TRANSACTIONS OF SECTION B. 607. 



mixtures of potassium nitrate and septic-tank effluent containing 25 parts of 

 nitric nitrogen per 100,000 in the case of the nitrated samples. The chief 

 general conclusions arrived at were as follows : — 



(1) Potassium nitrate (and no doubt any other nitrate likely to be produced in 

 a sewage effluent) is decomposed by the septic-tank effluent, and the nitric 

 nitrogen is evolved, sometimes entirely as free nitrogen, sometimes partly as nitric 

 oxide. Nitrous oxide may also be formed, but the evidence on that point is not as yet 

 conclusive.^ The action seems to vary with different samples of septic-tank effluent, 

 but in four of the eight experiments the theoretical quantity of nitrogen [corre- 

 sponding with the added nitrate] was found either as iree nitrogen alone or along 

 with nitric oxide. 



There is no evidence from the authors' experiments to show that any con- 

 siderable quantity of these gases is produced from either the free ammonia or the 

 organic nitrogen present in the septic-tank effluent. 



(2) The general character of the change is that of combustion, the oxygen of 

 the nitrate eventually appearing either partly or entirely in the form of carbonic 

 anhydride. 



(3) In some of the experiments a little nitrite was produced, but in the 

 majority none was found at their conclusion. 



(4) The destruction of the nitrate occurs with remarkable rapidity, and in 

 most of the experiments the whole of the added nitrate — equivalent to 2-5 parts of 

 nitric nitrogen per 100,000 of mixture — was decomposed in twenty-four hours. 



(5) In addition to the decomposition of the nitrate and evolution of carbonic 

 anhydride the fermenting liquid experiences other changes. 



There is always a loss of free ammonia, and as a rule a ffain in albuminoid 

 ammonia, but the former generally exceeds the latter. Thus in six experiments 

 the average loss of free ammonia amounted to 0*266 part per 100,000, while the 

 corresponding gain in albuminoid ammonia was only 0166, or rather more than 

 half the preceding figure. 



It may also be mentioned that in two of the experiments marsh gas was found, 

 not only in the septic tank effluent, but also in the nitrated fluid, and it is some- 

 what remarkable that more of the gas was found in the latter than in the former. 



Towards the end of the investigation the effects of aeration were studied. 

 Parallel experiments were made, in one of which the septic-tank effluent was 

 thoroughly aerated before mixing it with the nitrate, while in the other the 

 same quantity of nitrate was added to the non-aerated effluent. In both experi- 

 ments the whole of the nitrate was decomposed in twenty-four hours with evolu- 

 tion of the equivalent quantity of nitrogen gas. In the aerated sample no marsh 

 gas was formed, but a considerable excess of carbonic anhydride was produced, the 

 total volume per litre of fluid being 57"1 c.c, whereas the amount corresponding with 

 the added nitrate plus the oxygen dissolved from the air was 49'8 + 5"53 = 55-33. 

 In the non-aerated sample some marsh gaa was formed, but considerably less 

 carbonic anhydride. 



The results obtained in this investigation suggest in part, at all events, an 

 explanation of the production of free nitrogen in the ' contact ' beds commonly 

 employed in the purification of sewage. These during the period they are in con- 

 tact with air no doubt become charged with nitrates, and the latter are then de- 

 stroyed when the beds are filled with sewage. 



II. Cause of the Decomposition of Nitrates when in Contact with a 



Putrefying Liquid. 



In order to ascertain whether the action was brought about entirely by the 

 vital processes of micro-organisms, and was not due to enzymes excreted by them 

 or to purely chemical changes, an apparatus was devised in which the septic-tank 

 effluent was passed through a Chamberland filter and thence into a sterilised flask 



' Gayon and Dupetit found aU three gases. 



