256 Dr. H. Chick. The Process of Nitrification [Apr. 1, 



frequently looked for. It was possible, however, that, at the very first, 

 ammonia had been taken up by the filtering material, to saturation point, and 

 that afterwards no more absorption was possible until nitrification had 

 begun. Unfortunately no analyses were made at the very beginning, but 

 this gap was afterwards filled by special experiments with clean sterile 

 coke. 



The tendency of these observations was thus in opposition to any theory 

 of ammonia-absorption by the filtering material, and this opposition was con- 

 firmed by the following experiments, made to test the power of various solids 

 to absorb ammonium salts. The solids employed were barium sulphate, 

 sand, and ground-up " clinker," and the experiments were carried out as 

 follows : — 



A small quantity of a solid (1 gramme or 2 grammes), previously carefully purified, 

 was weighed out into a small flask, which was then exhausted to remove air films, which 

 might cause imperfect contact of solid and liquid.* A measured quantity (50 c.c. to 

 100 c.c.) of ammonium chloride solution was added through a tap-funnel, and the 

 whole left standing for 24 hours. The clear liquid was then drawn off by a pipette, 

 and the ammonia estimated in a small portion (1 to 5 c.c.,) ; this was first diluted to 

 about 500 c.c. with NH 3 -free water, and then distilled and nesslerised. The remaining 

 liquid was well shaken up and the muddy residue analysed similarly : a correction had to 

 be made for the volume of the solid, which was measured, after centrifugalisation, in a 

 graduated tube. In every case, as a control, a blank experiment was also made, similar 

 in every detail except that NH 3 -free water replaced the ammonium chloride solution. 

 The solids had previously undergone a careful purification by washing, and often, by 

 ignition also. The ammonium chloride solutions were exceedingly dilute, so as to approxi- 

 mate to the concentration of ammonia in ordinary sewage. 



In Experiments 1 to 3 (Table IX) the ammonia yielded, both by the clear 

 and the muddy portions of the liquid, was found to have diminished. It was 

 therefore supposed that boiling was insufficient to drive off any ammonia 

 which might have been absorbed by the solid ; accordingly, in Experiments 4 

 to 7, a small amount (10 c.c. N/1 KOH) of alkali was added before distil- 

 lation. In this case the analysis of the muddy portion of the liquid showed 

 a small amount of the ammonium salt to have been absorbed by the solid, 

 but nothing comparable to the effect required in a sewage filter. Moreover, 

 the slight removal of ammonia demonstrated would appear to be a chemical 

 rather than a physical phenomenon, alkali being necessary to free the 

 absorbed ammonia from the solid. 



In none of these experiments, however, was coke itself employed, and the 

 surface of solid was very small in comparison with the amount of liquid taken ; 



* In Experiments 6 and 7 (Table IX) the flask was not thus exhausted, and the agree- 

 ment of their results with those of previous experiments indicates that this precaution is 

 unnecessary. 



