476 
Sewage Precipitation 
[August, 
fresh body of evidence, which ought to do away for ever 
with the “conclusions” of the Rivers’ Pollution Commis- 
sioners and of their echoes. . . 
Prof. Dewar, F.R.S., and Dr. Tidy applied for permission 
to make a thorough investigation into the details. I heir 
request was granted, and early in the present year they 
began their researches. It must be distinctly understood 
that they were not in any way commissioned or instructed 
by the Native Guano Company. They were working for 
their own satisfaction, that they might be able to tell the 
public the truth on a most important question. Hence they 
were under no temptation to prophesy smooth things to the 
Native Guano Company. . , 
Another important point is the minute and _ continuous 
character of their scrutiny. For three consecutive months, 
whilst the process was carried on in the usual manner, the 
Works were placed completely at their disposal. Personally, 
or by a qualified and very skilful assistant, they were present 
daily, and had full liberty to examine the sewage the 
effluent the deposit, and the materials used, when, where, 
and how they pleased. This was a scrutiny much closer 
and more rigorous than that of the ex- Rivers Pollution 
Commissioners at Leamington, or even than that of the late 
Mr. Keates at Crossness. 
Prof. Dewar and Dr. Tidy express their formal opinion 
that “ isolated observations on the composition ot a sewage 
and of an effluent have no real value in testing the efficacy 
of a sewage process a view in which we most fully con- 
cur They add— “ Considering the faCt that the strength 
and composition of the sewage change so rapidly and fre- 
quently, it is manifest that conclusions deduced from a tew 
observations are worthless, and often misleading. _ Hence 
thev made three series of experiments, each continued lot 
twenty-four consecutive hours. Samples of the raw sewage 
and of the effluent were collected every half hour, and equal 
portions of four successive half-hourly samples were mixed 
together for analysis. . , . 
These three series of sampling were purposely carried out 
under different circumstances, as regards rainfall and cha- 
racter of sewage. It was the objedt of the inquirers to test 
the process under the most varying conditions, and see it it 
at any time failed to answer the purpose. 
The first series of samples was collected from 8 a.m. 
January 29th to 8 a.m. January 30th. The flow of sewage 
was abnormally large, and it was consequently very dilute. 
The suspended matter was only 18*8 grs. per gallon. 
