682 PROCEEDINGS OF SECTION H. 
with which it is constantly in contact, and there will thus be an 
end to the antiseptic properties of the system. 
Mr. Dibden further states that where it is advisable to increase 
the effect of the lime, experience has shown that either sulphate 
of alumina or sulphate of iron in various proportions are best 
adapted for the purpose. The actual result of the combination of 
lime and either of the salts of alumina or iron depends in a great 
measure to the manner in which the lime is applied. Experiments 
have shown the hme is best applied in the form of “milk of 
lime.” 
The discussion on the paper, from which the foregoing state- 
ments have been quoted, is remarkable for the diversity of opinion 
of the chemical experts as to the efficiency of chemicals in connec- 
tion with purification and precipitation of sewage. The general 
opinion, however, tends towards the fact that the questiom is 
capable of more extended investigation. Whatever difference of 
opinion may exist among chemists on the matter, sanitary 
engineers are decidedly of opinion that before the effluent can be 
considered up to a standard of purity to admit of it passing 
into fresh-water streams, it should, subsequent to chemical treat- 
ment, be passed over and filtered through land. 
Mr. Dibden, when investigating the question for the Metro- 
politan Board of Works, London, made over 500 different analyses 
with 23 samples of London sewage, and treated them 25 different 
ways, and the results of the investigation show that the use of a 
larger quantity of chemicals would, independently of the expense 
of dealing with the increased quantity of sludge—an important 
consideration—have cost a great deal more than their worth. 
The table accompanying Mr. Dibden’s paper shows that whether 
he added 4:7 grains of chemicals, or 108 grains per gallon, at 
costs respectively of 11d. per head and 4s. per head, it did not 
make more than 20 per cent. difference in the amount of organic 
matter removed. 
The quality of sewage dealt with at an outfall varies con- 
siderably during the twenty-four hours, and the difficulty with 
chemical treatment is to apply the chemicals at all times so as to 
obtain the desired result. 
A few experiments were made at the Botany Sewage Works in 
connection with the Sydney main drainage. The quantity of 
chemicals used in the first instance was the same recommended 
by Mr. Dibden for treating the London sewage, viz., 3:7 grains 
of lime per gallon. This amount of chemicals, when mixed with 
the sewage, had no visible effect in precipitation, and with regard 
to purification the liquid was to all appearances as foul as it was 
before the chemicals were put in. When it is considered that 
there are 70,000 grains in a gallon, it is obvious that the homeo- 
pathic dose of 4:7 grains would be useless for either clarification 
or purification. 
