FOOD DIGESTION AND RESPIRATION 57 



result of which, in the case of nitrogen compounds, is ammonia or 

 closely related compounds. Many of these micro-organisms are 

 responsible for certain diseases, owing to the production by them 

 of poisonous substances, " toxins? which have powerful actions on 

 physiological processes, differing according to the particular organ- 

 ism producing them. In order to avoid putrefaction, those bacteria 

 already present must be killed and access of others prevented. 

 This is the process known as "sterilisation" familiar in the domestic 

 operation of bottling fruit. To avoid the access to wounds of those 

 organisms causing disease was the object of the " antiseptic method " 

 introduced by Lister. In his time the hospitals were swarming 

 with noxious organisms to such an extent that it was necessary to 

 dress wounds with chemical substances destructive of bacteria. 

 With the steady progress of general destruction, the use of anti- 

 septic chemicals has become less necessary, and the sterilisation of 

 the hands and instruments usually sufficient. But it must not be 

 forgotten that the principle on which Lister worked was the 

 exclusion of infection by any method whatever. The modern 

 " aseptic " method is merely one form of his treatment, made 

 possible by the previous antiseptic methods. It is clearly a difficult 

 matter to find an antiseptic chemical which kills bacteria without 

 injury to the delicate new tissues growing in a wound, although 

 some progress has been made in this direction. The need of it 

 has been made evident by the wounds of the late war, which 

 naturally became infected with all kinds of organisms. 



There are then in the soil micro-organisms which convert the 

 remains of animals and plants into ammonia compounds. Urea 

 and other nitrogenous excreta are also converted into ammonia by 

 the same agency. Now, green plants and some fungi can make use 

 of ammonia as a source of nitrogen, but it is rather remarkable 

 that green plants do better with nitrates. Indeed they are said to 

 suffer from nitrogen starvation when ammonia is their only supply 

 of nitrogen. It would have been expected that this would more 

 readily yield the NH 2 groups required for the production of amino- 

 acids and proteins. 



A part of the ammonia is probably made use of by the plant, 

 but the greater part is oxidised in the soil to nitrates by certain 

 bacteria present therein. The first stage is the production of 

 nitrites by a particular group of organisms. Another group then 

 converts the nitrites into nitrates (E., p. 186). The green plant is 

 thus supplied with that form of nitrogen food which it can utilise 

 best. Animals then consume the plants as sources of protein, and 

 so the circle is completed. But not entirely, since during the 

 conversion of the residues to ammonia some of the nitrogen 

 is lost, apparently by the agency of oxidising bacteria, becoming 



