86 REPORTS ON THE STATE OF SCIENCE.—1918. 
On the other hand Rolants, employing a precipitation method 
for removal of colloids, found that while the percentage of the 
total oxidisable matter present in pseudo solution was slightly 
higher in septic tank effluent than in the sewage treated, the 
actual amount of colloidal matter (as measured by the perman- 
ganate test) was, if anything, rather less. On determining the 
organic nitrogen and organic carbon contained in the colloids 
present, which was considered to give a better appreciation of the 
colloid matter than does the oxygen absorption test, he found in 
every case a diminution of colloid matter in the septic tank 
effluent as compared with the original sewage. 
On the whole, it may be considered that septic action as 
exhibited during septic tank treatment has no material effect on 
the amount of matter in pseudo solution, the differences found 
being very little more, if any, than the errors in the method of 
estimation. 
The chief object of preliminary treatment in slate beds is to 
keep the solids deposited therein under aerobic conditions as far 
as possible, and Dibdin has shown that if operated in a proper 
manner a considerable destruction of the deposited sludge is 
brought about by the agency of bacteria, moulds, and higher 
organisms such as protozoa, worms, &c. 
While it is possible that contact with the slates will tend to 
flocculate, by surface action, some of the matter in pseudo solu- 
tion, the filters are too open to be very effective in this respect, 
and it is understood that the effluents obtained are still turbid 
and contain appreciable quantities of colloidal matters. 
It can be stated that with properly designed plant, having 
regard to the maximum permissible velocity of flow, a more or 
less complete removal of the grosser suspended solids can be 
obtained, and that, dependent on the character of the sewage, a 
considerable proportion of the more finely divided solids, includ- 
ing, doubtless, colloids in the ‘ gel’ condition, may be effected. 
It is, however, evident that none of these preliminary processes, 
except heavy chemical treatment, affect appreciably the organic 
matter in pseudo solution. 
The resultant effluent, therefore, contains practically the 
original sewage solids in solution together with a varying propor- 
tion of the finely divided suspended solids, and its subsequent 
purification famalnes the application of extended surfaces. 
This is usually effected by distribution on land or on artificial 
filters composed of various inert media of different grades, in 
which the liquid is held in contact with the filtering media 
(placed in water-tight tanks), or is allowed to percolate through, 
either intermittently or continuously. 
Briefly, it may be said by the adoption of any of these methods 
no difficulty need be experienced in obtaining a satisfactory 
effluent, provided due attention is paid to the design of the puri- 
fication works and to its capacity in relation to the volume and 
character of sewage treated, 
With the object of dealing with the mechanism of the complex 
changes, physical, chemical and biological, which take place 
