356 Messrs Purvis and Gourtauld, Decomposition and nitrification 
Experimental part. 
The sewage samples were obtained from the Cambridge sewage 
farm. It was analysed for the amount of the free and albuminoid 
ammonias, nitrates and nitrites already present, and then mixed 
with (1) .ammonia-free distilled water made faintly alkaline with 
soda solution, and (2) with distilled water alone, so that each solu- 
tion contained 1 per cent, of the unfiltered sewage. The solutions 
were placed in Winchester bottles : the neck of each was loosely 
closed by a plug of cotton wool : and they were kept at a uniform 
temperature of about 15° C. The bottles and their contents were 
shaken from time to time, so as to aerate the contents very 
thoroughly. They were allowed to incubate, and their contents 
estimated from time to time. The two ammonias were estimated 
by Wanklyn’s method, and the nitrites by means of meta- 
phenylene-diamine. 
With regard to the estimation of nitrates, Purvis and Coleman 
used the indigo method, and they found they could detect 
nitrogen as nitrates added to sea-water in strengths not less 
than 0 005 parts per 100,000, but that the results were unsatis- 
factoiy in any proportions below this. In the present experiments 
we have used the zinc-copper couple method as described in full 
detail by Dr McGowan and his assistants* - . It seemed as if the 
method would enable us to estimate smaller quantities of nitrates 
than the above. We closely followed the instructions in the 
Report, which are most clear and detailed. At the same time, 
and as mentioned by Purvis and Coleman in their paper, there is 
some possibility that such a couple not only would reduce the 
nitrates and nitrites to free ammonia, but that also other nitro- 
genous compounds, like the albuminoids, might be broken down 
into simpler forms, of which ammonia might be one. The final 
result would be that what ought to be considered ammonia 
derived from the reduction of nitrates and nitrites, is also ammonia 
produced from the decomposition of other compounds, and the 
figure obtained would be correspondingly high. 
However, the aim of the following experiments is not to deter- 
mine the smallest quantity of nitrate produced in any incubation 
of sewage, but to see whether the decomposition of sewage when 
incubated in a weakly alkaline solution and also with pure water 
compares with the results obtained by Purvis and Coleman when 
the sewage was incubated with saline solutions and with sea- 
water. 
In the first and second series of experiments, the sewage was 
turbid, and the coarser particles in suspension were allowed to 
* Royal Commission Report on Sewage Disposal, Yol. iv. Pt v. p. 17. 
