197 



arc iinohed. I'liere remains, then, lint one satisfactory expedient: 

 To let the reaction proceed by itself and to note the result. But here 

 also there are difficulties, because bacterial reactions of this sort are 

 necessarily slow in reaching equilibrium, and the time required by a 

 nicely balanced effluent is greater than can be allowed in routine 

 work. Some arbitrary period of timie, therefore, is usually adopted, 

 and it is in respect to this factor that the confusion arises. If sta- 

 bility is to be considered a definite qualitative characteristic of an 

 effluent, that characteristic should be determined by a test sufficiently 

 prolonged to insure equilibrium, but such procedure is not feasible for 

 obvious practical reasons, and it is not desirable, because it is not 

 enough simply to know that the available oxygen is sufficient or in- 

 sufficient to satisfy the demands of the bacteria that are working on 

 the organic matter. If the available oxygen is sufficient, there is per- 

 fect stability — a definite condition; if it is insufficient, there is still 

 stability in the quantitative sense — a relative stability determined by 

 the relation of the available oxygen tO' the total amount of oxygen 

 required by the organic matter for perfect stability. In practice the 

 latter condition is the one usually encountered." 



Owing to the varied meanings which are attached to the word 

 putrescibility, Phelps ('09, p. yy') has recommended the word stabil- 

 ity to designate "that desirable quality which is the usual object of 

 sewage purification — the transformation of the organic matter to such 

 a form that it is incapable of undergoing ofTensi\e putrefaction." 

 He argues that the term stability implies a positive characteristic 

 which is acquired during the purification process, while the term 

 putrescibility refers to a negative characteristic. Stability describes 

 that condition in which the available oxygen exceeds the required 

 oxygen. The term putrescibility has, however, been retained in this 

 paper owing to the fact that it is still largely in use in the literature 

 which deals with sewage investigation. 



From the economic point of view questions bearing directly upon 

 the putrescibility of sewage are of the greatest importance. The ul- 

 timate aim of all sewage disposal operations is to render the putres- 

 cible matter as stable as possible, and any factors which facilitate or 

 hinder this process are of considerable practical importance. Since, 

 then, these Imiibricillid worms occur in such great abundance in con- 

 nection with devices which are operated to overcome the putrescibility 

 of sewage their possible favorable or unfavorable relation to this proc- 

 ess is a pertinent subject of inquiry. 



The purpose of the following experiments was to discover, if pos- 

 sible, just what effect these organisms have on putrescibility. Tests 



