ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 89 
factors, is, on the whole, much longer than the few minutes 
spoken of by Dunbar, and may in some eases amount to more than 
one hour, which would allow time for appreciable bacterial action. 
Also that Harriette Chick, Fowler and Gaunt, and Stoddart have 
shown that absorption of ammonium salts does not take place under 
sterile conditions except with material such as clinker, &¢., which 
allows of chemical interaction between the liquid and the media. 
Stoddart has extended this observation to solutions of albumen and 
was unable to confirm Dunbar’s experiment which shewed 50 per 
cent. loss of albumen. He suggests either preliminary decomposition 
(prior to use) with formation of ammonia which would be partially 
fixed by the clinker, or direct chemical interaction as a possible 
explanation. It is obvious that experiments designed to shew purely 
absorptive phenomena should be carried out with material entirely 
inert, such as pure quartz as used by Fowler and Gaunt. 
Stoddart’s conclusion that there is no evidence of the preliminary 
absorption of the soluble constituents of sewage was based on the 
results of his experiments on the nitrification of solutions of (a) 
ammonium salts (0) urea (c) albumen and (d) of sewage deprived of 
its suspended matter by filtration through paper, in which the flow 
through his experimental filter (seeded with a vigorous growth of 
nitrifying organisms) was interrupted from time to time, and the 
liquid treated replaced by salt solution of known strength applied at 
exactly the same rate. 
The coincidence of a series of curves plotted with respect to the 
time taken to attain maximum nitrification and maximum chloride 
content was claimed to establish that preliininary absorption did not 
take place. 
Stoddart also objects to the absorption theory, developed as it was 
as the result of studies with contact beds, being applied to the modern 
percolating filters where the conditions differ very considerably. On 
the other hand, Dunbar considers that such altered conditions are 
more favourable to absorptive phenomena. 
For a full discussion of the difference between the theory of 
preliminary absorption of colloidal matter and of its direct coagulation 
and deposition in the filter, the reader is referred to a paper presented 
by Travis to the meeting of the Association of Managers of Sewage 
Disposal Works at Leicester, (July 4th, 1908), which appeared in the 
July LOth, 1908, issue of the Surveyor, and to the discussion thereon 
which ensued between the author and Liibbert, of the Hamburg 
Hygienic Institute. 
So far as the mechanism of the preliminary removal of the matter 
in colloidal solutions is concerned, both Dunbar and Travis agree 
that it is essentially the result of physical action ; mention should 
therefore be made of the work of Fowler and Mumford. They found 
that under strictly aerobic conditions, a bacteritm isolated from 
a body of water receiving colliery discharges, acting through its 
enzyme in the presence of small quantities of iron salts, was capable 
of coagulating sewage colloids in the course of a few hours. Such 
removal of colloid matter from pseudo-solution is effected without 
the aid of surface content other than is provided by the air bubbles 
and the matter precipitated. 
