688 



PROCEEDINGS OF SECTION I. 



In all cases when the ratio was under 1 every sample decom- 

 posed, this being in accordance with Johnston and others' con- 

 clusion. They, however, do not state a converse, but leave one 

 to infer that if the ratio is over 1 the samples will be stable. This 

 does not seem to be so, for 5 out of 12 with a ratio between 1 and 2 

 decomposed. With a ratio of 3 and over none of the samples went. 



Table VII shows the relation between the absolute amounts of 

 oxygen absorbed in three minutes and decomposition. When the 

 amount was over .4 all samples (three) decomposed ; when less than 

 .2 none (56) went. Between these extremes 9 out of 46 decomposed. 



In conclusion, then, I submit that a very fair idea may be 

 obtained as to the probability of decomposition occurring in a 

 sewage effluent by calculating the ratio between the oxygen 

 absorbed and the oxygen in nitrites and nitrates. This is a matter 

 of a few minutes only, and can be worked out by the following 

 formula : — 



Decomposition factor =N (as N2O3) x 1-71+N (as N2O5) x 2-86 

 -r- oxygen absorbed in 4 hours. 



I do not think that, as a general rule, an effluent could be satis- 

 factory unless tliis factor exceeds -7, because if nitrification be low 

 the probabilities are that dissolved oxygen will also be low, both 

 conditions resulting from insufficient aeration of the beds. Below 

 this figure the chances of decomposition are great and continue to 

 increase as the ratio falls, but above it apparently lies safety. 



5— .ON THE ORIGIN OF SULPHURETTED HYDROGEN AND 



SULPHIDE OF IRON IN BRACKISH LAGOONS. 



By Dr. H. G. CHAPMAN. 



Abstract. 

 From many brackish creeks and lagoons on the coast of New South 

 Wales sulphuretted hydrogen is given off more or less continuously, 

 and at times in large quantities. If a small amount of the mud at 

 the bottom of such brackish waters be added to sterilised sea water 

 the evolution of sulphuretted hydrogen begins in a few hours at a 

 suitable temperature. From such muds certain bacteria are readily 

 separated which grow freely at summer temperature. 



