ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 83 
of the albuminous bodies present, and ‘‘ Oxygen absorption ”’ 
from an acid solution of permanganate (usually — K MnQO,) 
80 
under stated conditions, which gives an indication of the amount 
of oxidisable matter present. 
For the purpose of controlling purification plants, in addition 
to the above tests, the amount of oxidised nitrogen (Nitrite and 
Nitrate) is determined in the effluent. 
Incubation will show whether the effluent is putrescible, and 
determination of the amount of dissolved oxygen absorbed by 
the effluent from aerated water, as recommended by the Royal 
Commission on Sewage Disposal as a Standard Test, will afford 
valuable information with respect to the matter still capable of 
fermentation, and consequently of the extent to which the 
oxidation change has been carried. 
With the object of obtaining information in relation to what 
happens to the colloidal matters during the purification process, 
at any rate so far as the ‘‘ hydrosols ’’ are concerned, these tests 
have been supplemented by submitting the sewage and effluents 
to dialysis and thus determining the proportion of dialysable to 
non-dialysable matter and their respective oxygen absorption 
and albuminoid ammonia content, see Rivers Committee Report, 
Manchester, 1901, Kréhnke & Biltz, Fowler & Ardern, Travis & 
Johnston, O’Shaughnessy & Kinnersley (1901-1906). 
An objectionable feature of this method of attack is the time 
required by the dialysis operation, which is usually not less than 
24 hours, even where the determination is stopped when equi- 
librium is attained (as measured by the chloride content) on 
either side of the membrane employed, during which period there 
are many possibilities of change of character in an unstable liquid 
like sewage. In the method employed by Johnston for the 
complete separation of the non-dialysable from the dialysable 
constituents of the sewage, a period of 6 days usually elapsed. 
The sewage and effluents were sterilised by the addition of acid 
with the idea of obviating changes due to bacterial action, but 
it is obvious that mere acidification is likely to alter the character 
of the samples under examination. 
The method proposed by Riibner of coagulating the colloidal 
matter by precipitation with acetate of iron surmounts this diffi- 
culty but introduces other errors with respect to the removal of 
organic matters in true solution. Fowler and others elaborated 
this test and called it the ‘‘ clarification test,’? while Rolants 
compared the effect of acetate of iron with that of calcium 
chloride and sodium phosphate, alumina, and powdered tale, 
respectively, the latter material being employed with the idea of 
avoiding precipitation of organic matter from crystalloidal 
solution. 
A difficulty common to both these methods of investigation is 
the character of the preliminary treatment of the sample, under- 
taken for the removal of the finely divided non-colloidal matter, 
The method of procedure is either to allow the sample to stand 
