ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 87 
during the oxidation or purification process, it is proposed to 
consider briefly the three main theories which have been advanced 
in explanation of the purification phenomena, more particularly 
with reference to the fate of the colloid matter. 
The earliest theory advanced with respect to the mechanism 
of the purification changes effected by the filtration of ‘ sewage’ 
was based on the work carried out at the Lawrence Experimental 
Station, Mass., in respect of the development of the intermittent 
sand filters as advocated by Frankland, as a practical means of 
sewage purification. 
According to this theory, the suspended solids apphed with 
the sewage or tank effluent are arrested mechanically in the 
filter by virtue of its fine pores, and subsequently oxidised, and 
the organic matter in solution is directly oxidised and mineralised 
by bacterial agency during the course of its passage through the 
filter with the accumulation within the filter of small quantities 
of ‘humus,’ which is very resistant to further fermentation. 
Presumably colloids are included in the organic matters in solu- 
tion, although they are not referred to specifically. 
This simple representation of the purification process received 
almost universal support, notably by Stoddart as recently as 1909. 
Dunbar assumed that it was not possible to bring about the 
complete oxidation of the complex organic matter present in 
solution in sewage during the short period of time taken by the 
liquid in passing through the filter, and as the result of a series 
of experiments extending over a considerable period (1897-1900) 
carried out by him and his colleagues at the Hamburg Hygienic 
Institute (vide ‘ Principles of Sewage Treatment’) he advanced 
what is known as the ‘‘ absorption theory ’’ of sewage purifi- 
cation. 
This theory explains the mechanism of the purification changes 
as follows :— 
1. The actual suspended matters are removed by attraction 
of the filtering media, &c. 
2. The dissolved matters (true and pseudo solution) are 
retained by absorption phenomena, which is 
accelerated and increased as the slimy matter, or what 
is termed the bacterial film, accumulates on the media 
as the filter becomes mature. 
3. That the matters thus retained are oxidised by chemical 
action (condensed oxygen, &c.), and by the agency of 
bacteria and higher forms of life. The resultant 
soluble products—mineral salts, amino-acids, &c.—are 
subsequently washed out of the filter by succeeding 
quantities of sewage treated. 
4. That this absorption process is prevented from ceasing 
by the action of micro-organisms, &c., in the presence 
of atmospheric oxygen. 
5. There is a residue of organic matter, &c., which is very 
resistant to further oxidation changes, which either 
accumulates in the filter or is discharged with the 
effluent. 
