Davy — Observations on Nitrification. 245 



the mixture had not been boiled previously to the complete filling, 

 corking, and sealing of the bottles, that the air dissolved in the 

 liquid was sufficient to cause the production of nitrites to some 

 extent. 



The quantity of animal matter which is held in solution in the 

 water, I find exercises a considerable influence over nitrification ; for 

 where it occurs in very large proportion, there the process either does 

 not take place at all, or is carried on much slower than in the more 

 dilute solutions. This I have proved by comparative experiments 

 with water mixed with different proportions of the same sample of 

 urine, or of solution of excrementitious matter, where I found that 

 nitrification occurred first in the more dilute mixtures ; and that where 

 there was much organic matter present, that the nitrites which might 

 ultimately be formed soon afterwards disappeared again by their 

 subsequent change or decomposition, whereas those that had been 

 produced in more dilute solutions have remained unchanged for a 

 considerable time. 



But the circumstance which I have found to exercise the greatest 

 influence over nitrification is that of temperature ; for I have observed 

 that in cold weather it is very slow in taking place, whilst in warm 

 it is much quicker, and that by the application of artificial heat the 

 process can be greatly accelerated. The correctness of this observa- 

 tion is borne out by the well-known fact, that it is from the soils of 

 different hot climates that we obtain our chief supply of nitrates. 



As to what may be the most favourable temperature for this pro- 

 cess, I have not yet been able to determine, owing to the difficulty, as 

 I am circumstanced, in maintaining continuously the same degree of 

 artificial heat ; but I have found that where the mixtures were placed 

 where they were kept at a temperature which varied from about 70° 

 to 80° F., that there, the process was carried on very quickly, and 

 that nitrites were soon abundantly formed, whereas in similar mix- 

 tures maintained at lower degrees of heat, or at the ordinary tempera- 

 ture, not a trace of those salts could be detected in the same time, 

 and that their presence was not discoverable till after a much longer 

 period. 



The foregoing observations have, I conceive, some important bear- 

 ings as regards the contamination of water with sewage, and the evi- 

 dence of such, derivable from the. occurrence in it of nitrites and 

 nitrates. For though the presence of those salts is undoubtedly in 

 many instances an indication of previous sewage pollution, still their 

 absence, taken by itself, cannot be relied on as a sure indication of the 

 freedom of the water from such contamination. For the circum- 

 stances present may have either been unfavourable to the formation, 

 of nitrites and nitrates, or have produced their subsequent rapid dis- 

 appearance — thus, for instance, the lowness of the temperature of the 

 water may have prevented their formation, or the quantity of organic 

 matter present may have interfered with their development, or have 

 led to their subsequent change and disappearance. Such, amongst 



