114 Bacteria in Relation to Country Life 



largely gaseous in character. The Mouras "Automatic 

 Scavenger" may thus be regarded as the predecessor 

 of the modern "septic tank/' an important feature of 

 all efficient sewage-purification plants. 



In 1895, Cameron, then City Surveyor of Exeter, 

 England, introduced his so-called "septic tank" as an 

 efficient medium for the treatment of sewage. The septic 

 tank constructed by him consisted of a cemented pit, 

 arched over, covered with sod, and provided with a vent 

 for the gases generated from the decomposing materials. 

 The crude sewage was introduced five feet below the 

 surface of the liquid, so as to disturb it as little as possi- 

 ble, and made to pass through the tank so slowly as 

 to occupy about twenty-four hours in the process. In 

 the absence of air and light, the anaerobic organisms 

 soon attained an intense destructive activity. A leathery 

 scum was observed to have formed on the surface, thus 

 excluding the air still more effectively, while from the 

 interior of the liquid, bubbles of gas arose in large vol- 

 ume and the solid matter was constantly undergoing 

 liquefaction. The sediment accumulated very slowly, 

 being scarcely sufficient in amount to require removal at 

 the end of a year. Subsequent experiments, conducted 

 elsewhere, demonstrated that it is not even necessary 

 to cover the septic tank, for the leathery scum at the 

 surface is evidently adequate for the exclusion of air. 

 On the whole, however, the covered tank is preferable 

 to the open tank, as furnishing more uniform conditions 

 for the undisturbed action of the bacteria. 



Progress in sewage-purification. In the period during 

 which the septic tank was gradually evolved from the 



