43^ Journal of AgricuUure. [ii July, 1910. 



A number of Australian waters cleared by chemical means have, 

 through the kindness of Dr. Bull, been examined at the Bacteriological 

 Laboratory of the University of Melbourne, and the reduction in the num- 

 ber of organisms present has been most pronounced. Not onlv dees the 

 settling mud remove bacteria, but it also carries down the eggs of hydatids, 

 thus giving the means of protecting stock against the most fruitful source 

 of hydatid infection — dirty drinking water. 



It is only the actively swimming water Crustacese which escape the 

 precipitating action of the mud, and they do not live long in the clean 

 water, owing to an absence of food supply. 



Tlie Water is also Chemically Pure. — None of the chemical added to 

 the water remains in it. It separates completely with the precipitate, and 

 the most delicate chemical tests fail to show a trace of it in the clean water 

 after the sedimentation is completed. 



The Action of Chemical Precipitants (Theoretical). 



The change in the muddy water produced by adding certain salts is not 

 that a combination or insoluble chemical compound is formed, but the 

 action is more difficult to explain, and requires certain theoretical assump- 

 tions. 



The e.xplanation which most satisfactorily meets the known facts is as 

 follows : — The fine mud particles are assumed to have minute negative 

 electrical charges associated with them. These negative electrical charge.s^ 

 keep the tiny particles asunder and prevent them uniting t(j larger and 

 heavier masses. If, however, the negative electrical charges are removed, 

 or, better, neutralized with equal positive charges, then fusion and collec- 

 tion of the fine mud into large flocculi occur. This explanation makes it 

 possible to understand the enormous effect produced by the small amounts 

 of chemicals usually employed in the chemical purification of water. 



The Action of a Chemical Precifitant. — This is determined by the fact 

 that the fine mud suspensions, met with in tank waters, carry electrical 

 charges of a negative character. 



AVhen a given salt M X is added in solution to such a water, the 

 negatively charged mud particles absorb or attract to their surface the M 

 or metal half of the salt which is electrically positive in character. Thus 

 is formed a mud particle of electro-negative character, as.sociated with a 

 metal " ion " of electro-positive character, and if the proportions be right 

 negative and positive neutralize one another, and the mud particles being 

 now without an electric charge to keep them apart, run together and 

 coalesce to large flocculi. 



Now it is found that, in such a ca.=e. the metal M is more efficient in its 

 action the higher its valency. A metal M^^ is better than M^ and better 

 still is a metal M^'^ The M^'^ metals are iron, a'uminium, and chromium, 

 and it is salts of these metals which act most efficiently. 



Thus ia) I !b. .Alummium Chloride will clear 2,500 gals, muddy water. 



(b) I lb. Ferric Chloride will (dear 1,000 — 2,500 gals, of muddy water. 



(/■;) I lb. Chromium Chloride will clear 1,000 — 2,000 gals, muddy water. 



{d) I lb. Alum will clear ?oo — i,oco gals, muddy water. 



Salts of the divalent m.etals, such as those of calcium, copper, zinc, 

 barium, magnesium, and ferrous iron. are. in terms of their chemical 

 equivalents, onlv about one-fourth as effective as the salts of the trivalent 

 metals. Also, several of them are poisonous in character, e.g., copper, 

 zinc, and barium amongst those mentioned. Calcium as lime may. how- 

 ever, be used, and owing to its great cheapness it dees not matter thnt it 



