140 BACTERIA IN AIR, SOIL, AND WATER. 



and therefore it is more easily broken up in the next stage of 

 the process. This consists of running the effluent from the 

 septic tank on to filter beds, preferably of coke, where a further 

 purification process takes place. By this method there is first 

 an anaerobic treatment succeeded by an aerobic ; in the latter 

 the process of nitrification occurs by means of the special bac- 

 teria concerned. The results are of a most satisfactory nature. 

 Sometimes the effluent from a sewage purification system 

 contains as many bacteria as the sewage entering, but, especially 

 by means of the septic tank method, there is often a marked 

 diminution. It is said by some that pathogenic bacteria do not 

 live in sewage, but in an effluent B. coli, B. enteritidis, and 

 streptococci have been constantly found, so that the observation 

 is of little value, and it is only by great dilution and prolonged 

 exposure to the conditions present in running water that such 

 an effluent can be again a part of a potable water. 



ANTISEPTICS. 



The death of bacteria is judged of by the fact that when they 

 are placed on a suitable food medium no development takes 

 place. Microscopically it would be observed that division no 

 longer occurred, and that in the case of motile species move- 

 ment would have ceased, but such an observation has only 

 scientific interest. From the importance of being able to kill 

 bacteria an enormous amount of work has been done in the way 

 of investigating the means of doing so by chemical means, and 

 the bodies having such a capacity are called antiseptics. It is 

 now known that the activity of these agents is limited to the 

 killing of bacteria outside the animal body, but still even this is 

 of high importance. 



Methods. These vary very much. In early inquiries a great point was 

 made of the prevention of putrefaction, and work was done in the way of 

 finding how much of an agent must be added to a given solution such as beef 

 infusion, urine, etc., in order that the bacteria accidentally present might not 

 develop ; but as bacteria vary in their powers of resistance, the method was 

 unsatisfactory, and now an antiseptic is usually judged of by its effects on pure 

 cultures of definite pathogenic microbes, and in the case of a sporing bacterium 

 the effect on both the vegetative and spore forms is investigated. The organ- 

 isms most used are the staphylococcus pyogenes, streptococcus pyogenes, 

 and the organisms of typhoid, cholera, diphtheria, and anthrax the latter 

 being most used for testing the action on spores. The best method to employ 



