter containing carbonaceous substances. [Sidenote: Aerobic fermentation or decomposition.] When this process of mineralisation is carried on in the presence of sufficient quantities of air it is called “aerobic fermentation,” or “decomposition,” which is generally characterised by the absence of strong smells. The process may then be called one of complete oxidation. [Sidenote: Anaerobic fermentation or putrefaction.] Where, however, the mineralisation proceeds in the absence of air the process is called “anaerobic fermentation,” or “putrefaction,” and it is then that very pronounced foul smells are emitted. The process may then be called one of incomplete oxidation. [Sidenote: Obligatory aerobes and anaerobes.] [Sidenote: Facultative anaerobes.] That class of micro-organisms which can only live in the presence of oxygen is called “obligatory aerobes,” and that which can only exist in the absence of this gas “obligatory anaerobes.” Between these two is the group of “facultative anaerobes,” which, while growing best with a plentiful supply of oxygen, are nevertheless able to exist with a very small amount, and even with none at all, although in this case their vitality is often much impaired. [Sidenote: Organic matters are first split up and then converted into mineral substances.]