606 REPORT— 1903. 



modern ' mad-stone ' of the Soutliern States. It is peculiar in form, and appears 

 somewhat as a water-worn pebble. 



Specimens of the current ' mad-stone ' were exhibited in illustration of the 

 paper. 



7. On the Reduction of Nitrates by Sewage. By Professor E. A. Letts, 

 D.Sc, Ph.D., R. F. Blake, F.I.C, and 3. S. Totton, B.A. 



The object of this investigation was to ascertain to what extent, and with what 

 rapidity, the nitrogenous constituents of sewage can be broken down by a suitably 

 arranged scheme of purification, so that their nitrogen is evolved in the gaseous 

 state. It is well known that in the most efficient systems of sewage purification 

 by natural methods the resulting efiluent is comparatively free from ammonia and 

 organic nitrogen, but is charged with nitrates. These latter, however, do not 

 correspond in amount with the quantities of the two former originally present, but 

 are always less, and two of the authors have shown that during the treatment of 

 sewage by the so-called ' contact ' or ' bacteria ' beds a considerable quantity of the 

 combined nitrogen escapes as free nitrogen.^ 



The researches of Gayon and Dupetit, Tacke, Adeney, and others have shown 

 that nitrates are themselves decomposed when in contact with sewage with evolu- 

 tion of nitrogen or its oxides, and carbonic anhydride. 



It therefore seemed possible that by a judicious combination of the two pro- 

 cesses, i.e. production and destruction of nitrates, a considerable proportion and 

 possibly most of the combined nitrogen present in sewage might be converted 

 into free nitrogen, and as a consequence the resulting effluent be deprived of its 

 fertilising properties in relation to vegetation. 



Such a result might appear useless, and even wasteful, under ordinary circum- 

 stances, but in the case of the sewage of Belfast, and probably in that of other 

 towns similarly situated, it is really necessary. At Belfast, at all events, one of 

 the authors has shown that the growth in enormous quantities of the green sea- 

 weed Ulva latissi7na and the resulting nuisance which occurs when it is washed 

 ashore and putrefies — as happens each summer — is directly due to the fertilising 

 properties of the sewage of the city, which is poured into the lough in an un- 

 treated condition.- 



The experiments conducted by the authors consisted of — (I.) a study of the 

 changes which occur when potassium nitrate is added to the effluent from a 

 septic tank, and the speed with which they occur ; (II.) an investigation as re- 

 gards the cause of these changes ; (III.) the action of pure cultures of specific micro- 

 organisms in broth containing potassium nitrate. 



I. The Chemical Changes occurnng ivhen Potassium Nitrate is added to the 

 Effluent from a Septic Tank, 



The method of experiment consisted in completely filling two similar bottles 

 (which were then tightly stoppered), one with the septic-tank effluent alone and 

 the other with the same effluent ^^2<s an accurately measured volume of a strong 

 standard solution of potassium nitrate (1 c.c. = 10 mgs. nitric nitrogen). Allowing 

 the two bottles so filled to remain at the temperature of the laboratory for a given 

 interval of time, their contents were examined quantitatively as regards (a) dis- 

 solved gases, (b) nitrates, (c) nitrites, {d) free and albuminoid ammonia. 



Practically all the experiments (which were eight in number) were made with 



' ' On the Chemical and Biological Changes occurring during the Treatment of 

 Sewage bv the so-called Bacteria Beds,' by Professor Letts, D.Sc, Ph D., and R. F. 

 Blake, F.C.S., B.A. Rep. 1901. 



2 This used to be the case, but works for the purification of the sewage are being 

 pushed on rapidly, and at the present time a considerable proportion is being 

 purified. 



